Oat–Vetch Hay as a Dry Season Feed for Grazing Heifers in the Peruvian Andes: Effects on Performance and Methane Emission Intensity

Abstract

The objective of this study was to assess the contribution of oat (Avena sativa L.) and common vetch (Vicia sativa) hay supplementation as a forage-based strategy to improve the environmental and productive performance of grazing systems in the high Andean zone through its effects on enteric methane (CH₄) emissions and live weight gain. Twenty heifers grazed native grasses, and only half of the group received the supplement. The experiment was conducted as a crossover design. Methane emissions were quantified through sulfur hexafluoride methodology. Native pastures were characterized by low protein content, while lignin was lower in the oat hay plus common vetch hay than in the native grass mixture. On average, heifers consumed 7 kg dry matter per day (p ≥ 0.05) and ingested 24% more crude protein when supplemented (p = 0.0001). Digestible and metabolizable energy intakes were also significantly higher in supplemented animals (p ≤ 0.05). Live weight change was positive for supplemented animals (245 g/d). Net CH₄ production ranged from 179.6 to 196.3 g/d (p = 0.183). However, when CH₄ emissions were expressed relative to crude protein or acid detergent lignin intake, supplemented diets were found to emit less than native grass-based diets (p ≤ 0.05). These results suggest that supplementation with oat hay plus vetch is a feeding alternative for heifers during the dry season in the Peruvian Andean region to increase animal productivity without affecting CH₄ emissions.

1. Introduction

In the last two decades, global warming generated by the increase in greenhouse gases (GHGs) trapped in the atmosphere has been considered one of the major environmental impact problems [1]. Therefore, through multinational agreements, a commitment was ratified to reduce the emissions of the main gases caused by human activity, which are: carbon dioxide (CO₂), nitrous oxide (N₂O) and methane (CH₄). The latter is the main product of enteric fermentation in ruminants, and according to the Global Methane Assessment published by the United Nations Environment Programme and the Clean Air Coalition, global estimates indicate a wide range of emissions, approximately 150 to 420 L of CH₄ per animal per day, reflecting substantial variation among production systems, animal productivity levels and dietary conditions [2].

Peru, in 2001, committed to take actions to mitigate GHG emissions, despite being considered a country with low per capita and total GHG emissions. According to the Ministry of Environment, Peru produced 210,404.42 Gg CO_2eq in 2019; this represented 0.42% of total global emissions [3]. The agriculture sector contributed 13.5% (28,478.34 Gg CO_2eq) of the total emissions in the country, of which 54% (15,432.44 Gg CO_2eq) corresponds to the contribution made by ruminants in the process of enteric fermentation [3]. According to recent data from the National Agricultural Survey (ENA 2025) of the National Institute of Statistics and Informatics (INEI) [4], Peru currently maintains a cattle population of approximately 5.5–5.8 million head, with the Andean region concentrating close to three-quarters of the national herd, predominantly under extensive grazing systems based on native pastures. The Andean region comprises approximately 22.3 million hectares of agricultural land, representing about 57.5% of the national agricultural area, most of which corresponds to permanent pastures and native rangelands used for livestock production [5]. In this region, cattle production systems are predominantly extensive and rely on low-quality forage resources, a condition that has been identified as a key factor contributing to low animal productivity and increased environmental pressure per unit of animal product [6].

Native high-Andean pastures constitute the main forage base of extensive grazing systems in the region and are composed of diverse communities of perennial grasses adapted to harsh environmental conditions. These grasslands are typically dominated by tussock species such as Stipa ichu (Kunth) and Festuca dolichophylla, which are highly resilient to low temperatures, drought and nutrient-poor soils and play a key role in soil protection and ecosystem stability [7,8,9]. Stipa ichu is characterized by a high fiber content and low digestibility, whereas Festuca dolichophylla generally exhibits greater biomass production and comparatively better forage potential for ruminants and South American camelids [9,10]. In addition to these dominant species, native pastures may include grasses of higher nutritional quality, such as Poa spp. and Bromus spp., which are associated with greater digestibility and protein content; however, their contribution to animal diets is often limited under extensive grazing conditions dominated by fibrous species [7,11]. Consequently, the predominance of low-quality native grasses can constrain animal performance and increase methane emission intensity in pasture-based systems [12,13].

In the process of carbohydrate metabolism, the rumen microbiota, composed of bacteria, fungi and protozoa through fermentation, generate short-chain fatty acids and by-products such as enteric CH₄ (85–90%). This CH₄ production represents an energy loss for the animal, as ruminants are unable to utilize this gas, and the majority of enteric methane (approximately 90–95%) is released through eructation, while a small fraction (<5%) is emitted via the posterior digestive tract; pulmonary exhalation contributes negligibly to total methane emissions [14]. It is known that the production of this gas varies according to the total dry matter intake and the quality of the diet [15]. Within the latter strategy, the use of concentrates, lipids, glycerol, nitrites and nitrates, essential oils, ionophores, fibrolytic enzymes and improved forages are the most promising reported in the literature [16,17].

Common vetch (Vicia sativa L.) is a legume that is widely distributed worldwide and is characterized by its high levels of carbohydrates, proteins and other secondary components [18,19] due to its high capacity to adapt to diverse environmental conditions. Oat hay, on the other hand, helps improve nutrient intake, digestibility, nitrogen balance and energy utilization compared to native grasses [20,21].

Therefore, the integration of oats and common vetch as a conserved forage supplement represents a grassland-based management strategy with the potential to enhance forage utilization efficiency and reduce the environmental footprint of grazing systems in the Andean highlands. Improving the quality and seasonal availability of forage resources is a key component of sustainable grassland management in high-altitude systems, where native pastures often limit animal performance and increase methane intensity.

We hypothesized that supplementation with oat plus common vetch hay would improve forage intake, dry matter digestibility and overall grazing system efficiency, thereby enhancing animal productivity while reducing enteric methane emissions per unit of output compared with native pasture grazing alone. Accordingly, the objective of this study was to evaluate the role of oat plus common vetch hay supplementation as a forage-based intervention on intake, dry matter digestibility, animal performance and enteric methane emissions intensity in grazing heifers managed under extensive grassland conditions in the central highlands of Peru.

2. Materials and Methods

2.1. Location

The experiment was performed from August and September 2021 at the Sociedad Agrícola de Interés Social Túpac Amaru in the production center of CONSAC, located in the province of Jauja-Junín at 3752 m.a.s.l. (11°43′58″ LS and 75°45′20″ LW). This period corresponds to the dry season in the central Peruvian Andes, when native pastures typically exhibit reduced nutritional quality and forage supplementation is commonly practiced by local producers. The area is characterized by an ambient temperature of 8.6 °C, 72% relative humidity and an average annual rainfall of 730 mm [22].

2.2. Description of the Animals and Measurement of Their Performance

The experiment was carried out under the protection of Peruvian Law No. 30407, which protects and cares for animals. Twenty heifers (Brown Swiss × crossbreed) were randomly selected from a total of sixty animals. At the beginning of the experiment, the heifers had an average age of 18 months and weighed 262 ± 17.49 kg. The selected heifers were dewormed with Ivermectin (1%) 15 days before the beginning of the experiment. Live weight was monitored on a conventional scale of the Torre Casilda^® brand (Bv. Lisandro de la Torre 1265 Santa Fe, Casilda, Argentina).

2.3. Diets and Experimental Design

Oats (Avena sativa L.) and common vetch (Vicia sativa sp.) were broadcast sown in October and November 2020 in proportions of 70:30, respectively, and fertilized with ammonium sulfate, ammonium phosphate and potassium chloride (80–60–60 kg NPK/ha) at the time of sowing, as reported by producers from the Agricultural Society of Social Interest Túpac Amaru at the CONSAC production center. Harvesting was carried out after 5.5 months at a height of 5–10 cm above ground level, and the forage was left to dry in the open air until moisture content was reduced by more than 80%.

The experiment followed a cross-over design with two dietary treatments. The control treatment consisted of animals grazing native grass pastures only, without supplementation. The supplemented treatment consisted of animals grazing native grass pastures plus hay supplementation of oats and common vetch, provided at a rate of 1% of body weight on a dry matter basis (2.55 kg DM of oat plus common vetch hay per animal per day). Supplementation was provided once daily during the morning hours, prior to grazing. During the first stage, only 10 of the 20 experimental units received the hay supplementation; in the second stage, the supplementation was exchanged among animals.

Animals were housed in pens during the night, and during the day they grazed freely in open native rangelands under continuous supervision by a herder. Pasture biomass was not measured because the grazing system was conducted in extensive open-field conditions, with animals moving across large areas of native pasture accompanied by a shepherd, which prevented accurate estimation of available herbage mass.

In addition, all animals were provided with a mixture of iodized mineral salt consisting of 100 g of common salt plus 50 g of a mineral–vitamin supplement (Rocsalfos^®) per animal per day. After hay supplementation, animals were allowed to graze selected paddocks for a period of 9 h and then returned to their pens. The native pastures were mainly composed of Stipa ichu (Ruiz & Pav.) Kunth and Festuca dolichophylla J. Presl, and grazing management was adjusted according to pasture availability. Each experimental stage included a 14-day adaptation period to the diet and equipment, followed by 5 days of methane and feces sampling. No washout period was included between treatments because animals remained on the same native pasture base and the supplements used were forage based. The 14-day adaptation period was considered adequate to stabilize rumen fermentation and methane production, thereby minimizing potential carryover effects between treatments.

2.4. Chemical Analysis and Digestibility

The grazing consumption simulation technique [23] was selected to collect forage samples for chemical composition analysis, while the sampling of the pasture offered to obtain the total available biomass of forage in the paddocks was determined by the Zig-zag method [24], using a 1 m × 1 m polyvinyl chloride tube quadrant. The above-described method was carried out during three continuous days per stage. With respect to the sampling of oat and vetch hay, availability and rejection samples were collected for 5 days.

After grass and hay samples were obtained in the field, dry matter (DM) was determined by moisture extraction in an oven at 60 °C for 48 h. They were then ground, sieved with a 1 mm sieve and stored at environment temperature for later analysis at the Feed Nutritional Evaluation Laboratory (LENA, Spanish acronym). The analyses performed were ash, crude protein (CP) according to AOAC [25] and fibrous components such as neutral detergent fiber (NDF), acid detergent fiber (ADF) and acid detergent lignin (ADL), using the ANKOM^® (Ankom Technology Corporation Fairport, New York, NY, USA) in accordance with the methodology described by Van Soest [26], and in vitro digestibility of organic matter (DOM) was determined with the Ankom DaisyII Incubat^® (ANKOM Technology, Macedon, New York, NY, USA) [27]. The gross energy (GE) value was determined by calorimetry (methodology 9831, ref. [28]); the digestible energy was calculated from the gross energy and DOM. Metabolizable energy (ME) was determined by the equation of Menke and Steingass [29]:

$$ME (\mathrm{M}\mathrm{J}/\mathrm{k}\mathrm{g} DM)=(-1.15+0.1600 DOM)$$

(1)

where ME is metabolizable energy; DOM is to digestibility of organic matter; and DM is feed dry matter.

2.5. Feed Intake

To determine the voluntary feed intake of each heifer, the external marker titanium dioxide (TiO₂) was used following the methodology described by Sampaio et al. [30] and Alvarado-Bolovich et al. [31]. The animals were administered the marker with a bolus lance (10 g/d) in the mornings wrapped in biodegradable paper. Fecal collection was approximately 200 g of feces per animal directly from the rectum twice per day (morning and evening) for 5 days. These were homogenized (one sample from each heifer per period) and refrigerated for subsequent TiO₂ analysis. In both periods, fecal samples were collected after 7 days of adaptation. Diet composition used for intake calculations was estimated from hand-plucked samples obtained through grazing simulation to represent the forage selected by the animals under field conditions; due to the extensive nature of the grazing system, pasture biomass was not measured directly, and intake estimates therefore reflect diet selection rather than measured biomass availability.

The equation performed by Glindemann et al. [32] was used to calculate fecal organic matter (FOM) production per day:

$$FOM production\left(\mathrm{g}/\mathrm{a}\right)={\displaystyle \frac{External markers intake (\mathrm{g}/\mathrm{d})}{External markers quantity in feces (\mathrm{g}/\mathrm{k}\mathrm{g} OM)}}\times 100$$

(2)

Likewise, the equation established by Lukas et al. [33] was used to find the value of the apparent digestibility of organic matter (aDOM) of grass consumed by heifers:

$$aDOM\left({\displaystyle \frac{\mathrm{g}}{\mathrm{k}\mathrm{g}}}\right)=a-107.7\times e^{\left[-0.01515 \times CPf \left(\frac{\mathrm{g}}{\mathrm{k}\mathrm{g}}OM\right)\right]}$$

(3)

where a is 72.86, a constant used for grazing animals, and CPf is the amount of fecal crude protein (R = 0.82).

Organic matter intake (OMI) per animal was estimated from the results obtained in Equations (2) and (3) [32]:

$$OMI\left({\displaystyle \frac{\mathrm{k}\mathrm{g}}{\mathrm{a}\mathrm{n}\mathrm{i}\mathrm{m}\mathrm{a}\mathrm{l}}}\right)={\displaystyle \frac{\mathrm{F}\mathrm{O}\mathrm{M} \mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \left(\frac{\mathrm{g}}{\mathrm{a}\mathrm{n}\mathrm{i}\mathrm{m}\mathrm{a}\mathrm{l}}\right)}{100-\mathrm{a}\mathrm{D}\mathrm{O}\mathrm{M}}}\times 100$$

(4)

Daily dry matter intake (DMI) per animal grazing was calculated as follows:

$$DMI\left({\displaystyle \frac{\mathrm{k}\mathrm{g}}{\mathrm{a}\mathrm{n}\mathrm{i}\mathrm{m}\mathrm{a}\mathrm{l}}}\right)={\displaystyle \frac{\mathrm{O}\mathrm{M}\mathrm{I} (\mathrm{k}\mathrm{g}/\mathrm{a}\mathrm{n}\mathrm{i}\mathrm{m}\mathrm{a}\mathrm{l})}{\mathrm{O}\mathrm{M}\mathrm{d} (\mathrm{g}/\mathrm{k}\mathrm{g} \mathrm{D}\mathrm{M})}}\times 1000$$

(5)

where OMd is the organic matter in the diet content.

The estimated DMI was multiplied by each of the nutritional or energy compounds in the diet to find their intakes (CPI, NDFI, ADF and GEI). Likewise, the apparent NDF digestibility (NDFD) of the grass ingested per animal was estimated with the daily NDF intake and the fecal NDF productions.

2.6. Enteric Methane Emissions

Enteric methane (CH₄) emissions were continuously collected near the animal’s mouth and nose, while an intraruminal device slowly and constantly released the tracer gas sulfur hexafluoride (SF₆). This inert gas simulates CH₄ emission in the rumen and has no impact on fermentation [34]. On Day 1 of the first experimental stage, animals were introduced into the rumen with a single bronze permeation capsule using a bolus launcher, containing 2.2 to 2.8 g of SF₆. The capsule remained in the rumen throughout the entire experimental period, and no additional capsules were introduced in subsequent stages. After completing a 14-day adaptation period, methane emission measurements were initiated using the SF₆ tracer technique. Prior to the dosing of the animals, the gas release rate (SF₆) was quantified for 8 weeks at the National Institute of Agrarian Innovation–INIA Santa Ana-Huancayo; for this purpose, the capsules were stored in a forced air oven at 39 °C, and their weight was recorded three times a week. The release rate and shelf life of each permeation capsule was estimated to use a linear regression equation between the time and weight variable, as indicated by Berndt et al. [35]. Overall, the coefficient of determination (R²) between these variables was 0.999, and their average permeation rate was 4.695 mg/d per capsule.

The exhaled gas from the nose was collected through a nylon hose (with an internal diameter of 0.3 cm) to a storage tube. Inside this hose was a Swagelok^® (Cleveland, OH, USA) particulate filter and a steel capillary of 0.01 internal diameter that allowed a constant sampling flow. In addition, the nylon hose was covered by another hose made of polyethylene that served to protect the collection line and in which there was a fabric filter at the beginning to prevent clogging of the system with animal feed. The container where the gas was collected was a tube made of polyvinyl chloride with an approximate volume of 700 mL. These containers were subjected to a vacuum test for 24 h before use to ensure their negative pressure.

On Day 1 of the measurements, the collection system was placed on each animal with the help of the ropes and remained there for the next five days. On the last day, the canisters and the flow line were removed; then, with the help of a syringe, the stored gas was removed and injected into vials, which were previously cleaned with ultra-high purity nitrogen, and a vacuum was performed [31,36,37].

In the laboratory, CH₄ and SF₆ concentrations were determined by gas chromatography (Agilent 7890B^®, Agilent Technologies, Santa Clara, CA, USA) from a 0.6 mL subsample of gas obtained from the vials. The chromatographic parameters used for these quantifications were described by Alvarado-Bolovich et al. [31] in their article.

Daily enteric CH₄ emissions per animal adjusted for environmental pressure were performed according to the equations of Williams et al. [38] and Deighton et al. [39]:

$$Methane\left({\displaystyle \frac{\mathrm{g}}{\mathrm{a}\mathrm{n}\mathrm{i}\mathrm{m}\mathrm{a}\mathrm{l}}}\right)=TL{SF}_6\left({\displaystyle \frac{\mathrm{g}}{\mathrm{d}}}\right)\times \left[{\displaystyle \frac{\left({CH}_{4 vs}-{CH}_{4 as}\right)}{\left({SF}_{6 vs}-{SF}_{6 as}\right)}}\right]\times \left[{\displaystyle \frac{\left(P1\right)}{(P2-P3)}}\right]$$

(6)

where TLSF₆ is the rate of gas release from the capsule into the rumen; CH₄vs and SF₆vs are the contents of both gases obtained by chromatography in the vials; CH₄as and SF₆as are the contents of methane and sulfur hexafluoride in the environment; and P1, P2 and P3 are the differential pressures after pressurizing with nitrogen, before and after gas sample collection, respectively.

Multiple methane intensity metrics (CH₄/DMI, CH₄/OMI, CH₄/DOMI, CH₄/CPI, CH₄/NDFI, CH₄/ADFI, CH₄/ADLI and Ym) were calculated to describe the relationship between enteric methane emissions and the intake of different nutritional fractions. These metrics allow for a more comprehensive assessment of dietary treatment effects on methane production. For the primary interpretation of results, CH₄/DMI and Ym were prioritized, as they are widely used and comparable across studies

2.7. Statistical Analysis

The experimental model was cross-over; that is, the treatments were randomly assigned to the animals, and to determine the effect of the treatments on the variables such as intake and production of CH₄, the PROC MIXED procedure was used with SAS 9.4 software [40]. The separation of means was performed using Tukey’s test with an alpha of 0.05.

The model is described as Yijk = μ + δi + βk + Pj + δi × Pj + eijk, where the following are true: Yijk: observation of heifer k under diet i in period j; μ: overall mean of the population; δi: effect of the i-th diet (i = 1 and 2); Pj: effect of the j-th period (j = 1 and 2); βk: effect of k-th animal (k = 1 … 20); δi × Pj: interaction between diet and period; and eijk: experimental error.

3. Results

3.1. Chemical Compositions of Forages and Diets

The chemical composition of the diets offered to the heifers in the CONSAC-SAIS Túpac Amaru production unit during the dry season is presented in Table 1. Organic matter content was similar among treatments, with values of 854.8 g/kg DM for native grasses (Diet 1), 856.4 g/kg DM for oat–vetch hay and 852.6 g/kg DM for the mixed diet consisting of native grasses plus oat–vetch hay (Diet 2).

Table 1. Chemical compositions of diets offered to heifers in the CONSAC-SAIS Túpac Amaru production unit during the dry season.

Parameter	Native Grasses (100%) Diet 1	Oat–Vetch Hay	Native Grasses Plus Oat–Vetch Hay (78.5% + 21.5%) Diet 2
Organic matter (g/kg DM)	854.8	856.4	852.6
Crude protein (g/kg DM)	51.22	87.20	58.80
Acid detergent fiber (g/kg DM)	377.0	387.8	378.2
Neutral detergent fiber (g/kg DM)	717.4	691.3	709.6
Acid detergent lignin (g/kg DM)	36.89	31.06	35.53
Gross energy (MJ/kg DM)	17.41	17.04	17.28
Metabolizable energy (MJ/kg DM)	7.530	8.910	7.800
DOM (g/kg DM)	401.5	386.3	397.0

Abbreviations: DM: dry matter; DOM: digestibility of organic matter.

Crude protein content differed markedly among diets. Native grasses alone showed the lowest crude protein concentration (51.2 g/kg DM), whereas oat–vetch hay had the highest value (87.2 g/kg DM). The inclusion of oat–vetch hay in the mixed diet increased crude protein content to 58.8 g/kg DM compared with native grasses alone.

Fiber fractions were generally comparable among diets. Acid detergent fiber ranged from 377.0 to 387.8 g/kg DM, while neutral detergent fiber varied between 709.6 and 717.4 g/kg DM. Acid detergent lignin content was lowest in oat–vetch hay (31.1 g/kg DM) and higher in native grasses and the mixed diet (36.9 and 35.5 g/kg DM, respectively).

Gross energy content showed little variation among treatments, ranging from 17.28 to 17.41 MJ/kg DM. In contrast, metabolizable energy was higher in oat–vetch hay (8.91 MJ/kg DM) compared with native grasses (7.53 MJ/kg DM), with the mixed diet presenting an intermediate value (7.80 MJ/kg DM).

Digestibility of organic matter (DOM) was slightly higher for native grasses (401.5 g/kg DM) and the mixed diet (397.0 g/kg DM) compared with oat–vetch hay alone (386.3 g/kg DM).

3.2. Nutrient and Energy Intake and Animal Productivity

Dry matter intake did not differ between treatments (p = 0.757. Table 2), with mean values of 7.07 kg/d for animals grazing native grasses (Diet 1) and 6.99 kg/d for those receiving native grasses plus oat–vetch hay (Diet 2). Similarly, organic matter intake was comparable between diets (6.12 vs. 6.05 kg/d; p = 0.740), as was digestible organic matter intake (3.06 vs. 3.20 kg/d; p = 0.170).

Table 2. Nutrient and energy intake and weight gains of heifers during the experiment.

Items	Native Grasses_Diet 1	Native Grasses Plus Oat–Vetch Hay_Diet 2	p-Value Diet	p-Value Period	p-Value- D * P	Root MSE
Intake of
Dry matter, kg/d	7.071	6.990	0.757	0.269	0.057	0.335
Organic matter, kg/d	6.123	6.052	0.740	0.100	0.063	0.233
Digestible organic matter, kg/d	3.064	3.211	0.170	0.969	0.027	0.204
Crude protein, g/d	334.1	442.1	0.0001	0.003	0.096	5.966
Neutral detergent fiber, kg/d	4.682	4.512	0.281	0.051	0.057	0.160
Acid detergent fiber, kg/d	2.461	2.451	0.508	0.160	0.034	0.112
Acid detergent lignin, kg/d	0.230	0.251	0.101	0.002	0.590	0.006
Gross energy, MJ/d	119.0	132.2	0.005	0.003	0.413	5.113
Digestive energy, MJ/d	59.51	69.83	0.0001	0.0001	0.397	2.554
Metabolizable energy, MJ/d	6.852	7.333	0.0001	0.0001	0.401	0.324
Animal productivity
Initial weight, kg	262.9	256.6	0.0009	0.078	0.211	11.23
Final weight, kg	256.9	261.5	0.0037	0.121	0.450	11.94
Weight change, g/d	−319.0	245.1	0.0010	0.901	0.396	11.43

Abbreviations: root MSE: root mean square; D * P: interaction between diet and period.

Crude protein intake differed significantly between treatments (p < 0.0001), being higher in animals receiving supplementation (442 g/d) compared with those fed native grasses only (334 g/d). Intakes of neutral detergent fiber, acid detergent fiber and acid detergent lignin did not differ between diets (p > 0.10).

Energy intake variables showed significant differences between treatments. Gross energy intake was greater in animals receiving the supplemented diet (132 MJ/d) than in those grazing native grasses alone (119 MJ/d; p = 0.005). Likewise, digestible energy intake was higher for Diet 2 than for Diet 1 (69.8 vs. 59.5 MJ/d; p < 0.0001). Metabolizable energy intake also differed significantly between treatments, being higher in Diet 2 (7.33 MJ/d) compared with Diet 1 (6.85 MJ/d; p < 0.0001).

Regarding animal productivity, initial body weight differed between treatments (p = 0.0009), with animals assigned to Diet 1 being heavier at the start of the experimental period (262.9 kg) than those assigned to Diet 2 (256.6 kg). Final body weight was higher in animals receiving the supplemented diet (261.5 kg) compared with those grazing native grasses only (256.9 kg; p = 0.0037). Consequently, average daily weight change differed significantly between treatments (p = 0.0010), with animals grazing native grasses experiencing body weight loss (−319.0 g/d), whereas supplemented animals showed positive weight gain (245.1 g/d).

The effect of the period was significant for the intake of digestible organic matter (kg/d; p = 0.027), acid detergent fiber (kg/d; p = 0.034) and acid detergent lignin (kg/d; p = 0.002). In contrast, no significant period effects were observed for the intake of dry matter, organic matter, crude protein, gross energy, digestible energy, metabolizable energy or for any of the animal productivity variables. The interaction between diet and period (D × P) was not significant (p > 0.05) for most of the evaluated variables. However, trends toward significance were observed for the intake of dry matter (p = 0.057), neutral detergent fiber (p = 0.057) and crude protein (p = 0.096).

3.3. Methane Emissions

Enteric methane (CH₄) emissions of heifers with and without supplementation are shown in Table 3. Total methane production expressed as grams per day did not differ between treatments (p = 0.183), with mean values of 196.3 g/d for animals grazing native grasses (Diet 1) and 179.6 g/d for animals receiving native grasses plus oat–vetch hay (Diet 2).

Table 3. Enteric methane emissions (g/d/animal) of heifers grazing only native pasture versus those supplemented with hay in the dry season.

Items	Native Grasses_Diet 1	Native Grasses Plus Oat–Vetch Hay_Diet 2	p-Value	p-Value Period	p-Value-D * P	Root MSE
CH4, g/d	196.3	179.6	0.183	0.945	0.504	37.59
CH4/DMI, g/kg	27.77	28.08	0.149	0.795	0.979	5.361
CH4/OMI, g/kg	32.08	32.45	0.141	0.608	0.994	6.283
CH4/DOMI, g/kg	64.15	61.34	0.072	0.848	0.898	11.12
CH4/CPI, g/kg	0.591	0.444	0.0001	0.344	0.661	0.100
CH4/NDFI, g/kg	41.94	43.62	0.212	0.503	0.981	8.512
CH4/ADFI, g/kg	79.80	81.79	0.179	0.693	0.885	15.79
CH4/ADLI, g/kg	853.5	785.2	0.036	0.093	0.7072	180.6
Ym, %	8.855	8.443	0.149	0.054	0.2080	0.235

Abbreviations: CH₄: methane; DMI: dry matter intake; OMI: organic matter intake; DOMI: digestible of organic matter intake; CPI: crude protein intake; NDFI: neutral detergent fiber intake; ADFI: acid detergent fiber intake; ADLI: acid detergent lignin intake; Ym: yield of CH₄; root MSE: root mean square; D * P: interaction between diet and period.

Methane yield expressed relative to dry matter intake (CH₄/DMI), organic matter intake (CH₄/OMI) and digestible organic matter intake (CH₄/DOMI) did not differ between diets (p > 0.05). Values of CH₄/DMI were 27.77 and 28.08 g/kg for Diet 1 and Diet 2, respectively. Similarly, CH₄/OMI averaged 32.08 g/kg for native grasses and 32.45 g/kg for the supplemented diet. Methane yield per unit of digestible organic matter intake showed a tendency to be higher in Diet 1 compared with Diet 2 (64.15 vs. 61.34 g/kg; p = 0.072).

In contrast, methane production expressed per unit of crude protein intake (CH₄/CPI) differed significantly between treatments (p < 0.0001), being greater in animals grazing native grasses only (0.59 g/kg) compared with those receiving the supplemented diet (0.44 g/kg). Methane yield relative to neutral detergent fiber intake (CH₄/NDFI) and acid detergent fiber intake (CH₄/ADFI) did not differ between treatments (p > 0.05).

Methane yield per unit of acid detergent lignin intake (CH₄/ADLI was significantly higher in the native grass diet compared with the supplemented diet (853.48 vs. 785.20 g/kg; p = 0.036). The methane conversion factor (Ym, %) did not differ between treatments (p = 0.149), with values of 8.86% for Diet 1 and 8.44% for Diet 2.

The effect of the period was not significant for daily methane production (CH₄, g/d) or for methane emissions expressed per unit of dry matter intake (CH₄/DMI), organic matter intake (CH₄/OMI), digestible organic matter intake (CH₄/DOMI), crude protein intake (CH₄/CPI), neutral detergent fiber intake (CH₄/NDFI) and acid detergent fiber intake (CH₄/ADFI). No significant period effect was also observed for methane yield (Ym). However, values close to the significance level were observed for CH₄ per unit of acid detergent lignin intake (CH₄/ADLI; p = 0.093) and methane yield (Ym; p = 0.054).

The interaction between diet and period (D × P) was not significant (p > 0.05) for any of the methane-related variables evaluated, including CH₄ (g/d), methane intensity expressed per unit of intake (CH₄/DMI, CH₄/OMI, CH₄/DOMI, CH₄/CPI, CH₄/NDFI, CH₄/ADFI and CH₄/ADLI) and methane yield (Ym).

4. Discussion

4.1. Chemical Compositions of Forages and Diets

The native pasture mixture was characterized by having a low CP content; this was about 41% less than oat plus vetch hay (51 vs. 87 g/kg, respectively). This coincides with authors such as Chancayauri [41] or Chino et al. [12], who reported a range of CP in high Andean pastures between 40 and 120 g/kg DM; according to the latter authors, the CP content is slightly lower in the dry season than in the rainy season, and this is also corroborated by Alvarado-Bolovich et al. [31] in their research. These results highlight the strong seasonal limitation of native grasslands in high Andean systems, where forage quality becomes a key constraint for nutrient supply and system productivity during the dry period.

As for oat plus vetch hay, these species added approximately 28 g CP/kg DM to the basal diet. This is in agreement with Assefa et al. [42] and Pereira et al. [43], who highlight the advantages of legumes such as vetch in taking up atmospheric nitrogen and providing higher protein content to diets than grass alone. From a grassland management perspective, the inclusion of legume–grass mixtures represents an opportunity to improve nutrient cycling and reduce dependency on external protein inputs, contributing to more resilient grazing systems

The acid and neutral detergent fiber (ADF and NDF) contents of the grass mixture and hay were very similar, with average values of 382 and 704 g/kg DM, respectively; this is in agreement with what was reported by Lee [44], who states that the cell wall content of forage resources is between 23 and 90%. Chemical analysis to quantify ADL yielded a slightly lower value for hay than for the pasture mixture (31.06 vs. 36.89 g/kg). The above values of NDF, ADF and ADL are similar to those reported for high Andean pastures [12,31]; however, they are high compared to the findings of Salman et al. [45] for younger harvested forages. This similarity in structural carbohydrate content reflects the advanced phenological stage of the forages, emphasizing the importance of harvest timing and forage conservation strategies to improve feed quality within grass-based systems.

Despite the contribution made by oat hay plus vetch in terms of gross and metabolizable energy, both parameters were similar in the two diets (125.5 and 7.1 MJ/kg DM, respectively). The metabolizable energy reported by Alvarado-Bolovich et al. [31] for native grasses is slightly lower than that obtained in the present investigation, perhaps due to differences in season, types of native grasses or their age.

The diets offered to the heifers had a DOM between 389 and 401 g/kg DM. These values were low if compared with previous research conducted in the area. For example, Alvarado-Bolovich et al. [31] obtained a DOM of 625 g/kg DM for the diet based on native’s grasses. Likewise, Salas [46] obtained a DOM of 638 g/kg DM in lactating cows and 642.8 g/kg DM in dry cows when he offered this type of forage. Cordero et al. [47], when they supplemented with oats to alpacas, obtained an effective degradation of 60.64% of DM. Due to the similarity of the diets in terms of structural carbohydrate content, no differences in DOM were observed in the present investigation, contrary to what was reported by Chino et al. [12] and Pereira et al. [43], who found an inverse relationship between both parameters when they included forages from the high Andean zone and common vetch in their treatments.

4.2. Nutrient and Energy Intake and Animal Productivity

The nutrient and energy intake and weight gains of the heifers during the experiment are presented in Table 2. On average, heifers consumed 7 kg DM per day, equivalent to 2.7% of their live weight. Like DM, OM and DOM intakes did not differ between diets; this is explained in part because the structural carbohydrate content was similar between both diets, as described by Arelovich et al. [48] and Gaviria-Uribe et al. [49], who state that the higher the content of wall compounds, the longer the retention time in the rumen and therefore the lower the voluntary feed intake. In addition to the above, it is possible that the amount of hay supplied or the proportion of the legume in the hay mixture was low to increase DMI, since there is evidence in the literature that supplementation with oats plus vetch increases DMI in cattle [43]. This response suggests that, under extensive grazing conditions, forage physical structure can limit intake regardless of supplementation, reinforcing the need to address pasture quality as a system-level constraint.

Heifers consumed 24% more CP when supplemented with oats plus vetch (p = 0.0001). A similar trend was obtained for crude, digested and metabolizable energy intake but in a lower proportion (on average 11%) with respect to the control diet. In spite of the above, crude protein intake was too low in both diets to support proper ruminal microbial activity, as low dietary protein levels limit nitrogen availability for fibrolytic bacteria and reduce organic matter digestibility [50,51,52].

The live weight change between the beginning and end of each stage was positive for the supplemented diet (245 g/d): this low gain is a consequence of the low nutritional intake, especially the low CPI. It should be noted that initial body weight was defined as the body weight at the start of each experimental period; therefore, the differences observed between treatments reflect a period effect inherent to the crossover design, associated with weight changes that occurred during Period 1, rather than a baseline imbalance between treatments. Similar results were reported by Mwendia et al. [53], who supplemented oats and vetch to cows and their milk production increased by 19.5% (this is equivalent to 1.4 kg/day); likewise, milk quality was improved between 16 and 18%. Consistently, Taffarel et al. [54] demonstrated that finishing steers fed a mixture of oats and vetch had a healthier intramuscular fat content than animals supplemented with corn meal. Collectively, these studies indicate that the inclusion of legumes in the diet can improve animal performance and, consequently, profitability, as also shown by Hernández-Ortega et al. [55] through the inclusion of vetch in cattle diets during the dry season. From an applied perspective, these findings are particularly relevant for livestock systems in the Peruvian highlands, where seasonal declines in pasture availability and quality compel producers to rely on dietary supplementation. In this context, even moderate improvements in forage quality through legume inclusion can contribute to enhanced animal productivity and long-term system profitability.

The significance of the period observed for some digestive fractions, particularly digestible organic matter and the more recalcitrant fiber components (ADF and lignin), suggests that temporal factors influenced nutrient utilization. These variations may be associated with changes in the quality of the native baseline forage between experimental periods, such as differences in forage maturity or fiber composition, which can affect digestibility without substantially altering total intake or animal productive performance. The absence of a period effect on most energy-related and productivity variables indicates that despite these digestive variations, the overall animal response remained relatively stable between periods. The lack of a significant diet × period interaction for most variables indicates that dietary effects were consistent across experimental periods, supporting the robustness of the experimental design and the stability of animal responses. The trends observed in some intake variables and fiber fractions suggest that the magnitude of the dietary effect may have varied slightly between periods; however, these variations were not sufficiently pronounced to generate a clear differential response between diets. Overall, these results indicate that the evaluated treatments did not depend on the period to express their main effects.

4.3. Methane Emissions

Net CH₄ production per animal ranged from 180 to 196 g/d (p = 0.183). Comparing these data with other experiments, it is observed that the results are slightly higher than the values reported by other authors for grass-based diets (on average 160 g/d) [56,57,58]. Perhaps this is due to differences in forage nutritional quality or consumption, as animals that consume more produce more net CH₄ [16,59,60]. In experiments with similar feeding conditions, Alvarado-Bolovich et al. [31] reported emissions between 232 and 322 g CH₄/d for animals weighing approximately 500 kg; likewise, Moscoso et al. [61] obtained a result of 421 g CH₄/day under native grazing conditions plus supplementation with silage based on oats in Brown Swiss cows in production stage. On the other hand, it has been evidenced in the literature that the inclusion of legumes, such as vetch, reduces CH₄ emissions [18] due to the contribution of secondary compounds [62,63]. However, this effect was not observed in the present study, which may be related to the low level of legume inclusion in the diet. In addition, the final botanical composition of the oat–vetch hay was not quantified, limiting an accurate estimation of the effective contribution of vetch and, consequently, its potential impact on CH₄ emissions.

When correcting methane production by DMI, this was on average 28 g CH₄/kg DMI, slightly higher than that established by Min et al. [64] in their meta-analysis, where they state that for each kg of DMI, 20.5 g of CH₄ is produced; perhaps these differences are due to the quality of the forages or the technique used. As expected, when similar amounts of NDF and ADF were consumed, CH₄ emissions related to these variables did not differ. This coincides with the findings reported by Meale et al. [65], who did not find a relationship between NDF content and CH₄ production when legumes or grasses were included in the treatments.

In contrast to the above, we found that the supplemented diet showed lower methane yields when expressed relative to crude protein or acid detergent lignin intake (p < 0.05). This indicates that CH₄ production per unit of CP or ADL ingested was lower in the oat–vetch-supplemented animals. These results evidence that the rate of CH₄ emission by ruminal fermentation is related to the physicochemical characteristics of the diet [66]. Chino et al. [12] reported a magnitude of correlation between CP or acid detergent lignin with in vitro CH₄ production between 0.42 and 0.55, respectively. This supports the concept that improving forage quality enhances fermentation efficiency and reduces methane intensity per unit of nutrient utilized, thereby improving resource use efficiency within grassland-based systems.

The average value of Ym (yield of CH₄) estimated for both diets was 8.6% (p ≥ 0.05); this value is similar (8.2%, n = 53) to that reported by Kaewpila and Sommart [67] in their meta-analysis for beef cattle ingesting low-quality feeds or by Patra [68] when cattle consumed diets of varying quality (5.84%, n = 142). Likewise, the latter author [68] found a significant correlation coefficient between methane (% of GEI) and soluble carbohydrate intake of 0.25, whereas with CP or NDF concentration in the diet, this Pearson correlation coefficient was −0.16 and 0.25, respectively. From a sustainability perspective, these values indicate that improving forage quality and digestibility represents a key pathway to reduce methane yield and enhance the environmental performance of grass-based livestock systems.

The absence of a significant period effect for most CH4 emission variables indicates that enteric CH₄ production and methane intensity remained relatively stable across the experimental periods. This stability suggests that temporal variation did not substantially affect rumen fermentation or the efficiency of substrate utilization for methanogenesis under the evaluated grazing conditions. The near-significant effects observed for CH₄ per unit of acid detergent lignin intake and methane yield (Ym) may reflect minor temporal variations in the most recalcitrant fiber fraction of the forage without leading to consistent changes in total methane emissions. Conversely, the lack of a significant diet × period interaction for all evaluated variables demonstrates that the response of methane emissions to dietary treatments was consistent across experimental periods. This indicates that the effects of native grasses alone or supplemented with oat–vetch hay on methane production and methane intensity were not influenced by period-dependent factors, supporting the robustness of the experimental design and suggesting a stable rumen fermentation response regardless of temporal variation.

4.4. Additional Considerations

This research addressed enteric methane as the main environmental outcome, given its importance in grazing-based livestock production. Nevertheless, the supplementation strategy assessed relies on cultivated forages (oats and vetch), which inherently adds other sources of greenhouse gas (GHG) emissions that were beyond the scope of this study. Operations such as broadcast sowing, the application of synthetic fertilizers and mechanized harvesting, drying and transportation contribute to CO₂ emissions through fossil fuel use, while nitrogen fertilizer manufacture and application are associated with emissions of N₂O, a potent greenhouse gas. Together, these inputs may represent a meaningful share of the total emissions associated with the supplemented system.

As a result, the overall GHG balance of the supplemented system may differ from estimates based solely on enteric methane emissions. Although forage supplementation has the potential to reduce methane emission intensity, these reductions could be offset, partially or fully, by upstream emissions from forage production and conservation. From a qualitative standpoint, the net environmental outcome will depend on the extent to which methane mitigation surpasses the additional emissions generated during forage establishment and management. An alternative approach could involve improving these forages and utilizing them through direct grazing rather than mechanical harvesting, which may lower emissions related to fuel use and transport while preserving comparable nutritional advantages. However, the present study did not evaluate whether the effects of supplementation persist beyond the dry season; therefore, future research integrating animal responses, pasture management and system-level greenhouse gas accounting should assess productive and nutritional responses across seasonal transitions to fully evaluate the mitigation potential of these strategies. These considerations emphasize the need to interpret methane mitigation results within a broader system-level GHG perspective and highlight the value of future studies incorporating more integrated assessments.

5. Conclusions

Oat hay plus vetch is an alternative feed for heifers during the dry season in the Peruvian Andes region because both feeds provided crude protein and energy in the diet based on native grasses, which directly influenced the positive change in live weight in the animals that received such supplementation. Likewise, although no effects on net methane production were observed, there was a reduction in emissions when related to the consumption of crude protein or acid detergent lignin.