Quality of Maize Silage After Using Meat Bone Meal as a Phosphorus Fertilizer in a Field Experiment

Abstract

The aim of this study is to determine the effects of increasing doses of meat and bone meal (MBM) and the year of the experiment on the feed value of maize silage. A three-year field experiment with silage maize was conducted. The following treatments were established: (1) zero-fert (no fertilization); (2) inorganic nitrogen (N), phosphorus (P), and potassium (K); (3) 1.0 t∙ha⁻¹ MBM; (4) 1.5 t∙ha⁻¹ MBM; (5) 2.0 t∙ha⁻¹ MBM. Both N and K were applied at constant rates, while P was applied at increasing rates: 0.0, 45, 68, and 90 kg∙ha⁻¹. Replacing conventional P fertilizer and, partially, N fertilizer with MBM in silage maize cultivation had a positive influence on the ensiled herbage, compared with the zero-fert treatment. The fermentation parameters and feed value of silage made from maize fertilized with MBM were comparable with the parameters of maize fertilized with inorganic NPK fertilizers. In turn, the content of crude protein (CP) and protein digested in the small intestine when energy is limiting (PDIE) was highest in the silage made from maize supplied with mineral fertilizer. The mineral composition of maize silage, i.e., the content of calcium (Ca) and magnesium (Mg) was modified by fertilization.

1. Introduction

Whole-crop maize (Zea mays L.) silage is an important source of feed for cattle in many dairy and beef production systems in regions where the local agronomic conditions are suitable for maize cultivation [1,2]. Maize is characterized by relatively stable grain yields under different environmental conditions, high energy content, and ensiling suitability. In addition, the inclusion of maize silage in dairy cow diets based on grass silage increases feed intake and milk yield [3,4]. As a result, the area under silage maize has increased considerably in many parts of the world, and maize silage has become one of the main components of the feed ration, alongside grass silage [5].

The quality of maize silage is determined by several factors, including fertilization, maturity at harvest, processing, and ensiling conditions, which affect the protein–starch matrix of grain and the feed value of whole maize plants [6,7]. Despite the high yield potential of maize and the fact that it is grown worldwide, fertilization remains the key factor in maize production in some regions. Nitrogen (N) fertilizer increases the N content of maize plants as well as crude fiber (CF) concentrations and energy values [8]. The results of studies investigating the effect of N fertilizer rates on maize protein content are inconclusive. According to Kim et al. [9], the response of maize to N fertilization may vary depending on water availability because the synergistic relationship between water and N fertilizer contributes to increasing yield and nutrient use efficiency. Maize plants are sensitive to phosphorus (P) deficiency. Adequate P supply promotes the development of an extensive and metabolically active root system, which provides the plant with water and nutrients, and offers effective protection against infection [10].

Waste management constitutes a significant environmental challenge in the contemporary world. The quantity of organic waste, including animal by-products, continues to increase, which renders their disposal increasingly challenging. As an animal by-product, meat and bone meal (MBM) offers a viable alternative to mineral and organic fertilizers owing to its high content of N (approx. 80 g∙kg⁻¹), P (approx. 50 g∙kg⁻¹), calcium (Ca; approx. 100 g∙kg⁻¹), micronutrients, and organic matter (OM) [11,12,13,14,15]. The composition of MBM is determined by the raw material and the proportions of the P- and Ca-rich bone fraction and the N-rich soft tissue fraction N [12]. The low carbon (C)/N ratio (4:1 to 5:1), indicates that MBM is characterized by a relatively high rate of N mineralization, compared with other organic fertilizers. Despite the high P content of MBM, a significant portion of this nutrient is present in the form of hydroxyapatite (bone fraction), which is not readily available to plants [12,14]. In turn, organic P (soft tissue) is rapidly converted to mineral forms that are readily available to plants. Presently, the recycling of P from waste is recognized as one of the fundamental tenets of the European strategy for sustainable P management. In many regions of the world, MBM is a locally produced organic by-product that can be used as a fertilizer, thereby promoting agricultural intensification to meet the mounting consumer demand [16]. Silage maize can be fertilized with organic by-products to support nutrient recycling and replace (completely or partially), expensive mineral fertilizers. The use of MBM to replace or supplement conventional mineral fertilizers and provide plants with macronutrients is justified for environmental and economic reasons. According to Jankowski and Nogalska [17], replacing mineral fertilizers with MBM decreased energy demand in winter oilseed rape production by 20–55%. Due to the relatively low price of MBM and the fact that the recommended dose of MBM is much lower than that of organic fertilizers (e.g., manure), fuel costs were considerably reduced.

The application of MBM as an organic fertilizer to maize grown for grain has been extensively researched [13,15,16,17,18,19,20,21], while only a few studies have investigated the efficacy of MBM as fertilizer in silage maize. Therefore, the following research hypothesis was formulated: MBM may replace mineral phosphate fertilizers in maize silage cultivation. The aim of the present study is to determine the effect of MBM on the feed value of maize silage during a three-year field experiment.

2. Materials and Methods

2.1. Experimental Site and Design

A three-year, small-area field experiment with silage maize was performed at the Agricultural Experiment Station in Tomaszkowo (NE Poland). The station is administered by the University of Warmia and Mazury in Olsztyn. The following experimental treatments were established: (1) zero-fert (no fertilization); (2) inorganic nitrogen (N), phosphorus (P), and potassium (K); (3) 1.0 t∙ha⁻¹ MBM; (4) 1.5 t∙ha⁻¹ MBM; (5) 2.0 t∙ha⁻¹ MBM. Both N and K were applied at constant rates, whereas P was applied at the following increasing rates: 0.0, 45, 68, and 90 kg∙ha⁻¹ (

Table 1. Annual rates of nitrogen (N), phosphorus (P), and potassium (K) applied with meat and bone meal (MBM) and mineral fertilizers (kg∙ha⁻¹) to silage maize (2014, 2019, and 2020).

Treatment	N	P	K
1. Zero-fert	0	0	0
2. Inorganic NPK 1	158	45	145
3. 1.0 Mg MBM + N79 2	158 (79 + 79)	45	145
4. 1.5 Mg MBM + N40 3	158 (118 + 40)	68	145
5. 2.0 Mg MBM 4	158	90	145

¹ Inorganic NPK—mineral fertilization; ² MBM + N₇₉—meat and bone meal with mineral nitrogen (79 kg N∙ha⁻¹) fertilizers; ³ MBM + N₄₀—meat and bone meal with mineral nitrogen (40 kg N∙ha⁻¹) fertilizers; ⁴ MBM—meat and bone meal fertilizer. Every year, mineral potassium fertilizer was applied in treatments 2, 3, 4, and 5 at the same rate of 145 kg K∙ha⁻¹.

). Each ton of MBM supplied around 79 kg N and 45 kg P. In order to achieve a constant level of N fertilization (158 kg N∙ha⁻¹), mineral N was applied with MBM at 40 kg∙ha⁻¹ (treatment 3) and 79 kg∙ha⁻¹ (treatment 4). Given the low levels of K present in MBM, mineral K was administered (treatments 2–5) at a constant rate (145 kg∙ha⁻¹) prior to planting. Meat and bone meal, supplied in a loose form, was thoroughly mixed with soil before application.

The following crop species were grown in the long-term field experiment (2014–2022): silage maize (2014, 2019 and 2020), winter wheat (2014/15 and 2017/18), winter oilseed rape (2015/16 and 2016/17), and hulled oats (2021 and 2022). Silage maize cv. PIONIER P8488 was sown on 5 May 2014, 29 April 2019, and 8 May 2020. Whole maize plants were harvested in the milk–dough stage on 22–24 September. Maize was cultivated in the same 20 plots, each measuring 20 m² (4 × 5 m). All agronomic and protective treatments were applied at optimal dates, according to the recommendations for silage maize. A comprehensive description of the data on soil and weather conditions, chemical composition of MBM, and silage maize fertilization can be found in Nogalska et al. [22].

2.2. Silage Preparation

In each year of the experiment, after harvest (separately for each plot), whole maize plants were chopped to a theoretical chaff length of 10 mm. Fresh chaff samples were packaged in 330 × 400 mm polyamide/polyethylene (PA/PE) vacuum bags (thickness—100 μm) using a vacuum-packaging machine (Vacutronic 2000, PP 5.4, ZTP Tepro, Poland), and ensiled. Each bag contained 550 g of fresh chaff with natural moisture content. Vacuum-sealed bags were stored in the shade for 120 days. Fermentation was carried out under laboratory conditions. All mini silos were stored under the same conditions, i.e., temperature—18–20 °C, humidity—55–60%. No microorganisms or other additives were used during the ensiling.

2.3. Chemical Analysis

At the end of the ensiling process, silage samples (collected separately in each year of the experiment) were dried to absolutely dry mass at 105 °C, weighed, ground, and then wet-mineralized in concentrated sulfuric (VI) acid with hydrogen peroxide (H₂O₂) as the oxidizing agent. Mineralized silage samples were analyzed for the content of P, K, Ca, and Mg. The chemical analyses and the equipment used in the experiment were described in detail in our previous study [22].

Silage samples were analyzed to determine the content of DM, crude ash (CA), crude protein (CP), extract ether (EE), and CF, according to AOAC procedures [23]. The content of neutral detergent fiber (NDF) acid detergent fiber (ADF), and acid detergent lignin (ADL) was analyzed as described by Van Soest et al. [24] using the ANKOM 220 Fiber Analyzer (ANKOM Technology Corp., Macedon, New York, United States).

Fermentation parameters were also determined. The pH of maize silage was measured using the HI 8314 pH-meter (Hanna Instruments, Woonsocket, Rhode Island, United States). The concentration of ammonia nitrogen (N-NH₃) was determined by direct distillation using the 2100 Kjeltec distillation unit (Foss Analytical A/S, Hilleröd, Denmark). The concentrations of lactic acid (LA) and volatile fatty acids (VFAs), including acetic acid (AA), propionic acid (PA), butyric acid (BA), isobutyric acid, valeric acid, and isovaleric acid, and ethanol concentration, were analyzed as described by Kostulak-Zielińska and Potkański [25], and Gąsior [26]. The analyses and the equipment used in the experiment were described in detail elsewhere [27].

2.4. Calculations and Statistical Analysis

The energy value of silage was expressed as the feed unit for milk production (UFL) and meat production (UFV). Protein value was expressed as the content of protein digested in the small intestine when nitrogen is limiting (PDIN) and when energy is limiting (PDIE) per kg DM. These parameters were calculated using PrevAlim 3.23 (INRAtion 3.0) software. The LA/AA ratio was calculated based on the concentrations of LA and AA.

The data on the quality of maize silage were processed statistically by two-way repeated-measures analysis of variance (ANOVA), using STATISTICA 13.3 software (StatSoft Inc., Tulsa, OK, USA). Increasing the MBM dose was the fixed grouping factor (five fertilization treatments), and the year of the study was the repeated measurement factor (three years). The random factor was the location of the plots where maize herbage was harvested for silage The significance of differences between the mean values of the analyzed traits of maize silage was determined by Tukey’s test at a significance level of p ≤ 0.05 and p ≤ 0.01.

3. Results

3.1. Chemical Composition of Maize Silage

Fertilization had a significant effect on the content of CP and ADL (p ≤ 0.01) in the experimental silage (

Table 2. Chemical composition and carbohydrate fractions of maize silage (mean ± standard error of mean).

Treatment (t)		DM 1	CA 2	CP 3	EE 4	CF 5	NDF 6	ADF 7	ADL 8	NFC 9
		g∙kg−1 FM 10	g∙kg−1 DM
Total average		319.7 ± 7.61	39.2 ± 0.79	64.9 ± 1.89	26.3 ± 1.05	220.9 ± 5.81	460.7 ± 9.53	285.4 ± 7.17	19.6 ± 0.78	408.8 ± 5.62
1. Zero-fert		312.1 ± 17.99	39.3 ± 1.48	52.4 C ± 3.18	24.4 ± 2.28	224.5 ± 14.74	468.5 ± 25.37	287.0 ± 16.09	17.9 AB ± 1.65	415.4 ± 23.31
2. Inorganic NPK		328.6 ± 18.96	40.9 ± 1.96	77.2 A ± 3.03	28.4 ± 2.33	216.5 ± 9.51	441.1 ± 13.75	269.5 ± 11.35	16.9 B ± 1.80	412.4 ± 17.63
3. 1.0 Mg MBM + N79		323.2 ± 18.21	35.5 ± 1.41	69.4 B ± 3.59	26.4 ± 2.61	223.8 ± 14.18	457.2 ± 22.34	283.9 ± 17.28	22.6 A ± 1.56	411.5 ± 21.54
4. 1.5 Mg MBM + N40		324.6 ± 15.45	38.1 ± 1.92	64.7 B ± 3.51	26.7 ± 2.31	212.2 ± 13.85	464.9 ± 22.93	286.0 ± 16.21	20.1 AB ± 1.34	405.6 ± 18.63
5. 2.0 Mg MBM		310.2 ± 16.61	42.1 ± 1.73	60.9 B ± 4.55	25.7 ± 2.47	228.0 ± 13.86	472.1 ± 22.89	300.9 ± 19.65	20.8 AB ± 2.11	399.2 ± 16.32
Annual mean (y)	2014	312.5 b ± 5.21	40.5 ± 1.62	79.3 a ± 2.06	36.0 a ± 0.67	169.4 c ± 4.31	373.2 c ± 7.71	221.8 c ± 6.12	21.7 ab ± 1.36	471.1 A ± 6.87
	2019	262.1 c ± 7.53	39.5 ± 0.96	61.7 b ± 2.38	23.5 b ± 0.99	259.9 a ± 7.22	524.0 a ± 9.68	335.5 a ± 7.87	22.4 a ± 1.01	351.4 C ± 5.41
	2020	384.7 a ± 7.47	37.6 ± 1.47	53.9 c ± 2.42	19.5 c ± 1.03	233.6 b ± 4.04	485.1 b ± 6.93	299.1 b ± 4.99	14.8 b ± 1.02	404.0 B ± 4.58
Interaction (t × y)		ns 11	ns	ns	ns	ns	ns	ns	ns	ns

¹ DM—dry matter; ² CA—crude ash; ³ CP—crude protein; ⁴ EE—extract ether; ⁵ CF—crude fiber; ⁶ NDF—neutral detergent fiber^{; 7} ADF—acid detergent fiber; ⁸ ADL—acid detergent lignin; ⁹ NFC—non-fiber carbohydrates; ¹⁰ FM—fresh matter; ¹¹ ns—not significant. Values marked with the same letters in columns, within factors are not significantly different: a, b, c at p ≤ 0.05; A, B, C at p ≤ 0.01.

). The concentration of CP was highest in the inorganic NPK treatment, and lowest in the zero-fert treatment (p ≤ 0.01). No significant differences in the CP content of silage were found between treatments fertilized with increasing doses of MBM. However, these treatments differed significantly (p ≤ 0.01) from the remaining treatments. The proportion of ADL was significantly higher in the silage made from maize fertilized with 1.0 Mg MBM + N₇₉, relative to the inorganic NPK treatment, whereas no significant differences were noted between the other treatments.

Significant differences in the chemical composition and carbohydrate fractions of maize silage were observed across years of the experiment (Table 2). The content of CP and EE in silage decreased throughout the study (p ≤ 0.05). The silage made from maize harvested in 2019 had the highest concentrations of CF, NDF, ADF, and ADL (p ≤ 0.05), and the lowest content of NFC (p ≤ 0.01) and DM (p ≤ 0.05).

3.2. Macronutrient Content of Maize Silage

In all five fertilization treatments, the P and K contents of whole-crop maize silage remained stable (1.73–1.96 g P, 11.4–13.5 g K DM), and it was not significantly modified by the application of fertilizers (

Table 3. Macronutrient content (g∙kg⁻¹ DM) of maize silage (mean ± standard error of mean).

Treatment (t)		P	K	Ca	Mg
Total average		1.85 ± 0.05	12.2 ± 0.68	1.18 ± 0.09	0.97 ± 0.011
1. Zero-fert		1.90 ± 0.07	11.2 ± 0.89	0.86 B ± 0.14	0.90 b ± 0.12
2. Inorganic NPK		1.73 ± 0.05	12.2 ± 0.86	1.44 A ± 0.19	1.00 a ± 0.15
3. 1.0 Mg MBM + N79		1.83 ± 0.09	12.0 ± 1.21	1.02 B ± 0.17	0.91 b ± 0.12
4. 1.5 Mg MBM + N40		1.96 ± 0.06	12.4 ± 1.24	1.07 B ± 0.16	0.94 ab ± 0.13
5. 2.0 Mg MBM		1.84 ± 0.07	13.5 ± 1.19	1.49 A ± 0.14	0.95 ab ± 0.13
Annual mean (y)	2014	2.05 a ± 0.04	8.4 C ± 0.35	1.05 ± 0.19	1.15 A ± 0.04
	2019	1.78 b ± 0.05	15.3 A ± 0.59	1.14 ± 0.13	0.95 B ± 0.05
	2020	1.73 b ± 0.05	13.4 B ± 0.41	1.35 ± 0.04	0.81 B ± 0.05
Interaction (t × y)		s 1	ns 2	ns	ns

¹ s—significant; ² ns—not significant. Values marked with the same letters in columns, within factors are not significantly different: a, b at p ≤ 0.05; A, B, C at p ≤ 0.01.

). However, the Ca and Mg contents of silage were affected by fertilization (p ≤ 0.01 and p ≤ 0.05, respectively). The concentration of Ca was significantly higher in the inorganic NPK treatment (1.44 g∙kg⁻¹ DM) and the 2.0 Mg MBM treatment (1.49 g∙kg⁻¹ DM) than in the other treatments. Similarly to Ca, the Mg content in silage was significantly higher in the inorganic treatment (1.00 g∙kg⁻¹ DM).

Significant differences in the content of P, K, and Mg in maize silage were noted between years of the study (Table 3). The silage with the highest P content was made in 2014 (p ≤ 0.05), and the silage with the highest K content was made in 2019 (p ≤ 0.01). The silage made in 2014 had the highest Mg content (p ≤ 0.01), relative to 2019 and 2020. A significant interaction between the type of fertilizer and the year of the study was found for P concentration in maize silage (Figure 1).

3.3. Fermentation Parameters and Feed Value of Maize Silage

The rate of LA fermentation was similar in all treatments regardless of fertilization and year of the study (

Table 4. Selected fermentation parameters and the content of net energy and protein digested in the small intestine in maize silage (mean ± standard error of mean).

Treatment (t)		pH	N-NH3 1 g∙kg−1 TN 13	LA 2	AA 3	PA 4	BA 5	VFA 6	Et 7	LA:AA 8	UFL 9	UFV 10	PDIN 11	PDIE 12
				g∙kg−1 DM 14
Total average		4.41 ± 0.04	45.0 ± 2.02	97.9 ± 4.12	27.9 ± 1.88	0.05 ± 0.02	1.86 ± 0.26	30.4 ± 1.63	1.17 ± 0.32	2.72 ± 0.44	0.91 ± 0.05	0.81 ± 0.05	48.1 ± 2.05	64.1 ± 3.46
1. Zero-fert		4.46 ± 0.04	46.6 ± 3.01	98.9 ± 6.79	30.1 A ± 1.41	0.070 ± 0.02	2.83 ± 0.39	33.3 a ± 1.45	1.00 ± 0.32	3.34 ± 0.39	0.91 ± 0.05	0.80 ± 0.04	33.5 ± 2.36	61.3 B ± 4.58
2. Inorganic NPK		4.38 ± 0.04	41.5 ± 3.93	97.1 ± 9.04	28.4 AB ± 1.85	0.047 ± 0.01	1.48 ± 0.58	30.4 ab ± 1.36	1.05 ± 0.25	3.48 ± 0.85	0.92 ± 0.06	0.81 ± 0.05	49.6 ± 3.38	66.8 Aa ± 5.78
3. 1.0 Mg MBM + N79		4.38 ± 0.08	45.8 ± 2.54	86.7 ± 5.52	27.1 B ± 1.32	0.059 ± 0.01	1.57 ± 0.54	29.2 ab ± 1.87	1.16 ± 0.35	3.22 ± 0.54	0.92 ± 0.07	0.81 ± 0.05	44.3 ± 2.45	65.3 A ± 4.32
4. 1.5 Mg MBM + N40		4.39 ± 0.06	44.4 ± 2.36	102.6 ± 7.32	26.1 B ± 1.69	0.045 ± 0.01	1.93 ± 0.46	29.0 b ± 1.39	1.31 ± 0.41	3.93 ± 0.41	0.92 ± 0.05	0.82 ± 0.05	41.3 ± 2.48	64.5 AB ± 4.25
5. 2.0 Mg MBM		4.47 ± 0.06	46.8 ± 2.02	104.2 ± 4.91	28.2 AB ± 2.32	0.047 ± 0.01	1.46 ± 0.46	30.1 ab ± 1.58	1.31 ± 0.35	3.75 ± 0.54	0.90 ± 0.06	0.80 ± 0.06	71.4 ± 2.78	62.8 Bb ± 4.81
Annual mean (y)	2014	4.35 b ± 0.06	43.7 ± 2.52	100.8 ± 6.78	28.8 a ± 1.58	0.010 b ± 0.01	3.28 a ± 0.32	33.1 A ± 1.97	1.04 ± 0.41	3.56 ± 0.41	0.97 A ± 0.09	0.88 A ± 0.07	50.9 ± 3.14	70.0 A ± 3.34
	2019	4.51 a ± 0.04	46.8 ± 1.94	96.4 ± 3.92	28.6 a ± 1.59	0.065 a ± 0.02	1.61 b ± 0.35	30.5 B ± 1.32	1.24 ± 0.26	3.39 ± 0.61	0.87 Bb ± 0.04	0.76 Bb ± 0.05	58.9 ± 1.59	61.4 B ± 4.45
	2020	4.38 b ± 0.02	44.5 ± 2.06	96.6 ± 5.09	26.5 b ± 2.12	0.086 a ± 0.03	0.68 b ± 0.23	27.5 C ± 1.58	1.22 ± 0.31	1.22 ± 0.32	0.90 Ba ± 0.03	0.79 Ba ± 0.04	34.4 ± 1.96	60.9 B ± 3.23
Interaction (t × y)		ns 15	ns	ns	ns	ns	ns	ns	ns	ns	ns	ns	ns	ns

¹ N-NH₃—ammonia nitrogen; ² LA—lactic acid; ³ AA—acetic acid; ⁴ PA—propionic acid; ⁵ BA—butyric acid; ⁶ VFA—sum of volatile fatty acids, i.e., acetic acid, propionic acid, butyric acid, isobutyric acid, valeric acid, isovaleric acid; ⁷ Et—ethanol; ⁸ LA:AA—ratio of lactic acid to acetic acid; ⁹ UFL—the feed unit for milk production; ¹⁰ UFV—the feed unit for meat production; ¹¹ PDIN—protein value was calculated as the content of protein digested in the small intestine when nitrogen is limiting; ¹² PDIE—protein digested in the small intestine when energy is limiting; ¹³ TN—total nitrogen; ¹⁴ DM—dry matter; ¹⁵ ns—not significant. Values marked with the same letters in columns, within factors are not significantly different: a, b at p ≤ 0.05; A, B, C at p ≤ 0.01.

). Significant differences were noted in AA concentration in maize silage, which was significantly higher in the zero-fert treatment than in 1.0 Mg MBM + N₇₉ and 1.5 Mg MBM + N₄₀ treatments (p ≤ 0.01). The silages differed also in the concentrations of VFAs, and significant differences were found between the zero-fert treatment and the 1.5 Mg MBM + N₄₀ treatment (p ≤ 0.05). Fertilization-induced differences in the content of PDIE in maize silage were evaluated according to the INRA system (p ≤ 0.05). The silages made in inorganic NPK and 1.0 Mg MBM + N₇₉ treatments were characterized by higher PDIE content than those made in zero-fert and 2.0 Mg MBM treatments (p ≤ 0.01).

Differences in silage acidity were observed between years of the experiment. The silage made in 2019 had a significantly higher pH than those produced in 2014 and 2020 (p ≤ 0.05). The silage made in 2014 had the highest concentrations of AA, BA, and VFAs, and the lowest PA content (p ≤ 0.05). The silage produced in 2019 had the highest concentrations of AA and PA, and significantly lower concentrations of BA and VFAs (p ≤ 0.05). The silage made in 2020 had the lowest concentrations of AA, BA, and VFAs, and the highest PA content (p ≤ 0.05).

The energy value of maize silage varied across the years of the study. The values of UFL and UFV were highest in 2014 (p ≤ 0.01). The silage made in 2019 was characterized by the lowest energy value, 0.87 UFL and 0.76 UFV (p ≤ 0.05). The PDIE content and energy value were significantly highest in the silage made in 2014 (p ≤ 0.01). No interaction between the experimental factors was found for the analyzed fermentation parameters or the feed value of silage, determined according to the INRA guidelines.

4. Discussion

4.1. Chemical Composition of Maize Silage

The nutritional value of maize silage is determined by numerous factors. These include the plant genotype, which encompasses such characteristics as maturity type, cell wall type, starch type, and endosperm type. In addition, agronomic conditions, such as soil type and fertilization, and growth conditions, such as temperature, irradiation, and soil moisture content, also play a role. The maturity of maize herbage at harvest and harvesting practices, such as chop length and grain processing, should also be considered. Finally, the feed value of silage is influenced by ensiling conditions, including the use of additives [5,7,28].

A comparison of the chemical composition of maize herbage analyzed in our earlier experiment [22] and the silage made from this plant material in the current study revealed that ensiling induced an increase in the average concentrations of CA (7.66 g∙kg⁻¹ DM), CP (2.64 g∙kg⁻¹ DM), EE (4.14 g∙kg⁻¹ DM), CF (30.46 g∙kg⁻¹ DM), NDF (28.79 g∙kg⁻¹ DM) and ADF (43.42 g∙kg⁻¹ DM). The concentration of ADL in silage was higher than in herbage (analyzed in a previous study) only in MBM-fertilized treatments.

The average nutrient content of maize silage and variation in its chemical composition and quality parameters have been discussed in the literature [1,5]. In comparison with the mean values reported by Khan et al. [5], the silages made in the current study had a similar content of DM (338 g∙kg⁻¹ fresh matter, FM) and ADL (18.3 g∙kg⁻¹ DM), lower concentrations of CP (73.7 g∙kg⁻¹ DM), EE (34.4 g∙kg⁻¹ DM), and a higher proportion of NDF (399 g∙kg⁻¹ DM) and ADF (220 g∙kg⁻¹ DM). The values obtained in this experiment remained within the typical ranges for maize silage (202–568 g DM, 57–124 g CP, 22–47 g EE, 207–659 g NDF, 152–334 g ADF, 12–26 g ADL per kg DM) reported by Khan et al. [5].

The high nutritional value of the silages produced in the current study was confirmed by the results obtained by Tharangani et al. [1]. At a comparable DM content of silage (321.2 g∙kg⁻¹ FM, range of 274.6 to 423.7 g∙kg⁻¹ FM), the content of EE (30.1 g∙kg⁻¹ DM), ADF (281.1 g∙kg⁻¹ DM), ADL (24.5 g∙kg⁻¹ DM) was similar, and NDF content was higher (413.5 g∙kg⁻¹ DM), whereas CP concentration was below the mean value (81.2 g∙kg⁻¹ DM) and the reference range of values (66.1–111.8 g∙kg⁻¹ DM) proposed by Tharangani et al. [1].

The proportion of NDF in the experimental silage was within the recommended range of 400–500 g∙kg⁻¹ DM [1]. In the silage produced in 2019, the percentage of NDF was slightly higher than the recommended levels (326.3–509.7 g∙kg⁻¹ DM). According to Ferraretto et al. [6], the NDF content of whole-plant maize silage is an important consideration in ruminant nutrition because it influences dietary energy intake and animal productivity. In this experiment, NDF content in silage was close to the Nutrient Requirements of Dairy Cattle [29], i.e., 451 ± 47.2 g∙kg⁻¹ DM. Kruse et al. [30] conducted a three-year field experiment, which demonstrated that maturity at harvest affected variability in NDF content and NDF composition, i.e., the share of individual structural carbohydrates (cellulose, hemicellulose, and lignin) in whole-plant maize silage. According to the cited authors [30], factors such as maturity type and environmental conditions have a minor effect on the quantity and quality of NDF in maize silage. Khan et al. [28] found, based on multivariate analyses, that the high variability in the NDF content of whole-plant maize silage resulted primarily from considerable differences in plant maturity at harvest.

The maturation of maize plants during grain filling is associated with an increase in DM and starch content, and a decrease in NDF and CP concentrations. These changes occur unevenly in individual parts of the plant, i.e., leaves, cobs, and stems, which is also an important consideration. The NDF content of stems increases with advancing maturity, whereas NDF concentration in the entire crop decreases due to a more rapid increase in the proportion of grains in the DM of whole plants, compared with the NDF content of stems. As a result, a negative relationship (R² = 0.669) between starch concentration and NDF content can be observed in maize silage. Therefore, a deliberate delay in the harvesting process to attain a high starch content of maize silage will result in a reduction in NDF content, particularly digestible NDF content [28,31]. In a study by Khan et al. [5], a substantial increase in starch concentration and a considerable decrease in NDF content were observed when the DM content of maize silage increased from <250 g∙kg⁻¹ FM to 250–290 g∙kg⁻¹ FM. The processes of ear growth and nutrient accumulation in grains occur rapidly in the early grain-filling stage. However, their rate decreases as the maturation process progresses. Consequently, ensiling maize at an early maturity stage and DM content <250 g∙kg⁻¹ FM can be expected to decrease the starch/NDF ratio. Thus, maturity at harvest influences not only the chemical composition of silage, but also its digestibility and fermentation quality [5]. The relationships described above were also observed in this study although all maize plants were harvested in the milk–dough stage in all years of the experiment. Maize harvested in 2019 was characterized by significantly lower DM content and significantly higher NDF content than maize harvested in 2014 and 2020.

Maize silage is relatively low in CP, and CP content and digestibility decrease with advancing maturity [5]. Abeysekara et al. [32] found that genetic variation had a significant effect on the concentrations of total CP (62–89 g∙kg⁻¹ DM) and CP fractions in maize, which is consistent with previous findings [33,34]. In the work of Peng et al. [35], maize cultivars were characterized by different molecular structures of proteins, which were correlated with protein solubility and rumen degradability. High negative values of the degraded protein balance in the rumen were noted for maize silage, pointing to a high N deficiency in the rumen relative to the amount of degraded energy when maize silage was the sole source of nutrients for dairy cattle [35].

According to Mandić et al. [36], whole-crop CP content is significantly influenced by N fertilization. In turn, Phelps et al. [37] found that N fertilizer induced an increase in total N content only in some maize cultivars. The CP content of maize increases in response to N fertilization due to the role of N in protein synthesis [36]. In addition, N is the main factor limiting plant growth and development. Soil N availability, as well as weather conditions and agronomic practices, play an important role in maize cultivation. Another important aspect is the CP content of maize leaves, as they are associated with metabolic activity such as photosynthesis and high N concentration [38].

4.2. Macronutrient Content of Maize Silage

The macronutrient content of silage plays an important role in animal nutrition, especially in high-yielding cows with high mineral requirements [39]. In the current study, P concentration in maize silage was close to the INRA norms for ruminants [40], whereas Ca concentration was more than twice lower than the reference value. Maize silage analyzed by Cooke et al. [3] contained around 30–50% more Ca, P, and Mg, and its K content was similar to that noted in this experiment.

In the present study, an interaction between fertilization and the year of the experiment was found for the P content of maize silage. In successive years of the experiment, P concentration decreased to different extents at different fertilization levels. The greatest decrease in P content was noted in the 1.0 Mg MBM + N₇₉ treatment where the rate of P fertilizer was lowest (45 kg∙ha⁻¹) and the proportion of mineral N (50%) in the total constant N rate (158 kg∙ha⁻¹) was highest. In the second and third years, the decrease in P concentration was at the level of the zero-fert treatment. The decrease in the P content of maize silage observed throughout the study accompanied the decrease in CP content. It should be noted that P affects N metabolism in plants by decreasing the content of low-molecular-weight N and increasing the content of protein N and exogenous amino acids. In turn, progressive P deficiency hinders the conversion of sugars to starch or cellulose, which leads to the synthesis of anthocyanins [10]. The comparison of the macronutrient composition (P, K, Ca, and Mg) of the maize herbage achieved in our experiment [22] to the silage made from it described in this study, revealed a decrease in the content of these components after ensiling. The greatest decrease in silage content (on average) was in Mg (12%), then P (6.6%), K (3.1%), and Ca (2.5%).

Maize requires large amounts of nutrients (NPK), and their availability can significantly affect the chemical composition of plants. Interactions between chemical compounds can be observed in the soil, leading to their mobilization (synergism) or fixation in unavailable forms (antagonism). Phosphorus uptake by plants is determined by the presence of nitrate ions in the soil. Nitrogen fertilization enhances Ca and Mg uptake, and limits K uptake. The uptake of most nutrients continues until the beginning of grain filling [41]. However, maize should be supplied with adequate amounts of P during the first weeks of growth (critical period). At the beginning of the growing season, the response to P deficiency is stronger under high N fertilization levels, acidic soil pH, and low air temperature. Sabir et al. [42] demonstrated that adequate N supply is one of the factors contributing to leaf area growth, and a high photosynthetic rate can improve other growth parameters and, consequently, crop yield and quality.

4.3. Fermentation Parameters and Feed Value of Maize Silage

Fermentation parameters are important predictors of maize silage quality because acidity and the concentrations of VFAs and N-NH₃ affect feed intake and palatability [43]. The value of pH is one of the key quality attributes of maize silage, and it was high in the present experiment, compared with that reported by Kung et al. [44] (3.7–4.0). Silage acidity is influenced by many factors, mainly LA concentration and the buffering capacity of the ensiled crop. A lower pH of maize silage, relative to that determined in this study, was also obtained by Khan et al. [5] (3.9) and Tharangani et al. [1] (4.01). Above-normal pH levels in silage may be due to too high buffering capacity, which is generally observed in legumes rather than in maize, and limited fermentation due to e.g., low ambient temperature [44].

Typical LA concentrations in commonly fed maize silages range from 30 to 60 g∙kg⁻¹ DM [44]; the value noted in this experiment was, on average, twice higher. A wider range of values was reported by Tharangani et al. [1], 36.7 to 72.1 g LA∙kg⁻¹ DM. According to Kung et al. [44], the highest LA concentration in maize silage can be achieved with a DM of 351–400 g∙kg⁻¹, which was not confirmed in this study. Above-average LA concentration in maize silage can be attributed to the composition of epiphytic microbiota and the levels of water-soluble carbohydrates (WSCs) that provide sufficient amounts of substrate for LA bacteria [45]. The composition of epiphytic bacterial communities, the abundance of heterofermentative genera in the community, and the extent of the transition from hetero-fermentation to homo-fermentation in the initial stage of the ensiling process are important determinants of silage quality [46], and they could affect silage fermentation in the present experiment.

Acetic acid is the second most abundant acid in silage [45], and its recommended range for silage made from whole maize plants is 10 to 30 g∙kg⁻¹ DM [44], which was also observed in this experiment. The AA content of silage, determined in the present study, is adequate and sufficient to inhibit yeast growth. Such a concentration of AA also contributes to improving the aerobic stability of silage [47]. High-quality silage is characterized by a low concentration (<0.1% DM) or absence of PA, which was also observed in the experimental silage. A low proportion of PA in silage is indicative of the absence of activity of Clostridium propionicum bacteria [44].

Well-fermented silage should not contain BA. The presence of this acid is associated with the metabolic activity of Clostridium bacteria, which leads to considerable DM losses and poor energy recovery [48]. Unfortunately, BA was detected in the silage produced in this experiment. According to Mills and Kung [49], silages containing BA have lower than normal LA concentrations, higher than normal pH, and higher than normal concentrations of AA and N-NH₃, which was not observed in the current study. Increased pH may be due to the activity of Clostridium spp., and the noted concentration of N-NH₃ was below the recommended values (50–70 g∙kg⁻¹ N_total) [44].

The concentrations of AA and N-NH₃ are the most important indicators of maize silage quality. In this experiment, they were consistent with the recommended values, or lower. The LA/AA ratio is commonly used as a qualitative indicator of fermentation. In well-fermented silages, this ratio ranges from 2.5 to 3.0 [44]. In the silages made in this experiment, the LA/AA ratio was above 3.22 and up to 3.93, due to the high abundance of homolactic acid bacteria that produce only LA [45]. In general, the quality of silages with a higher LA/AA ratio should not be considered as low. However, such silages may be more aerobically unstable than those with normal LA/AA ratios because low AA concentration and high LA concentration may be insufficient to inhibit the activity of lactate-assimilating yeasts [44].

The energy value of the silages produced in this study was comparable with the values recommended by INRA [40] for whole-crop maize silage with a DM content of 30–35%: UFL = 0.92 and UFV = 0.82. The lower values noted in the silage made in 2019 can be attributed to sugar losses during fermentation and BA fermentation [45]. The average recommended values of protein units in the INRA system [40] were higher (76 g PDIN, 76 g PDIE) than those noted in the present study. These differences resulted from different weather conditions between years of the experiment, which affected plant maturity at harvest [50].

5. Conclusions

The use of MBM to replace or supplement conventional mineral fertilizers and provide plants with macronutrients, and to replace manure in organic farms that do not raise animals, is justified for environmental and economic reasons. This solution offers numerous benefits, including rational waste management, nutrient recycling, supply of organic matter to the soil, and reduced use or elimination of expensive mineral NP fertilizers. Particular attention should be paid to the recycling of P from MBM, which is one of the key principles of the European strategy for sustainable P management.

The present field experiment revealed that MBM is a good organic fertilizer for silage maize. Replacing conventional P fertilizer and, partially, N fertilizer with MBM in silage maize cultivation had a positive influence on the ensiled herbage, compared with the zero-fert treatment. The fermentation parameters and feed value of silage made from maize fertilized with MBM were comparable with the parameters of maize fertilized with inorganic NPK fertilizers. In turn, the content of CP and PDIE was highest in the silage made from maize supplied with inorganic NPK fertilizer. The mineral composition of maize silage, i.e., the content of Ca and Mg, was modified by fertilization. Significant differences in the macronutrient content of maize silage were found across years of the study and under different weather conditions.