The Effect of Different Altitude Conditions on the Quality Characteristics of Turnips (Brassica rapa)

Abstract

The turnip (Brassica rapa) is a multipurpose crop traditionally utilized for food, fodder, and medicinal materials in China. However, it remains unclear how it adapts to harsh environments in Xizang. To clarify the gap, this study investigates the impact of altitude on the nutritional profile of the “Zhongke 1” turnip across five altitude gradients (3300 m, 3600 m, 4270 m, 4300 m and 4450 m). Comprehensive post-harvest analyses of key nutritional parameters were conducted to evaluate quality variations at different altitudes. The results indicated that both the relative fodder value and relative grass quality of the turnips reached levels comparable to high-quality forage grasses. Additionally, the sugar and soluble carbohydrate content of the turnips exhibited a distinct pattern, initially declining and then rising with altitude. Total digestible nutrients exceeded 60%, while the crude protein content remained above 14% across all elevations. The lignin content in the belowground part of the turnip at 4450 m was more than two times higher than at 3300 m (6.59% vs. 2.96%). Notably, most nutritional indicators remained stable even at the highest elevation of 4450 m, highlighting the strong adaptability of turnips to the diverse environmental conditions of Xizang. The study further identified soil chemical properties, rather than temperature or precipitation, as the primary factors driving nutritional variations across altitudes. In conclusion, the turnip variety showed significant potential as a high-quality and high-productivity forage crop in high-altitude regions more than 4000 m above sea level. This study is of great significance for understanding the altitude adaptability of turnip quality and promoting the development of animal husbandry in the high altitude area of the Xizang Autonomous Region.

1. Introduction

The Xizang Autonomous Region, situated at the heart of the Tibetan Plateau, is one of China’s five major pastoral regions [1]. It spans approximately 80,000 km² of grassland, accounting for one-third of the country’s total grassland resources [2], with animal husbandry serving as a central economic pillar [1]. However, the development of grassland resources is significantly constrained by the region’s high altitude, low temperatures, and short growing seasons [3]. Currently, various studies concentrated on the plant–soil interactions in the grasslands of the Tibetan Plateau [4,5,6], but the insufficient supply of high-quality forage has become a major obstacle to the growth of livestock industry there [7]. In recent years, as livestock farming expands and ecological conservation demands increases, the Tibetan grassland industry faces the dual challenge of improving both forage yield and quality [8]. Consequently, identifying forage species well adapted to the unique environmental conditions is crucial for the sustainable development of Tibetan animal husbandry [9].

Altitude is one of the most critical environmental factors affecting plant growth, morphology, and yield [10]. High-altitude environments generally impose stress conditions, such as low temperatures and strong ultraviolet radiation, which can alter photosynthesis, biomass allocation, and metabolic pathways in plants [11,12]. While some forage crops exhibit increased secondary metabolite accumulation at higher altitudes, leading to enhanced nutritional or medicinal properties, others experience suppressed growth and reduced biomass production [13]. For example, studies on medicinal plants indicate that increasing altitude can enhance antioxidant enzyme activity and flavonoid accumulation [14]. However, research on Brassica species has shown that high-altitude conditions may lead to lower biomass and a reduced mineral content [15]. Therefore, understanding how altitude affects the growth and nutritional composition of turnips (Brassica rapa) is essential for optimizing cultivation strategies and maximizing their potential as livestock feed.

The turnip (Brassica rapa), locally known as “Niu Ma” in the Xizang Autonomous Region, has a long history of cultivation in the region [1]. Due to its cold resistance and barren resistance, this crop has been widely cultivated in high-altitude areas of the Xizang Autonomous Region [16]. The turnip has become an important food and fodder resource for local farmers and herders, playing a vital role in alleviating the effects of natural disasters and food shortages [17]. Previous studies have highlighted the turnip’s value as a food, fodder, and medicinal crop, noting that its carbohydrate and protein contents surpass that of radishes [18]. Furthermore, turnips have been shown to enhance human tolerance to hypoxia and reduce the risk of cardiovascular diseases [19,20]. The belowground parts of the turnip are rich in vitamin C, glucosinolates, dietary fiber, and polyphenolic compounds, which exhibit antioxidant, anti-inflammatory, and anticancer activities [21,22]. Epidemiological studies have linked regular turnip consumption to reduced risks of cardiovascular diseases and metabolic syndrome [23]. As reported by Xinhua News Agency [24], recent agricultural trials in the Xizang Autonomous Region have achieved a fresh weight yield of over 75 tons per hectare for turnip varieties. Field studies conducted in Nagqu (e.g., a city in Xizang) revealed that both fresh biomass and dry fodder yields of turnips significantly outperformed those from natural grasslands [9,25].

Despite these agronomic advantages, scientific understanding of turnip’s altitudinal adaptation remains fragmented. The quality of turnips directly influences their economic and ecological value as forage [25,26]. In recent years, there has been growing scholarly attention on optimizing the nutritional profile of turnips. Most research has focused on agronomic traits, gene cloning, hypoxia tolerance, and nutritional composition in low-altitude regions [27,28,29]. However, studies on the quality of turnips grown in various high-altitude regions of Xizang remain limited. To address this research gap, the present study conducted a comprehensive evaluation of the nutritional quality of both above- and belowground parts of turnips across five altitude gradients (3300 m, 3600 m, 4270 m, 4300 m, and 4450 m) in the Xizang Autonomous Region. Based on the current research background of turnips, we hypothesized that the nutritional quality of turnips has a most suitable altitude and would be suppressed at very high altitudes. The findings aim to provide a theoretical foundation for promoting turnip cultivation in high-altitude areas.

2. Materials and Methods

2.1. Plant Material and Field Experiment

A field experiment was conducted at five experimental sites at different altitudes in the Xizang Autonomous Region of China during the 2023 growing season. The climatic and environmental characterization of the five locations are listed in

Table 1. The climatic and environmental characterization of the five locations at different altitudes in Xizang, China.

Location	Longitude	Latitude	Altitude (m)	MAT (°C)	MAP (yr−1)	Frost-Free Period
Linzhi County	94°44′ E	29°46′ N	3300	8.5	654	175
Lhasa City	91°20′ E	29°40′ N	3600	7.5	425	125
Rutog County	79.70° E	33.39° N	4270	0.1	73.4	95
Damxung County	90°45′ E	29°31′ N	4300	1.3	456.8	62
Nagqu City	92°06′ E	31°16′ N	4450	−2.1	406.2	0

Note: MAT: mean annual temperature; MAP: mean annual precipitation.

.

The turnip variety used in this study was “Zhongke 1”, developed through selective breeding by the Institute of Tibetan Plateau Research and the Kunming Institute of Botany, Chinese Academy of Sciences, in Nagqu. Before sowing, the soil required a fine preparation (flat, soft, moisture preservation) and basal fertilization (10,000 kg ha⁻¹ of farmyard manure). The experiment was arranged in a randomized complete block design with three replications. Each experiment plot consisted of 50 m² (10 m long × 5 m wide). Each experimental field was plowed to a depth of 30 cm, and ridge planting was used. Each ridge had a height of approximately 10 cm and a spacing of 20 cm between ridges. Each ridge was designated for a single variety, with three replicates per ridge. Within each row, plants were spaced 30 cm apart, and six seeds were sown per planting hole. Irrigation was applied uniformly based on soil moisture conditions, and urea fertilizer was administered twice during the growth period.

2.2. Quality Trait Analysis

Turnip samples were thoroughly rinsed with deionized water to eliminate surface contaminants, dehydrated to constant weight in an oven at 65 °C. Then, the samples were ground to pass through a 0.42 mm sieve and mixed uniformly. Nutritional components were analyzed using the following standardized methods: GB/T 6438-2007 for ash content, GB/T 20805-2006 for lignin, GB/T 20806-2006 for neutral detergent fiber (NDF), and NY/T 1459-2007 for acid detergent fiber (ADF) [30,31]. Near-infrared reflectance spectroscopy (NIR) was employed to evaluate other nutritional components (crude protein, crude fiber, crude fat) of turnip forage (GB/T 18868-2002) [32]. The mineral content was extracted according to NIR described by Clark et al. (1987) [33].

The relative feed value (RFV) and relative forage quality (RFQ) of the “Zhongke 1” turnip was calculated using the following equations [34,35,36]:

Digestible dry matter (DDM) = 88.9 − 0.779 × ADF (% DM)

(1)

Dry matter intake (DMI) = 120/NDF (% DM)

(2)

Relative feed value (RFV) = (DDM (% BW) × DMI (% BW))/1.29

(3)

Relative forage quality (RFQ) = (DMI (% BW) × TDN (% DM))/1.23

(4)

where 1.29 denotes the expected digestible dry matter intake as % of body weight.

2.3. Statistical Analysis

Data organization and calculations were performed using Excel 2017, while correlation analyses were conducted with SPSS 27.0. One-way ANOVA and Tukey’s test were conducted for each crop quality of turnip at different altitudes. Results were presented as mean ± standard deviations of triplicate determinations. Graphical visualizations were generated using Origin 2021. Statistical significance was established at p < 0.05.

3. Results

3.1. Nutritional Quality of Turnip at Different Altitudes

The nutritional value of the aboveground parts of turnips varies significantly across different altitudes. The ash content ranged from 10.35% to 15.61%, with the highest value observed at 3600 m, higher than those at other altitudes (except 4300 m; p < 0.05; Figure 1a). The highest crude protein (CP) content (22.77%) was recorded at 3300 m, followed closely by 22.43% at 4270 m (Figure 1a). In contrast, the lowest CP content (14.09%) was observed at 4300 m, significantly lower than those at other altitudes (p < 0.05; Figure 1a). The acid detergent fiber (ADF) content ranged from 14.80% to 20.49%, while the amylase-treated neutral detergent fiber (aNDF) content ranged from 22.70% to 27.44%. Both ADF and aNDF levels exceeded the classification standards for premium-grade alfalfa (Table S1). The non-fibrous carbohydrate (NFC) content was highest at 4300 m (44.74%) and lowest at 4450 m (41.16%). Total digestible nutrients (TDN) exceeded 60.50% at all altitudes, with the highest value (67%) observed at 3300 m (Figure 1a). Mineral and trace element concentrations in the aboveground parts of turnips varied with altitude. The calcium (Ca) content ranged from 0.96% to 2.84%, peaking at 3600 m, significantly higher than those at other altitudes (p < 0.05). Similarly, phosphorus (P) and magnesium (Mg) concentrations were the highest at 3600 m (0.48% and 0.61%, respectively). In contrast, the potassium (K) content was the highest at 4300 m (1.59%; Figure 1b). The water-soluble carbohydrates (WSC) and lignin contents of the aboveground parts exhibited an initial decline followed by an increase with rising altitude (Figure 1c). Sugars and fructans initially decreased, then increased with altitude. The lowest values of soluble carbohydrates (7.64%) and lignin (3.45%) were recorded at 4270 m, significantly lower than at other altitudes (p < 0.05), with noticeable increases at 4450 m (Figure 1c). Net energy for lactation (NEL), maintenance (NEM), and gain (NEG) followed consistent trends across altitudes. Specifically, NEL ranged from 1.47 to 1.62 Mcal/kg, NEM from 1.46 to 1.65 Mcal/kg, and NEG from 0.74 to 0.93 Mcal/kg. The highest NEL value was recorded at 4270 m, while NEM and NEG were the highest at 3300 m (Figure 1d).

The quality of the belowground parts of turnips also varied across altitudes. The ash content did not show significant variation across altitudes (Figure 2a). However, the TDN content decreased with increasing altitude, reaching its lowest value of 59.50% at 4450 m, significantly lower than those (3300 m and 3600 m) at other altitudes (p < 0.05). The CP content exhibited an initial increase followed by a decline, peaking at 36.99% at 3600 m, higher than at other altitudes (p < 0.05) and exceeding the CP content of premium-grade alfalfa (i.e., alfalfa was used as a comparison standard). The NFC content followed a similar trend, initially decreasing and then increasing with altitude, reaching its lowest point at 3600 m (35.63%), lower than at other altitudes (4270 m and 4300 m; p < 0.05), before slightly declining again at 4450 m. The mineral content in the belowground parts showed a general decrease in P, K, and Mg levels with increasing altitude, while the Ca content increased (Figure 2b). Respectively, Ca, K, Mg, and P contents at different altitudes ranged from 0.85% to 1.63%, 1.05% to 1.79%, 0.18% to 0.37%, and 0.17% to 0.56%. The fat content exhibits a trend of an initial decrease followed by an increase with rising altitude, reaching the lowest value at 4300 m. Lignin and fructan contents consistently increased with altitude, peaking at 6.59% and 1.51% at 4450 m. No significant variations were observed in sugars and WSC across altitudes (Figure 2c). The NEL, NEM, and NEG in the belowground parts decreased with increasing altitude, reaching their lowest values at 4450 m. The values were 1.52 Mcal/kg, 1.44 Mcal/kg, and 0.72 Mcal/kg, respectively (Figure 2d).

3.2. Forage Value of Turnip at Different Altitudes

The relative feed value (RFV) and relative forage quality (RFQ) are critical indicators for assessing forage quality. The RFV of the aboveground parts of turnips was highest at 3300 m, with significantly lower values at 4300 m and 4450 m (p < 0.05). Conversely, the RFV of the belowground parts peaked at 3600 m, significantly exceeding values at other altitudes (p < 0.05). Overall, the RFV displayed a decreasing trend with increasing altitude, reaching its lowest point at 4450 m (Figure 3). Regarding RFQ, the aboveground parts exhibited the lowest values at 4300 m and 4450 m, lower than at other altitudes (p < 0.05). For the belowground parts, RFQ was highest at 3600 m and lowest at 4450 m (Figure 3). Therefore, both the RFV and RFQ demonstrated a general decline as altitude increased.

3.3. Physical and Chemical Properties of Soils at Different Altitudes

Soil physicochemical properties, which significantly influence forage growth and quality, varied markedly across different altitudes (

Table 2. Physical and chemical properties of soils at five experimental sites at different altitudes in Xizang, China. Different lowercase letters indicate significant difference among altitude gradients (p < 0.05).

Parameters	3300 m	3600 m	4270 m	4300 m	4450 m
pH	7.14 ± 0.17 c	8.18 ± 0.16 a	7.42 ± 0.12 bc	7.66 ± 0.06 b	7.29 ± 0.13 bc
SOC (%)	2.80 ± 0.32 a	0.95 ± 0.02 c	1.13 ± 0.12 c	1.73 ± 0.13 b	8.00 ± 0.13 c
AP (mg/kg)	122 ± 4.65 a	58.10 ± 14.04 b	30.43 ± 1.49 c	22.03 ± 2.63 c	13.53 ± 0.77 c
AK (g/kg)	0.25 ± 0.05 a	0.05 ± 0.003 b	0.08 ± 0.01 b	0.10 ± 0.01 b	0.11 ± 0.02 b
TP (%)	0.25 ± 0.02 a	0.10 ± 0.01 b	0.08 ± 0.003 b	0.07 ± 0.01 b	0.03 ± 0.003 c
TN (%)	0.26 ± 0.02 a	0.11 ± 0.003 c	0.13 ± 0.01 c	0.18 ± 0.01 b	0.10 ± 0.01 c
TK (%)	1.89 ± 0.08 c	2.24 ± 0.04 a	1.71 ± 0.05 d	2.06 ± 0.02 b	1.96 ± 0.02 bc
NH4+-N (mg/kg)	4.08 ± 1.12 a	0.38 ± 0.23 b	5.36 ± 1.03 a	1.02 ± 0.14 b	1.09 ± 0.49 b
NO3−-N (mg/kg)	11.89 ± 1.21 b	20.83 ± 7.03 b	63.26 ± 1.20 a	9.26 ± 0.60 b	9.93 ± 0.72 b
MBC (mg/kg)	986.62 ± 63.19 a	392.00 ± 34.67 c	463.76 ± 88.72 c	746.49 ± 23.81 ab	560.42 ± 138.82 bc
MBN (mg/kg)	78.26 ± 5.32 a	37.29 ± 2.89 c	44.02 ± 9.75 c	71.82 ± 2.40 ab	51.96 ± 11.84 bc
Zn (mg/kg)	105.42 ± 2.74 a	73.78 ± 1.33 b	77.12 ± 4.28 b	81.09 ± 7.24 b	28.49 ± 6.62 c
Cu (mg/kg)	18.61 ± 1.88 b	19.02 ± 1.99 b	29.16 ± 1.92 a	3.80 ± 0.06 d	13.10 ± 1.85 c
Mn (%)	0.13 ± 0.01 a	0.07 ± 0.001 c	0.09 ± 0.01 b	0.08 ± 0.01 bc	0.03 ± 0.005 d
CaO (%)	7.19 ± 0.28 b	4.19 ± 1.04 d	11.36 ± 0.28 a	5.94 ± 0.14 bc	5.53 ± 0.14 cd

Values are presented as mean ± SE of three replicates. Note: SOC—soil organic carbon, AP—available phosphorus, AK—available potassium, TP—total phosphorus, TN—total nitrogen, TK—total potassium, MBC—microbial biomass carbon, MBN—microbial biomass nitrogen.

). Principal component analysis (PCA) indicated that the first two principal components accounted for approximately 80% of the total variance in soil parameters, with PC1 explaining 51.0% and PC2 28.8% (Figure 4). The main factors contributing to PC1 variation included total nitrogen, organic carbon, and available potassium, while calcium oxide, nitrate nitrogen, and ammonium nitrogen were more influential in PC2 (Table S2). The Pearson correlation analysis revealed significant negative correlations between soil concentrations of zinc, organic carbon, available P, total P, and total nitrogen with altitude (p < 0.05; Figure 5).

The TDN in the aboveground parts of turnips showed a negative correlation with both the soil total K content and pH (

Table 3. Correlation analysis between soil chemical properties and nutritional quality of aboveground parts of turnip.

Index	NH4+-N	NO3−-N	MBC	MBN	TK	Zn	Cu	CaO	TP	TN	SOC	AP	AK	pH
Fat	−0.117	0.284	−0.597	−0.496	−0.258	−0.873	0.013	0.184	−0.887 *	−0.766	0.538	−0.895 *	0.538	−0.082
ADF	−0.768	−0.633	0.075	0.163	0.903 *	0.205	−0.683	−0.782	0.008	0.085	−0.318	0.06	−0.318	0.684
Ca	−0.785	−0.322	−0.461	−0.431	0.938 *	−0.081	−0.274	−0.749	−0.238	−0.406	−0.292	−0.086	−0.292	0.936 *
P	−0.1	0.198	−0.4	−0.522	0.463	0.338	0.46	−0.214	0.256	−0.167	−0.586	0.413	−0.586	0.695
K	0.305	0.285	0.188	0.185	0.053	0.896 *	0.168	0.248	0.493	0.471	−0.963 **	0.439	−0.963 **	0.335
Mg	−0.55	−0.192	−0.449	−0.528	0.789	−0.007	0.087	−0.635	0.014	−0.35	−0.255	0.21	−0.255	0.799
TDN	0.864	0.291	0.592	0.508	−0.887 *	0.331	0.373	0.722	0.525	0.601	0.138	0.393	0.138	−0.914 *

Note: SOC—soil organic carbon, AP—available phosphorus, AK—available potassium, TP—total phosphorus, TN—total nitrogen, TK—total potassium, MBC—microbial biomass carbon, MBN—microbial biomass nitrogen, ADF—acid detergent fiber, TDN—total digestible nutrients. * indicates significant differences at the p < 0.05 level, and ** indicates significant differences at the p < 0.01 level.

). The K content in the turnips was positively correlated with the zinc content in the soil but was negatively correlated with soil organic carbon and available K contents (Table 3). The Ca content showed a significant positive correlation with both the soil total K content and pH (Table 3). ADF in the aboveground parts of turnips showed a negative correlation with soil total K content. Additionally, the fat content in the aboveground parts of turnips demonstrated a significant negative correlation with soil P (both total and available) (Table 3). Furthermore, lignin and fructans in turnips were negatively correlated with the zinc content in the soil, and ADF in the belowground parts of turnips negatively correlated with soil pH (

Table 4. Correlation analysis between soil chemical properties and nutritional quality of belowground parts of turnip.

Index	NH4+-N	NO3−-N	MBC	MBN	TK	Zn	Cu	CaO	TP	TN	SOC	AP	AK	pH
Fat	−0.004	0.028	−0.158	−0.351	0.338	0.291	0.504	−0.249	0.482	0.006	−0.273	0.651	−0.273	0.361
CP	−0.42	−0.229	−0.257	−0.377	0.715	0.16	0.147	−0.593	0.254	−0.14	−0.274	0.443	−0.274	0.662
Lignin	−0.321	−0.069	−0.388	−0.281	−0.102	−0.942 *	−0.263	−0.08	−0.824	−0.646	0.763	−0.827	0.763	−0.171
Sugars	0.524	0.577	−0.015	0.113	−0.691	0.032	0.089	0.768	−0.307	0.011	−0.168	−0.487	−0.168	−0.34
Fructan	−0.466	−0.211	−0.349	−0.222	0.047	−0.932 *	−0.417	−0.227	−0.832	−0.622	0.753	−0.831	0.753	−0.087
ADF	0.193	−0.389	0.696	0.72	−0.456	−0.148	−0.372	0.112	0.167	0.467	0.673	0.042	0.673	−0.884 *
aNDF	−0.102	−0.264	0.162	0.262	−0.322	−0.66	−0.427	0.026	−0.452	−0.13	0.829	−0.539	0.829	−0.596

Note: SOC—soil organic carbon, AP—available phosphorus, AK—available potassium, TP—total phosphorus, TN—total nitrogen, TK—total potassium, MBC—microbial biomass carbon, MBN—microbial biomass nitrogen, aNDF—amylase-treated neutral detergent fiber, ADF—acid detergent fiber, CP—crude protein. * indicates significant differences at the p < 0.05 level.

).

3.4. Effect of Different High-Altitude Areas on Correlation Analysis of Turnip Quality

The nutritional quality parameters of the aboveground parts of turnips exhibit significant correlations with environmental variables (

Table 5. The correlation analysis of the quality of the aboveground parts of the turnip with environmental factors.

	Altitude	Jun	Jul	Aug	Jun–Aug	Jun	Jul	Aug	Jun–Aug
Ash	−0.429	0.394	0.338	0.278	0.347	0.303	0.774	0.986 *	0.896 *
Fat	0.946 *	−0.762	−0.842	−0.795	−0.804	0.109	−0.035	−0.432	−0.202
CP	−0.191	−0.328	−0.236	−0.244	−0.28	−0.572	−0.969 **	−0.765	−0.940 *
Lignin	−0.092	−0.47	−0.211	−0.156	−0.306	−0.852	0.066	−0.022	−0.237
Sugars	−0.438	−0.198	0.05	0.07	−0.047	−0.77	0.287	0.374	0.076
Fructan	−0.454	−0.215	−0.068	−0.092	−0.139	−0.518	0.484	0.683	0.386
WSC	−0.866	0.435	0.659	0.657	0.573	−0.586	−0.02	0.257	−0.05
ADF	−0.33	0.444	0.46	0.446	0.455	0.205	0.908 *	0.943 *	0.898 *
aNDF	0.761	−0.23	−0.206	−0.107	−0.187	0.094	0.195	−0.344	−0.071
Ca	−0.249	−0.059	−0.057	−0.101	−0.073	0.003	0.775	0.916 *	0.772
P	−0.64	−0.015	−0.02	−0.114	−0.048	−0.177	0.102	0.577	0.282
K	−0.542	0.776	0.633	0.554	0.677	0.563	0.133	0.464	0.455
Mg	−0.493	−0.193	−0.128	−0.188	−0.176	−0.336	0.412	0.729	0.434
NFC	−0.153	0.923 *	0.84	0.85	0.889 *	0.639	0.329	0.251	0.446
TDN	−0.068	0.218	0.249	0.273	0.247	−0.112	−0.843	−0.824	−0.784

Note: TDN—total digestible nutrients, NFC—non-fiber carbohydrates, aNDF—amylase-treated neutral detergent fiber, ADF—acid detergent fiber, WSC—water-soluble carbohydrates, CP—crude protein, Jun—June, Jul—July, Aug—August, Jun–Aug—Monthly average from June to August. * indicates significant differences at the p < 0.05 level, and ** indicates significant differences at the p < 0.01 level.

). Specifically, the fat content demonstrates a significant positive correlation with altitude (p < 0.05; Table 5). The NFC show significant positive correlations with both the mean temperature in June (Figure S1a) and the average monthly temperatures from June to August (p < 0.05). The CP content is significantly negatively correlated with July precipitation (p < 0.01; Figure S1b) and with the average precipitation from June to August (p < 0.05; Table 5). The ADF content is positively correlated with precipitation in July and August and the average precipitation from June to August (p < 0.05). The Ca content is significantly positively correlated with August precipitation (p < 0.05). The ash content exhibits a highly significant positive correlation with August precipitation (p < 0.01) and a significant positive correlation with the average precipitation from June to August (p < 0.05). Regarding the belowground parts of turnips, the lignin content is significantly positively correlated with altitude (p < 0.05), whereas K and P are both significantly negatively correlated with altitude (p < 0.05;

Table 6. The correlation analysis of the quality of the belowground parts of the turnip with environmental factors.

	Altitude	Jun	Jul	Aug	Jun–Aug	Jun	Jul	Aug	Jun–Aug
Ash	0.614	0.191	0.151	0.232	0.196	0.441	0.458	−0.04	0.289
Fat	−0.775	−0.101	0.012	−0.059	−0.058	−0.577	−0.173	0.318	−0.076
CP	−0.683	−0.072	0.031	−0.032	−0.032	−0.445	0.272	0.665	0.314
Lignin	0.887 *	−0.715	−0.712	−0.637	−0.699	−0.067	0.082	−0.385	−0.184
Sugars	0.637	0.147	−0.072	−0.056	0.024	0.749	−0.205	−0.464	−0.096
Fructan	0.855	−0.642	−0.629	−0.551	−0.618	−0.042	0.241	−0.244	−0.042
WSC	0.357	0.37	0.142	0.13	0.236	0.785	−0.263	−0.362	−0.056
ADF	0.236	0.106	0.232	0.324	0.211	−0.22	−0.32	−0.637	−0.516
aNDF	0.731	−0.341	−0.285	−0.185	−0.281	−0.081	−0.081	−0.58	−0.352
Ca	0.805	−0.466	−0.473	−0.399	−0.453	0.111	0.495	0	0.227
P	−0.903 *	0.205	0.31	0.238	0.247	−0.416	0.049	0.543	0.174
K	−0.939 *	0.68	0.765	0.713	0.723	−0.127	0.224	0.62	0.366
Mg	−0.734	0.234	0.327	0.275	0.276	−0.259	0.504	0.842	0.551
NFC	0.327	0.349	0.2	0.215	0.27	0.549	−0.473	−0.614	−0.336
TDN	−0.812	0.436	0.401	0.305	0.391	0.026	−0.118	0.409	0.17

Note: TDN—total digestible nutrients, NFC—non-fiber carbohydrates, aNDF—amylase-treated neutral detergent fiber, ADF—acid detergent fiber, WSC—water-soluble carbohydrates, CP—crude protein, Jun—June, Jul—July, Aug—August, Jun–Aug—Monthly average from June to August. * indicates significant differences at the p < 0.05 level.

). None of the belowground nutritional parameters show significant correlations with monthly average temperature or precipitation (Table 6).

4. Discussion

The nutritional value of forage serves as a primary determinant of pasture quality [37]. This study demonstrated that the nutritional quality of turnips (Brassica rapa) varies significantly across different altitudes. Some nutrient properties increased while some properties decreased with altitudes, which did not confirm our hypothesis. Notably, the CP content in the aboveground parts of turnips initially decreased and then increased with altitude. The fructan content in the belowground part of the turnip increased with altitude. This finding aligned with previous investigations on turnip varieties cultivated below 4000 m, where elevated sugar accumulation correlated positively with increasing altitude [17]. This adaptive mechanism may be attributed to the dual physiological roles of sugars as both energy reserves and structural components in plant stress responses [35]. The Tibetan plateau is identified by high altitudes with strong light intensity and low air temperature, and the plants there generally accumulate more soluble carbohydrates, such as fructans, during the short growing season [38,39,40]. This accumulation not only helps plants adapt but may also improve the palatability of turnips [41]. The fat content in forage is generally low, and in this study, the fat content of the aboveground parts of turnips ranged from 1.10% to 2.23%, which meets the energy requirements of livestock [42]. Compared to national alfalfa grading standards, the CP content in the aboveground parts of turnips at altitudes of 3300 m and 4270 m exceeded that of premium alfalfa (Table S1). However, at 4300 m, the CP content was the lowest (14.09%), corresponding to the third-grade alfalfa standard. In the belowground parts of turnips, the CP content at 3600 m surpassed that of premium alfalfa (Table S1). The ADF and aNDF are key indicators of forage quality [43]. The ADF content ranged from 14.80% to 20.49%, and the aNDF content ranged from 22.70% to 27.44%, both of which exceeded the premium alfalfa standards (Table S1).

Under low temperature, soil in the Tibetan plateau generally appears to have low gross mineralization and nitrification rates [44], and the mineral content is another crucial determinant of forage feed value [45]. In this study, the mineral content in both the above- and belowground parts of turnips exhibited variability across altitudes. Specifically, K, P, and Mg contents in the aboveground parts decreased with altitude, while the Ca content in the belowground parts increased with altitude. Ca is an essential mineral involved in structural, metabolic, and signaling processes within organisms, and it plays a critical role in livestock growth and development [46].

The energy value of forage is a fundamental measure of its nutritional quality. The results from this study showed that the net energy for lactation, maintenance, and gain in the aboveground parts of turnips did not differ significantly across altitudes. However, in the belowground parts, these energy values exhibited a decreasing trend with increasing altitude. Notably, the NDF content increased with altitude, which directly affects the digestibility of the forage [47]. This suggests that while the aboveground parts of turnips maintain relatively consistent energy content, the belowground parts become less energy-dense as altitude increases, likely due to changes in the fiber content and forage digestibility.

Relative feed value (RFV) and relative forage quality (RFQ) are key indicators used to assess the quality of forage [41]. Higher RFV and RFQ values indicate better forage quality [48]. This study found that the RFV and RFQ in both the above- and belowground parts of turnips significantly decreased with increasing altitude. Despite this, the quality of turnip forage at higher altitudes remained superior to that of many high-quality pastures, suggesting that this variety of turnip is well adapted to the environmental conditions of high-altitude regions. In conclusion, turnips grown at higher altitudes exhibited significant changes in nutritional composition, with some nutrients (e.g., crude protein and calcium) increasing with altitude, while others (e.g., NFC and energy content) decreased. These findings not only elucidate the species’ remarkable high-altitude adaptability but also validate its potential as a sustainable forage resource for the farmland ecosystem of the Tibetan Plateau.

5. Conclusions

The turnip variety “Zhongke 1” was specifically bred for cultivation in the Tibetan region and demonstrated excellent adaptability to the high-altitude environment, showing promising nutritional value. Overall, the mineral content in both above- and belowground parts of turnips decrease with increasing altitude. Nutritional indicators such as fats and soluble carbohydrates increased at 4450 m. Total digestible nutrients (TDN) exceeded 60% at all altitudes. The CP content of turnips exceeded 14%, peaking at 36.99% in the belowground parts at 3600 m. The lignin content exhibited an altitudinal dependency, peaking at 4450 m. Notably, the lignin content in the belowground part of the turnip was more than two times higher than at 3300 m (6.59% vs. 2.96%). The study revealed that altitude primarily influenced turnip quality through its effect on the physicochemical properties of the soil. Notably, the average temperature and precipitation during the growing season did not have a significant impact on the quality of the belowground parts of the turnip. In conclusion, the findings from this study provided sound evidence supporting the promotion and cultivation of “Zhongke 1” turnips in the high-altitude regions of Xizang, making them a valuable forage resource for these areas.