BACKGROUND
Early weaning in yaks is intended to improve cow feed utilization and shorten postpartum intervals, but it can impair calf growth after separation from the dam. Yak production on the Qinghai–Tibet Plateau is constrained by harsh environmental conditions, long cold seasons, and limited pasture resources, all of which may worsen the performance of early-weaned calves. Probiotics and exogenous enzymes have been used in other livestock species to improve feed utilization, gastrointestinal health, and growth, but their effects have been inconsistent across studies and had not been evaluated clearly in grazing yak calves. This study therefore tested whether supplementing milk replacer with Bacillus licheniformis alone or with a combined probiotic-enzyme preparation could improve growth performance, body size, serum biochemical indices, and serum hormone profiles in early-weaned grazing yak calves.
METHODS
The trial was conducted from July to October at Damxung Co., Lhasa, China (30.5° N, 91.1° E), at an average altitude of 4200 m, with an average annual temperature of 1.3 °C and average annual precipitation of 456.8 mm. Thirty two-month-old male yaks with an initial body weight of 38.89 ± 1.45 kg were fed milk replacer solution daily at 3% of body weight and randomly assigned to 3 treatment groups, with n = 10 in each group. The control group received no supplementation. The T1 group received 0.15 g/kg Bacillus licheniformis containing 2 × 10^10 CFU/g. The T2 group received 2.4 g/kg of a combination containing 0.4 g/kg Bacillus licheniformis (2 × 10^10 CFU/g), 1.0 g/kg yeast (1 × 10^10 CFU/g), and 1.0 g/kg of xylanase, cellulase, and glucanase mixed in a 1:1:1 ratio, with activities of 20,000 U/g, 1500 U/g, and 6000 U/g, respectively. Calves grazed alpine meadow during daytime for 60 days and were individually fed milk replacer before and after grazing at 0800 and 2000 h. Body weight was recorded on d 0, 30, and 60, and average daily gain was calculated for 0–30 d, 30–60 d, and 0–60 d. Body length, hip height, and heart girth were measured on d 0 and d 60. Blood was collected before morning feeding on d 0 and d 60 for measurement of globulin, blood urea nitrogen, glucose, non-esterified fatty acids, insulin-like growth factor-1, epidermal growth factor, growth hormone, insulin, and cortisol. Statistical analysis used one-way analysis of variance with Duncan’s post hoc test; p < 0.05 was considered significant.
KEY RESULTS
Baseline and follow-up body weights did not differ significantly among groups. Body weight on d 0 was 40.64 kg in controls, 40.13 kg in T1, and 40.07 kg in T2 (p = 0.956). On d 30, body weight was 44.27 kg, 44.63 kg, and 45.7 kg, respectively (p = 0.775). On d 60, body weight was 48.09 kg, 49.42 kg, and 51.47 kg, respectively (p = 0.367). Despite the lack of significant body weight differences at individual time points, average daily gain improved with supplementation. From 0–30 d, average daily gain was 121.21 g in controls, 150.00 g in T1, and 187.78 g in T2 (p = 0.009), with T2 significantly higher than control. From 30–60 d, average daily gain was 127.27 g, 159.72 g, and 192.22 g, respectively (p = 0.006), again with T2 significantly higher than control. Across 0–60 d, average daily gain was 124.24 g in controls, 154.86 g in T1, and 190.00 g in T2 (p < 0.001). Over this full period, both T1 and T2 were significantly higher than control, and T2 was significantly higher than T1.
Body size measurements did not differ significantly among treatments. On d 60, body length was 59.69 cm in controls, 61.63 cm in T1, and 62.16 cm in T2 (p = 0.126). Hip height was 64.54 cm, 63.72 cm, and 64.15 cm (p = 0.815), and heart girth was 78.51 cm, 78.48 cm, and 78.42 cm (p = 0.932).
Serum biochemical variables also showed no significant treatment effects. On d 60, globulin was 40.99 mg/mL in controls, 41.49 mg/mL in T1, and 45.18 mg/mL in T2 (p = 0.455). Blood urea nitrogen was 27.12 mmol/L, 24.86 mmol/L, and 25.24 mmol/L (p = 0.627). Glucose was 7.57 mmol/L, 6.80 mmol/L, and 7.26 mmol/L (p = 0.524). Non-esterified fatty acids were 667.63 μmol/L, 583.54 μmol/L, and 599.50 μmol/L (p = 0.377).
The most notable physiological differences were in serum hormones at d 60. Insulin-like growth factor-1 was 156.25 ng/mL in controls, 208.69 ng/mL in T1, and 319.38 ng/mL in T2 (p = 0.001), with T2 significantly higher than both control and T1. Epidermal growth factor was 429.82 pg/mL, 608.88 pg/mL, and 729.04 pg/mL (p = 0.007), with T2 significantly higher than control. Growth hormone was 7.94 ng/mL, 11.77 ng/mL, and 14.12 ng/mL (p = 0.012), with T2 significantly higher than control. Insulin did not differ significantly at d 60: 26.32 mIU/L, 22.66 mIU/L, and 29.76 mIU/L (p = 0.764). Cortisol was 105.42 ng/mL in controls, 80.79 ng/mL in T1, and 93.49 ng/mL in T2 (p = 0.049), with control significantly higher than T1.
CLINICAL IMPLICATIONS
In this 60-day animal feeding study, both Bacillus licheniformis alone and a combined probiotic-enzyme preparation improved growth rate in early-weaned grazing yak calves, but the combined formulation produced the strongest effect, especially over the full 0–60 d period. The lack of change in standard serum biochemical markers suggests the growth response was not accompanied by major detectable shifts in basic metabolic blood indices. The concurrent increases in growth hormone, insulin-like growth factor-1, and epidermal growth factor in T2 provide a plausible biological explanation for the superior average daily gain. Bacillus licheniformis alone may also reduce weaning-associated stress, as reflected by lower cortisol versus control. Because the T2 group combined multiple active components, the study could not isolate which ingredient drove the added benefit, and the authors also noted missing data on diarrhea and nutrient digestibility. Even so, the results support the practical use of probiotic or probiotic-enzyme supplementation in milk replacer to improve production performance in early-weaned yak calves under plateau grazing conditions.