Performance, nitrogen metabolism, and mammary gland development of grazing HolsteinxGyr crossbred heifers supplement with different levels of rumen undegradable protein
Authors:   , Oliveira Gabriella Maria Mota de 
Published: January 2025

The objective was to assess the impact of supplementing with increasing levels of RUP on the performance, N metabolism, and mammary gland development of grazing dairy heifers. Twenty-eight crossbred Holstein×Gyr dairy heifers (initial BW of 278 ± 50.4 kg) were grazed intermittently on Megathyrsus maximus cv. BRS Quênia grass pasture for an experimental period of 84 d, divided into four sampling sub- periods of 21 d each. The experiment followed a randomized block design, consisting of three treatments with increasing levels of RUP and a control group (CON). The supplemented groups received 30% (RUP30), 48% (RUP48), or 66% (RUP66) of RUP in their supplement. The supplement was provided at 0.5% of the animals' BW, with a consistent CP content around 24% across all treatments. Sampling of pasture, feces, and urine was performed on four consecutive d in each sub-period. On d 0 and 19 of each sub-period, the animals were weighed, and biometric measurements were recorded. Ultrasonic mammary gland images were taken on d 0, 42, and 84. Blood samples were collected on the same d to measure the concentrations of IGF-1, albumin, total proteins, urea, and glucose. At d 0, 42, and 84 the development of the reproductive tract was assessed by transrectal palpation using an ultrasound device. Liver tissue was sampled on d 0 and 84. Supplemented animals had higher DMI, relative DMI (g/kg of BW), and nutrient intake when compared to CON animals. A quadratic effect of RUP level was observed for supplement DMI, pasture DMI, DMI, and nutrient intake with higher values noted in the RUP48 treatment. The supplemented animals had a significant increase in the digestibility of DM, CP, and OM. There was a linear increase in the digestibility of NDF and OM across the RUP level. Supplemented animals achieved higher BW, ADG, thoracic circumference, and rump height when compared to the CON animals. A quadratic effect of RUP level was observed for BW and ADG, with higher values in the RUP48 treatment. Supplemented animals had higher N intake, urinary and fecal N excretion, and microbial CP synthesis when compared to CON animals. A quadratic effect of RUP level was observed for N intake and fecal N excretion, with higher values in the RUP48 treatment. Supplemented animals had a lower pixel count in the mammary gland when compared to the CON animals and there was no effect of RUP level on this variable. Supplemented animals had higher levels of urea and blood IGF-1 when compared to CON animals. A quadratic effect of RUP level was observed for total serum protein, with higher levels in the RUP48 treatment. Supplementation resulted in higher BCS, greater mean horn diameter, and improved reproductive tract tone and score. An increasing linear effect was observed for uterine tone across the RUP levels, with higher values in the RUP66 treatment. Supplemented animals had higher liver expression of Glutamic-oxaloacetic transaminase 1 (GOT1) enzyme. Additionally, a quadratic effect was observed for GOT1 expression across the RUP levels, with lower expression in RUP48 and RUP66 treatments. In conclusion, a RUP level of 48% in the feed supplement is the optimal recommendation for grazing Holstein×Gyr crossbred dairy heifers, as it generally improves performance and N metabolism. Keywords: weight gain,; protein metabolism,; protein supplementation