Effects of rumen-undegradable protein supplementation on performance, nitrogen metabolism, and mammary gland development of grazing heifers.

This study examined the effects of supplementation with increasing levels of RUP on performance, nitrogen metabolism, and mammary gland development in grazing dairy heifers. Twenty-eight 5/8 crossbred Holstein × Gyr dairy heifers (initial BW of 278 ± 50.4 kg) grazed on Megathyrsus maximus cv. BRS Quênia grass pastures for an experimental period of 84 d, divided into 4 subperiods of 21 d each. The experiment followed a completely randomized design in a 4 × 3 factorial arrangement, with 4 supplementation strategies (control [CON], or supplementation with 30% [RUP30], 48% [RUP48], or 66% [RUP66] RUP) and 3 initial BW classes (light, medium, and heavy). The supplement was administered at 0.5% of the animals' BW with a consistent CP content of ∼24% in all treatments. Sampling of pasture, feces, and urine was performed on 4 consecutive days in each period. On d 0 and 19 of each period, the animals were weighed and biometric measurements were recorded. Ultrasound images of the mammary gland and blood samples were taken on d 0, 42, and 84. On 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, DMI related to BW (g/kg of BW), and nutrient intake when compared with CON animals. An effect of RUP level was observed for supplement intake, pasture intake, and total DMI, with higher values noted in the RUP48 treatment. The supplemented animals had a significant increase in the digestibility of DM, CP, and OM. We observed a linear increase in NDF and OM digestibility across RUP level. Supplemented animals achieved higher BW, ADG, thoracic circumference, and rump height when compared with 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 with 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 with the CON animals, and no effect of RUP level was observed on this variable. Supplementation resulted in greater mean horn diameter and improved reproductive tract tone and score. An increasing linear effect was observed for uterine tone across the RUP levels. Nonsupplemented 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.