Abstract
Supplementing animal sourced 50% rumen undegradable protein (RUP) improved pregnancy rates in grazing heifers. Grazing heifers offered a plant sourced 36% and 50% RUP supplement had no differences in puberty attainment or antral follicle counts (AFC). The effects of source and level of RUP on serum metabolites and AFC in dry-lot developed heifers remain unclear. The objective of this study is to assess source and level of RUP on circulating metabolites and AFC in developing heifers. Heifers (n = 20) were randomly assigned to one of four treatments: 1) 36% RUP plant source (n = 5; 36P), 2) 50% RUP plant source (n = 5; 50P), 3) 36% RUP animal source (n = 5; 36A), and 4) 50% RUP animal source (n = 5; 50A). Heifers were individually fed for 77 d utilizing a Calan gate feeding system. Blood samples were collected every 14 days and were analyzed for glucose, urea, and insulin concentrations. Antral follicle counts were evaluated via transrectal ultrasonography 50 d after diet treatments began. Glucose, insulin, and urea concentrations were analyzed as repeated measures using PROC MIXED (SAS v9.4). Antral follicle counts were analyzed using PROC MIXED (SAS v9.4). Glucose was not impacted by source or level (P ≥ 0.21), however their subsequent interaction tended to influence glucose (P = 0.06). Heifers supplemented with 50A tended to have increased glucose compared to 50P heifers, with 36A and 36P having similar glucose levels with each other. Insulin and urea were not impacted by source, level, or their subsequent interaction (P ≥ 0.25). Day influenced glucose with reduction on d 77 compared to all other sampling time points (P < 0.001). Insulin was affected by day (P < 0.001) with increased concentrations on d 64 compared to all other sampling time points. Urea concentrations increased throughout the study compared to d -15 (P < 0.0001). Level × day, source × day, and level × source × day did not affect glucose and insulin (P≥ 0.14). Urea tended to be affected by level × day (P = 0.08) with 36 and 50 level heifers tending to have increased urea after treatments began compared to 36 and 50 level heifers on d -15. Source × day impacted urea (P < 0.001) with animal and plant sourced RUP having increased urea after treatments began in comparison to animal and plant sourced RUP on d -15. Source, level, and their interaction did not affect AFC (P ≥ 0.53). In conclusion, a 50% animal sourced RUP diet increases glucose concentrations but does not promote follicular activation. The increase in serum glucose concentrations observed in 50A heifers may be beneficial in improving pregnancy rates. USDA is an equal opportunity provider and employer.