Use of Blood Urea Nitrogen Concentration to Determine Time and Level of Protein Supplementation in Wintering COWS ,

Abstract

Measurement of blood urea nitrogen (BUN) may be useful as a tool for making real-time adjustments in protein supplementation for grazing or wintering beef cattle. The objective of our research was to test this approach by comparing performance of cows wintered using a predetermined standard supplementation program with cows wintered under similar conditions but where level and timing of protein supplementation was guided by monitoring BUN concentrations. Two herds on each of five production units at Deseret Ranches of Florida (110 to 210 pregnant cows per herd), assigned to either standard (CO) or BUN-guided (BG) winter supplementation treatments, with 25 cows selected from each of the 10 herds for monitoring BUN concentration. The CO treatment consisted of feeding a cottonseed meal-based cube (33% CP) and molasses according to standard ranch protocol. The BG treatment was the same, except that time and amount of cube feeding was guided by results of BUN analyses. Cube feeding was increased when a herd sample mean BUN reached < 7 mg/dL or when 25% of the sampled individuals had BUN concentrations of < 6 mg/dL. Less cube was fed to BG cows than to CO cows (41.2 vs 51.5 kg/cow; P<.10). Herd mean calf weaning weights were not affected by treatment, and were 230.6 and 228.6 kg for CO and BG, respectively. Rebreeding percentage was greater (P<.01) for BG (87.0%) than for CO (82.1 %), but treatment effect on rebreeding percentage varied among production units. In the sample groups, there was no effect cirfre-atment on cow body weight change, but decrease in body condition score was greater (P<.01) with the BG treatment than with the CO treatment; also, there was no effect of treatment on calving interval. These data suggest that BUN can be used to guide protein supplementation of wintering cows, but practical application of BUN monitoring may be limited to infrequent sampling for diagnostic purposes in specific situations.