AbstractsBiology & Animal Science

The efficiency of utilization of dietary nitrogen can be monitored using milk urea nitrogen (MUN). Overfeeding or underfeeding of protein can be identified through the observation of deviations from target MUN concentrations. This will assist in lowering feed costs of dairy farms, and improving nutrition management of herds. Higher efficiency of utilization of dietary nitrogen might result in a reduction in environmental pollution. Non-genetic factors affecting variation in MUN were herd-test-day (HTD), lactation stage and year of calving. The contribution of HTD was the highest, ranging from 58.56% to 63.18% in parity 1 to 3. Lactation stage had the second largest contribution to the MUN variation. Differences in least squares means for MUN in various years of calving were observed. The heritability estimate for MUN was 0.09±0.01 in the first parity, and remained constant at 0.11±0.01 in the second and third parity. Heritability estimates for milk, fat and protein yield ranged from 0.40±0.01 to 0.43±0.01, 0.21±0.01 to 0.26±0.01, and 0.32±0.01 to 0.38±0.01, respectively. These estimates were within acceptable ranges for South African Holstein cattle. Genetic correlations between MUN and milk production traits were low and positive, ranging from 0.01±0.003 to 0.10±0.004 across parities. Phenotypic correlations ranged from 0.02±0.11 to 0.16±0.07, being generally higher than the genetic correlations. The positive associations between MUN and milk production traits are undesirable as the dairy cows would be less efficient in utilizing dietary protein and may result in increased environmental pollution. The genetic trend for MUN was 0.44, 0.007 and 0.049 mg/dl in the first, second and third parity, respectively. Results of the current study indicate that MUN has potential as a management tool in South African Holstein dairy herds. It might be a good indicator of the efficiency of dietary protein utilization of dairy herds, and has practical advantage as it is currently collected by the national dairy herd recording and improvement scheme.