In an observational study of nine commercial dairy herds over two springs (four in 1990 and five in 1991) where cows grazed pasture alone, negative associations were found between blood (or milk) urea levels and milkfat and milk protein production, and positive associations of urea levels with pasture protein to soluble carbohydrate (N/Sol.CHO) ratios. Statistical analyses using principal component analysis of two datasets from spring 1990 and spring 1991 indicated that a high proportion of variation (73% in 1990, and 33% in 1991) in the variables measured was explained by the above relationships. Analysis of variance of the PC scores showed that these relationships were between-farm effects and not related to seasonal change with time (P<0.001). Additional data from 35 dairy herds in September 1991 also supported the associations found. Reproductive performance in the nine herds (conception rate and submission rate to AB mating) was also associated negatively with urea levels. Variations in the nutrient composition of spring pasture during the spring are presented. The correlation between N/Sol.CHO ratio and milk urea levels was high (r=0.69). Further work is needed to clarify cause and effect relationships.

MB, Blackwell

Proceedings of the New Zealand Society of Animal Production, Volume 53, , 37-40, 1993
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