Abstract/Summary

There are increased concerns regarding N pollution of air and ground water from grazing cattle. Although a number of studies have investigated mitigation strategies for N output from dairy cows fed conserved forages and concentrates, similar research on fresh-cut grass in addition to production parameters is limited. Therefore the current study, using 3 dietary treatments and incorporating 2 genotypes, was designed to evaluate the effects of concentrate crude protein (CP) level on animal production and N utilization efficiency (NUE) of lactating dairy cows. Twelve multiparous cows (6 Holstein and 6 Holstein × Swedish Red) were used in a change-over study with three 25-d periods and 3 diet treatments; low, medium and high CP concentrate (14.1, 16.1 and 18.1% respectively, dry matter (DM) basis) fed at 32.8% DM intake in combination with good quality zero-grazed perennial ryegrass (18.2% CP, DM basis). Each period consisted of an adaption phase (18-d) housed as a single group, 1-d adaption in individual stalls and a 6-d measurement phase with feed intake and feces, urine and milk output recorded. There was no significant interaction between cow genotype and concentrate CP level on any animal performance or NUE parameters. Total DM intake, milk yield and composition and NUE were not affected by dietary treatment. However, increasing concentrate CP level increased (i) N intake by 42 g/d and excretion in urine and manure, by 38 and 40 g/d, respectively, and (ii) the ratio of urine N over manure N. Feeding high CP, rather than low CP concentrate, increased milk urea N (MUN) content by 3.6 mg/dL and total MUN output by 1.08 g/d. Crossbred cows had lower grass DM intake, total DM intake, total N intake and consequently energy-corrected milk yield. However, cow genotype had no significant effect on NUE or MUN parameters. Equations have been developed to predict urine N excretion using MUN output as sole predictor or in combination with dietary CP level. The present study indicated that when grazing cows are fed on good quality pasture, feeding concentrates with a protein content as low as 14.1% may not negatively affect productivity. In addition, reducing concentrate CP concentration may be a successful method of reducing urinary N excretion of lactating dairy cattle on pasture-based systems, but further research is needed to investigate long-term effects of supplementary concentrate CP content on milk production.