Effect Of Grazing Management On Herbage Protein Concentration, Milk Production And Nitrogen Excretion Of Dairy Cows In Mid-Lactation

The objective of this experiment was to use diurnal and temporal changes in herbage composition to create two pasture diets with contrasting ratios of water-soluble carbohydrate (WSC) and crude protein (CP) and compare milk production and nitrogen-use efficiency (NUE) of dairy cows. A grazing experiment using thirty-six mid-lactation Friesian x Jersey cows was conducted in late spring in Canterbury, New Zealand. Cows were offered mixed perennial ryegrass and white clover pastures either in the morning after a short 19-day regrowth interval (SR AM) or in the afternoon after a long 35-day regrowth interval (LR PM). Pasture treatments resulted in lower pasture mass and greater herbage CP concentration (187 vs. 171gkg(-1) DM) in the SR AM compared with the LR PM but did not affect WSC (169gkg(-1) DM) or the ratio of WSC/CP (10gg(-1)). Cows had similar apparent DM (175kg DM cow(-1)d(-1)) and N (501gNcow(-1)d(-1)) intake for both treatments. Compared with SR AM cows, LR PM cows had lower milk (185 vs. 212kgcow(-1)d(-1)), milk protein (069 vs. 081kg cow(-1)d(-1)) and milk solids (172 and 189kgcow(-1)d(-1)) yield. Urinary N concentration was increased in SR AM, but estimated N excretion and NUE for milk were similar for both treatments. Further studies are required to determine the effect of feeding times on diurnal variation in urine volume and N concentration under grazing to predict urination events with highest leaching risk.