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. 171 g kg⁻¹ DM) in the SR AM compared with the LR PM but did not affect WSC (169 g kg⁻¹ DM) or the ratio of WSC/CP (1·0 g g⁻¹). Cows had similar apparent DM (17·5 kg DM cow⁻¹ d⁻¹) and N (501 g N cow⁻¹ d⁻¹) intake for both treatments. Compared with SR AM cows, LR PM cows had lower milk (18·5 vs. 21·2 kg cow⁻¹ d⁻¹), milk protein (0·69 vs. 0·81 kg cow⁻¹ d⁻¹) and milk solids (1·72 and 1·89 kg cow⁻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.