Effects of herbage mass and herbage allowance on bite mass of grazing dairy cows

Dry matter intake (DMI) of herbage is a product of bite rate and bite mass and is a determinant of the performance of grazing ruminants. This research tested the effect of herbage allowance and herbage mass on average bite mass of individual dairy cows grazing perennial ryegrass herbage (Lolium perenne L.). Two experiments were conducted, one in each of winter (Experiment 1) and spring (Experiment 2). In each experiment, 24 Holstein-Friesian dairy cows grazed individual perennial ryegrass plots over 10 d. In each experiment cows grazed one of six treatments: low or medium herbage mass (1900 or 2200 kg DM.ha 1 in Experiment 1 and 2200 or 3000 kg DM.ha 1 in Experiment 2) combined with either low, medium or high herbage allowance (15, 25 or 40 kg DM.cow 1.d(-1) in Experiment 1 and 20, 30 or 40 kg DM.cow 1.d(-1) in Experiment 2). Grazing behaviour was measured with jaw movement sensors and DMI was determined by herbage disappearance measured with a rising plate meter daily, and at hourly intervals during the first 4 h of grazing after the morning milking. Average bite mass (g DM.bite(-1)) per day and per hour was determined as the mass of herbage that had disappeared in each period (g.d(-1) or g. h(-1)) divided by number of prehension bites in that period. The combined data from two experiments was analysed by Linear Mixed Model and it was found there was no effect of herbage mass (P = 0.686) on bite mass per day. However, cows offered high herbage allowance had greater (P < 0.001) bite mass per day (0.62 g DM.bite(-1)) than cows offered the lowest herbage allowance (0.38 g DM.bite1) across both experiments. The effect of herbage allowance was similar across both herbage masses. Bite mass per hour was the highest (P < 0.001) in the first hour of grazing in both experiments with average bite mass per hour across two experiments in the first hour of grazing (0.69 g DM.bite (-1)) greater than in the second (0.40 g DM.bite(-1)), third (0.35 g DM.bite(-1)) and fourth (0.32 g DM.bite(-1)) hours of grazing. We conclude that herbage allowance is a significant factor in determining bite mass and should be included in models aimed at predicting bite mass and hence daily herbage intake of grazing dairy cows.