Effect of rotationally grazing perennial ryegrass white clover or perennial ryegrass only swards on dairy cow feeding behaviour, rumen characteristics and sward depletion patterns

The effect of sward type, grass only (GO) or grass white clover (GWc), on lactating dairy cow feeding behaviour, herbage depletion rate and rumen function was investigated in rotationally strip-grazed swards at a daily herbage allowance of 17 kg dry matter (DM)/cow/day, in two experiments. In 2011, eight rumen-cannulated cows were blocked into two groups and allocated to each sward type for a 14-day period in a change-over design (2 sward types×2 periods) on three occasions: late spring, summer, and autumn. Feeding behaviour, rumen pH and rumen volatile fatty acids and ammonia contents were measured. Following a continuous design, in 2012 feeding behaviour and herbage depletion rate measurements were undertaken at similar times to those in 2011 over 2-3 weeks in late spring, summer and autumn. Twenty-six (spring) and 36 (summer and autumn) cows were used. Grazing sward height (GSH) was measured five times/day. The percentage of grazed vegetative units, and the extended tiller height, free leaf lamina, tiller DM weight and tiller leaf DM weight of perennial ryegrass (ryegrass) tillers were estimated four times/day. During late spring, summer and autumn, sward white clover content (DM basis) was 7.5%, 8.8% and 30.9%, respectively, in 2011, and 18.0%, 29.7% and 30.6%, respectively, in 2012. In 2011 cows had similar grazing times on both sward types. Cows on GWc spent less time ruminating than cows on GO. Cows had similar total volatile fatty acids on both swards but the isoacids and d-lactic acid percentages, ammonia content and rumen pH were higher in autumn for cows grazing GWc. In 2012, cows on GWc grazed for longer in late spring, ruminated for less time in summer and for less time at night in autumn compared to cows grazing the GO sward. Both sward types had similar post-grazing sward heights and sward height depletion rates, except during morning grazing in autumn, when GWc had a greater sward height depletion rate. A similar percentage of grazed ryegrass tillers was observed between sward types, but the percentage of ryegrass grazed was greater than the percentage of white clover grazed in the GWc swards. The ryegrass tillers in the GWc swards were smaller than those in the GO swards but had similar depletion rates. The GWc swards influenced cow feeding behaviour and rumen characteristics with increased effect in autumn because of an increase in white clover content and a decrease in ryegrass quality.