Abstract
The preference of lactating dairy cows for grazed herbage or maize silage (MS), simultaneously offered ad libitum in the field, was examined at two sward heights (SH 4-6 and 8-10cm) and two concentrate levels (CL 0 and 6kg day-1) in a 2x2 factorial arrangement within a completely randomised experimental design. The experiment lasted 35 days and was carried out in spring using 24 multiparous Holstein Friesian cows. On average, the cows proportionately spent more time grazing than eating MS (0.85:0.15) and even though the higher rate of intake (RI) of dry matter (DM) of MS compared with grazed herbage (76 versus 26g DM min-1), the proportion of total DM intake as herbage was higher compared to that of MS (0.56:0.44). The higher crude protein and low fibre content of grazed herbage appeared to have a higher priority of choice than RI, as the cows chose to graze for longer (grazing time 385 min, MS feeding time 67min) despite the lower RI of herbage. The low proportion MS intake indicated that RI was a secondary factor of choice. Concentrate supplementation had a greater depressing effect on herbage intake than on MS intake. These results suggest that the animals reduce the intake of feed with lower RI when the labor associated to eat is decreased. The factors influencing the choice for herbage over maize silage remain unclear.
Highlights
Changing the proportion of time available to graze herbage or to eat maize silage offered indoors affects the relative intake of each feed, but the benefit to dairy cow performance of offering maize silage was only found to be significant when sward height was limiting (Hernandez-Mendo and Leaver, 2004; 2006)
It may provide a means of alleviating the high rate of decline in milk yield of 2-3% per week of grazing dairy cows (Leaver, 1982), which is mainly due to low dry matter intake
Time spent grazing is of central importance to models of optimal diets, which assume that animals choose an optimal diet within sets of constrains (Rutter et al, 2004) including total feeding time, the digestive capacity of the animal, and the rate of intake
Summary
Changing the proportion of time available to graze herbage or to eat maize silage offered indoors affects the relative intake of each feed, but the benefit to dairy cow performance of offering maize silage was only found to be significant when sward height was limiting (Hernandez-Mendo and Leaver, 2004; 2006). Offering maize silage in the grazing paddock as an instantaneous alternative to grazing should give an insight into diet preference and provide a better understanding of the factors influencing feed intake. It may provide a means of alleviating the high rate of decline in milk yield of 2-3% per week of grazing dairy cows (Leaver, 1982), which is mainly due to low dry matter intake. It is necessary to have an understanding of diet preferences in order to predict how much and from which diet animals select
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