The effect of varying the height and bulk density of the leafy zone of the sward on the ingestive behaviour of six 11 month old Red deer hinds and seven 14 month old Romney ewe hoggets was examined. Deer and sheep, confined to metabolism crates were randomly allocated 21 sward height x density combinations consisting of 7 heights (3-21 cm) x 3 bulk densities (0.19-0.75 mg DM/cm3). The Sorghum bicolor swards were grown in 42 x 30 cm trays and had a deep, stem free leafy zone. The required height of leaf was made accessible to animals by raising or lowering each sward relative to a horizontal grid of fine rods forming 3 cm x 3 cm squares below which animals could not graze. The two lower bulk densities were engineered by snipping out alternate rows, or rows and columns of plants. After grazing for at least 20 bites, bite depth, bite weight, grazed stratum bulk density, bite volume and bite area were calculated. The ingestive behaviour of deer and sheep was very similar in relation to height and bulk density. The positive effect of height on bite depth was on average 11 times as large as the negative effect of bulk density. Height had little or no effect upon bite area, while bulk density had a moderate, negative effect. On average a 100% increase in height or bulk density respectively resulted in a 64% vs only 21% increase in bite weight; reflecting the fact that bite volume increased in relation to H (39%) but decreased in relation to bulk density (18%). Thus height was the major determinant of bite volume and bite weight via its influence on bite depth.
Proceedings of the New Zealand Society of Animal Production, Volume 51, , 117-122, 1991
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