Partial preferences of sheep and goats for ryegrass and white clover

A study was conducted to investigate partial preference for Trifolium repens and Lolium perenne, and the effect of relative heights of swards on the preference and selection of sheep and goats. Three Coopworth 1 year old ewes and three Saanen/Anglo-Nuvian male one year old goats were selected for the first experiment (November 1996) and three replacement animals of similar description for the second experiment (January 1997). The treatments consisted of a combination of 2 pasture species (grass, clover) and 3 heights (5, 9 and 13 cm), giving 12 paired contrasts, where each was a combination of two swardlets offered to one penned animal. Number of bites and dry matter removed were measured for each swardlet, as well as the total time allowed for grazing. At equal availability, both sheep and goats showed a strong preference for clover, both species removing between 67 and 78% clover from a clover/grass contrast. At varying height, within the same pasture species, both animal species preferred higher swards in grass contrasts. In clover contrasts goats had a stronger increase in intake with height than sheep, the percentage eaten from the longer sward for sheep was 56%, 59%, 81 % and for goats 54%, 72%, 88% at 13/13 cm, 9/13 cm and 5/13 cm contrasts, respectively. With combinations of pasture species and height, goats had a weaker preference for clover and stronger preference for height than sheep. In the 13 cm grass/altering clover contrasts, as clover height decreased to 9 and 5 cm, goats markedly increased the proportion of their intake from grass to 52 and 86 % respectively, while sheep only increased their intake of grass to 38 and 47 % respectively. Both sheep and goats had higher intake rates at the combinations of long clover/changing height grass than any other of the combinations tested. The trends observed in preferences were the same in both experiments, but more extreme in the second experiment. This may be related to the greater difference in digestibility between grass and clover in the second experiment. The conclusions of this study were contrary to popular belief. The within pasture species response of goats and sheep to changing height was very similar in grass monoculture. In clover monocultures goats showed a greater preference for the longer swardlet than sheep. In clover/grass and height combinations, sheep were willing to graze clover to a shorter height than goats, which switched to higher grass before sheep. The "preference for a mixture" concept was confirmed in this study for sheep and goats. It was not possible to support any given theory for selection of a mixed diet with complete confidence. It was suggested that a combination of the theories reviewed, involving the concept of "Maximum Ecological Fitness" could be a possible explanation for the behaviour observed. A model which illustrates the effect of changing grass/clover availability on ecological fitness was proposed.