Effects of parasitic status and level of feeding motivation on the diet selected by sheep grazing grass/clover swards

An experiment was carried out to determine the effects of parasitic status and level of feeding motivation on the diet selected by sheep grazing peren nial ryegrass/white clover swards. Twenty-four female Scottish Blackface we aned lambs were divided into four treatment groups each of six animals. Pre -grazing treatments were imposed to create two levels of feeding motivation , low (fed ad libitum) and high (0.60 of an libitum diet) and two parasitic states, parasitized (daily dose of 2500 L-3 Ostertagia circumcincta) and n on-parasitized. Each treatment group was placed in one of four 0.35 ha fiel d plots and allowed to graze a mixed grass/clover sward. Mean clover height was manipulated pre-grazing by cutting so that it was less than the mean g rass height. This maximized the probability that clover in the diet of shee p was due to selection. No fertilizer was added to the sward in order to en sure that the nitrogen content of the clover was higher than that of the gr ass. Each treatment group was rotated around the four plots on a daily basi s for 2 weeks. Sheep with a high feeding motivation strongly and significan tly selected clover and had a higher proportion of clover in their diet com pared with sheep with low feeding motivation. Parasitic status had no signi ficant effect on the diet selection of sheep; but the level of parasitic bu rden within parasitized sheep was positively correlated with the proportion of clover in the diet. Parasitized sheep spent less time grazing each day and had lower daily herbage intakes compared with non-parasitized sheep. Th is was a result of a reduction in the duration of feeding bouts and not a r educed number of bouts per day. Sheep with the higher feeding motivation ha d a greater number of feeding bouts per day and this resulted in those anim als having a greater daily herbage intake. Level of feeding motivation had greater effects on the diet selection of sheep than parasitic status. Sheep with a high feeding motivation are able to select strongly for clover in t heir diet when there is low overall clover availability and are capable of dramatically affecting the species composition of a sward. This has implica tions to the maintenance of a desired grass/clover ratio in grazed swards.