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 perennial ryegrass/white clover swards. Twenty-four female Scottish Blackface weaned 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 non-parasitized. Each treatment group was placed in one of four 0.35 ha field 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 grass height. This maximized the probability that clover in the diet of sheep was due to selection. No fertilizer was added to the sward in order to ensure that the nitrogen content of the clover was higher than that of the grass. Each treatment group was rotated around the four plots on a daily basis for 2 weeks. Sheep with a high feeding motivation strongly and significantly selected clover and had a higher proportion of clover in their diet compared with sheep with low feeding motivation. Parasitic status had no significant effect on the diet selection of sheep; but the level of parasitic burden 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. This was a result of a reduction in the duration of feeding bouts and not a reduced number of bouts per day. Sheep with the higher feeding motivation had a greater number of feeding bouts per day and this resulted in those animals 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 their diet when there is low overall clover availability and are capable of dramatically affecting the species composition of a sward. This has implications to the maintenance of a desired grass/clover ratio in grazed swards