Intake and selection for white clover by grazing lambs in response to gastrointestinal parasitism

Abstract

Lambs indoors seek to compensate the metabolic protein deficiency caused by intestinal parasites by selecting a diet with a higher concentration of protein. A study was conducted to determine if lambs grazing at pasture would enhance their nutrient intake in response to parasitism, by selecting more white clover from a mixed grass–clover pasture. The trial used a split-plot design with two grass–clover mixtures as main plots, two levels of parasite infection as the split-plot, and two replications. Lambs with a low level of infection (suppressive drenched with anthelmintic at 2-weekly intervals to restrict parasite infection; SD) or with a moderate level of infection (drenched whenever group mean faecal egg count reached a trigger level 1000 eggs per g fresh faeces; TD) grazed together on either perennial ryegrass ( Lolium perenne L.) and white clover ( Trifolium repens L.) (RWC), or on browntop ( Agrostis capillaris L.) and white clover (BWC). Plots were continuously stocked with trial lambs from early-summer (January) to early-winter (June). Additional put-and-take lambs were used to maintain sward surface height at 5 cm. A sub-group of 12 lambs, comprised of 4 SD and 8 TD, similar in mean liveweight to their cohorts, was used to determine individual herbage intake, diet composition and diet in-vivo digestibility over a 10-day period in autumn. This period was timed to coincide with the rise towards the trigger level in the egg count of TD lambs. Compared with lambs grazing BWC, those grazing RWC had higher liveweight gain (280 vs. 156 g day −1; P=0.06) and higher herbage intake (1.57 vs. 1.17 kg DM hd −1 day −1; P=0.003). The proportion of clover in the diet was higher for TD than for SD lambs on BWC (31% vs. 24%), but not on RWC, where the diet contained a higher proportion of white clover (51%) for both levels of parasitism (species×drench interaction, P=0.02). For each treatment, the proportion of white clover in the diet was higher than in the sward. The higher proportion of clover (30% vs. 20% of green DM) and lower bulk density of the RWC swards compared with BWC (180 vs. 310 kg DM ha −1 cm −1 of pasture canopy height) assisted RWC lambs in selecting a higher proportion of clover. However, with this higher proportion of clover in their diet, parasitism did not stimulate selection. It is suggested that in the complex foraging environment outdoors, grazing lambs can detect metabolic signals associated with parasitism and make a behavioural response to mitigate the harmful nutritional effects.