. 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. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Diet selection; Grazing; White clover; Herbage intake; Gastrointestinal parasites; Lambs
1. Introduction
Gastro-intestinal parasitism disrupts protein metabolism and depresses feed intake Ž
. Poppi et al., 1986; Poppi et al., 1990 . Recent studies with healthy lambs offered feed
choices indoors shows that they can select a diet that meets their protein requirements Ž
for fast growth and avoid excessive protein intake Hou et al., 1991; Kyriazakis and .
Oldham, 1993 . Further, in response to intestinal parasitism, they will select a diet with Ž
. higher protein content Kyriazakis et al., 1994 . These results from studies indoors
suggest a nutritional basis for food choices. However, the environment for foraging is more complex outdoors, yet links between parasitism and nutrition have considerable
importance for both experimentation and management. The ability of grazing animals to detect a metabolic protein imbalance resulting from parasitism, and to respond to it by
changing diet selection among pasture species, has not been tested.
Ž Sheep alter grazing behaviour in response to faecal contamination of pasture Hutch-
. ings et al., 1998 . In that study, parasitised sheep grazed more selectively from artificial
swards of ryegrass than non-parasitised sheep by taking smaller bites at a slower rate interpreted by those authors as an indication of parasite avoidance behaviour. This
suggests that parasitised sheep outdoors are capable of adjusting grazing behaviour, although, in this case, not to improve nutrient intake. However, together these and the
above quoted studies suggest that sheep might be capable of exploiting differences between plant species to improve their nutrient intake. For lambs grazing mixed
grass–clover pasture, there is an opportunity to alter dietary protein in response to parasitism by selecting for plant species with a higher protein content, or species that
Ž .
can be eaten at a faster rate. White clover Trifolium repens L. has protein character- istics that may satisfy the nutrient demands arising from parasitism. It has consistently
high nitrogen in its leaves, and while it does not necessarily have a higher nitrogen Ž
. concentration in the leaves than ryegrass
Lolium perenne L. , more of this nitrogen Ž
. reaches the lower gastro-intestinal tract Ulyatt et al., 1988 . Sheep can eat white clover
Ž .
at a faster rate than ryegrass when offered as monocultures Penning et al., 1991b . This would be an added advantage for increasing protein intake, but selective grazing to
obtain a desired component from a mixed-species pasture involves a cost to the animal Ž
. Parsons et al., 1994b . This cost is normally incurred through reduced short-term rate of
intake as an animal searches for desired food items, and results in either lower daily intake, or a need for longer grazing time to achieve the same daily intake. The amount
of a preferred food item eaten by sheep is influenced by availability, in both the Ž
. horizontal and vertical dimensions Edwards et al., 1996b . As the proportion of a
preferred component in the pasture decreases, the cost of selective grazing increases and Ž
the proportion in the diet is likely to decline see Parsons et al., 1994b; Thornley et al., .
1994 . The objective of this study was to test the hypothesis that in response to intestinal
parasitism, grazing lambs will increase the proportion of white clover in their diet as a means of increasing protein intake, and that the extent of this selection for clover will
depend on the grass species available as the dietary alternative.
2. Animals, materials and methods
