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Effects of the total, vertical and horizontal availability of the food resource on diet selection and intake of sheep

Published online by Cambridge University Press:  27 March 2009

G. R. Edwards ,
J. A. Newman ,
A. J. Parsons  and
J. R. Krebs
G. R. Edwards
Affiliation:
NERC Unit of Ecology and Behaviour, Department of Zoology, University of Oxford, South Parks Road, Oxford 0X1 3PS, UK
J. A. Newman
Affiliation:
NERC Unit of Ecology and Behaviour, Department of Zoology, University of Oxford, South Parks Road, Oxford 0X1 3PS, UK
A. J. Parsons
Affiliation:
Institute of Grassland and Environmental Research, North Wyke, Okehampton, Devon, EX20 2SB, UK
J. R. Krebs
Affiliation:
NERC Unit of Ecology and Behaviour, Department of Zoology, University of Oxford, South Parks Road, Oxford 0X1 3PS, UK
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Summary

Plant species mixtures vary in total herbage mass (total availability) and the fractional cover (horizontal availability) and bulk density or height (vertical availability) of the component plant species. The expected diet selection response of sheep to changes in these three components of availability was tested in an artificial food environment by releasing flocks (of three ewes) onto a vegetation-free field that contained patches (plastic bowls) of preferred and less preferred sheep pellets for 15 min and observing their behaviour. One hundred patches (bowls) were laid out at equidistant intervals on a 50 × 50 m field. The patches were filled with either cereal (preferred) or lucerne (less preferred) pellets in different amounts to create in a factorial design two levels of total availability (10000 g or 15000 g of pellet in the field), two levels of horizontal availability (20 or 50 of the 100 patches contained cereal pellets, the rest contained lucerne pellets) and two levels of vertical availability (the mass of pellet in the cereal patches was either 10 or 80% relative to the mass of pellet in the lucerne patches). All patches were positioned at random in the field. Although, in all cases, the proportion of cereal pellets in the diet was higher than that expected if the sheep had foraged at random, it was sensitive to the horizontal and vertical availability of the food resource. The proportion declined as the horizontal availability declined from 50 to 20 patches, but only at 10% not 80% vertical availability. The lower proportion for this treatment combination arose because sheep ate from a higher proportion of the lucerne patches that they encountered and rejected more of the cereal patches that they encountered. Total availability did not affect diet selection. From these results it is pointed out why conclusions of diet selection experiments that do not consider how the food alternatives are distributed horizontally and vertically can be equivocal. The effect of variable levels of selective grazing in relation to changing horizontal and vertical availability of plant species on the species composition of plant communities is considered.

Type
Animals
Information
The Journal of Agricultural Science , Volume 127 , Issue 4 , December 1996 , pp. 555 - 562
Copyright
Copyright © Cambridge University Press 1996

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