WHAT CONSTRAINS DAILY INTAKE IN THOMSON'S GAZELLES?

We tested whether cropping or digestion by Thomson's gazelles ( Gazella thomsoni) constrains daily energy intake under sward conditions normally encountered during the growing season. Distinguishing between these alternatives is important in un- derstanding grass-grazer interactions and modeling grazer energetics. Grazing trials on artificial swards showed that gazelles had a monotonically saturating functional response, but that relationships between grazing rate and forage density changed with grass height. Grazing rate was positively related to biomass on short swards, yet there was no significant relationship for tall swards. Bite mass and bite rate also differed in their relationship to biomass across sward heights, with the strongest relationships being found on short swards. Bite rate and bite mass were inversely related, as predicted by current theory for dense grass swards. Voluntary energy intake on a daily basis was a positive function of the digestible energy content of forage, but a negative function of sward biomass. Therefore, our results indicate that daily energy intake is constrained by digestive processes on swards with biomass .25 g/m 2 , whereas intake is constrained by cropping processes at lower sward biomass. Our data additionally suggest that variation in bite rate and bite mass with sward height could permit a small ruminant to select high-quality grass, thereby achieving high energy gain on immature swards.

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