Effects of sward structure on the diet selected by guanacos (Lama guanicoe) and sheep (Ovis aries) grazing a perennial ryegrass‐dominated sward

Abstract

The effect of three different vertical structures of a perennial ryegrass‐dominated sward, defined by pseudostem heights (cm) — 1·3 low (L), 2·5 medium (M) and 3·5 high (H) — on the bite depth and selection of plant parts within the grazed horizon of the sward by sheep and guanacos was studied. The bite depth (cm) was similar between sheep and guanacos across the different swards (L, 1·5; M, 3·6; H, 3·8) and was related to pre‐grazing sward surface height rather than to pseudostem height. There were differences in diet composition between species that were related to differences in selection for plant parts. Sheep had a higher proportion of green leaf in the diet than did guanacos in L (0·84 vs. 0·71, P < 0·05) and M (0·75 vs. 0·59, P < 0·05) swards, and possibly selected this plant part in H swards in which the proportion of green leaf in the grazed horizon was low (0·45–0·50). Guanacos had a higher proportion of dead leaf and sheath in the diet than did sheep in L (0·23 vs. 0·09, P < 0·05) and M (0·30 vs. 0·18, P < 0·05) swards, and possibly selected this plant part on these swards, in which the proportion of green leaf in the grazed horizon was high (0·70–0·95). The proportion of pseudostem in the diet of sheep and guanacos was similar across all swards (0·03) and was generally much lower than that in the grazed horizon (0·01–0·18). Guanacos had a higher proportion of dead stem in the diet than did sheep (0·06 vs. 0·02, P < 0·05) across all swards, but the proportion was similar to that in the grazed horizon of each sward.

Methane emission by alpaca and sheep fed on lucerne hay or grazed on pastures of perennial ryegrass/white clover or birdsfoot trefoil

“…On Lotus, alpaca preferred primarily the senescent grass material (weeds), but because of the low availability of this material, Lotus stem and leaf materials were also eaten. The differences between species in selective grazing were very evident in this study and agree with the results from other studies with alpaca and sheep (Sharp et al 1995) or guanacos and sheep (Bakker et al 1997;Fraser & Gordon 1997;Fraser 1998).…”

Defoliation pattern, foraging behaviour and diet selection by lactating dairy cows in response to sward height and herbage allowance of a ryegrass‐dominated pasture

“…The short and tall swards of this experiment were prepared by applying different cutting intervals on the same initial sward, over a period of 8 weeks. Bakker et al (1998) found that different cutting treatments (height and interval) applied over a period of 9 weeks in spring did not significantly alter the tiller density of a perennial ryegrass sward. Hence tiller density is considered not to have caused variation in the relationships between sward height, bite depth and bite area within the grazed horizon.…”

Sward structural resistance and biting effort in grazing ruminants

“…In practical terms obtaining a measure of the peak bite fracture force is of little importance, and it is the average force applied for a given bite that needs to be assessed. Furthermore, rejection of the Summit Force hypothesis is supported by the consistent findings that, across a wide range of body sizes, animals do not always fully exploit the available depth of regrowth [sheep and guanacos (Lama guanicoe) [6], bison (Bison bison) [10], cattle [27,33]]. …”

“…By contrast Canigano et al [15] did not find evidence for an intraspecific relationship between body mass and grazing depth in cattle, despite a wide contrast in body mass (256-608 kg). Furthermore, there has been consistency in bite depth for inter-species variation in body mass; cattle vs. sheep [55], sheep vs. guanacos (Lama guanicoe) [6]. Given these inconsistencies in the literature on intraand inter-specific species depth of penetration responses we are left wondering about the real costs involved in grazing [42].…”