Comparative studies of effects of sward structure on ingestive behaviour of sheep and goats grazing grasses and legumes : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy at Massey University, Palmerston North, New Zealand
A review of the literature indicated that the ingestive behaviour and herbage intake of grazing animals are strongly influenced by characteristics of sward canopy structure. However, there is limited comparative information on the behavioural contrasts of animals grazing grasses and legumes, and little comparative information on the ingestive behaviour of sheep and goats. The projects which form the basis of this thesis concentrated on these two aspects of ingestive behaviour using an indoor crate grazing technique, in which animals were individually confined in metabolism crates, and offered prepared turves. Three experiments were designed and were run over a period from 1989 to 1991 at the Ballantrae Research Station of the New Zealand Pastoral Agriculture Research Institute (AgResearch). In the first experiment, five grasses and four legumes were each sampled at two stages of growth to produce a range of canopy structures. Two sub-sets of data were generated. One sub-set (restricted data set) embraced four forages (two grasses and two legumes) at two maturity stages across two experimental years (4 forages x 2 stages x 2 animal species x 2 years). This sub-set was intended to assess the effects of the variation in animal body size with increasing maturity (Chapter 4). Another sub-set (enlarged data set) involved nine forages at two maturity stages over the second experimental year. It involved an attempt to examine the effect of sward canopy structure induced by a range of forage conditions on ingestive behaviour of sheep and goats (Chapter 5). The second experiment was designed to dissociate the effects of sward height from the confounding effects of plant growth habit and maturity stage by creating contrasting heights within a forage species, and by sampling vegetative swards only. This experiment involved four forages (two grasses and two clovers) grazed by four sheep and four goats (Chapter 6). The third experiment tested the comparability and conformability of ingestive behaviour between indoor crate gazing and field grazing using sheep, and between oesophageal fistulated and intact sheep using a field cage grazing technique (Chapter 7). Major conclusions may be drawn as follows: Bite weight was strongly influenced by bite depth. There was a substantially greater response in bite depth than in bite area to variation in sward conditions. Bite depth was a major spatial component of bite volume, and hence bite weight. Bite rate declined as bite weight increased, and intake rate was determined as the combination of bite weight and bite rate. Ingestive behaviour of the grazing animals was strongly influenced by the characteristics of the sward canopy structure. Sward height usually had a much more dominant impact than did other sward variables, and bite depth had a much greater response than did other behaviour variables. Very close positive relationships existed between sward height and bite depth, bite volume, and hence bite weight, but the relationship between sward height and bite rate was negative. There was no statistically significant relationship between bite area and either sward height or other attributes. However, there was a significant interaction between animal species (sheep vs goats) and sward categories (grasses vs legumes) in this process, as outlined below. Chapter 5 (enlarged data set of Experiment 1) revealed that when the leguminous swards were grazed by sheep, sward bulk density had a substantially greater effect on ingestive behaviour than did sward height. In contrast, the ingestive behaviour of goats grazing legumes was influenced largely by sward height, though the effect of bulk density was substantially increased compared with that in grasses. Both animal species had a greater response in bite weight than in bite depth when grazing legumes. Chapter 6 (Experiment 2) showed that sward height always had the most important effect in both grasses and legumes irrespective of animal species. Appreciation of the effect of bulk density on ingestive behaviour after the dominant effect of sward height was accounted for, depended very strongly upon the establishment of independent variation in sward height and bulk density over the range of test swards. Sward height had a dominant effect and bulk density had no significant effect where independent variation in height was achieved. Where the variation in sward height was confounded with bulk density, although sward height still had a dominant effect, the interactive effect of sward height and bulk density was significant in some cases, depending on the sward categories (grasses or legumes) and animal species. Animals usually had smaller bite dimensions on legumes than on grasses. However, the reduced bite depth, hence bite volume of legumes compared to grasses could be compensated for by a greater bulk density within the grazed strata, a smaller effort required to harvest herbage and less plant components slipping out of the teeth, leading to a greater bite weight. Other differences between sheep and goats were identified as follows: Sheep were generally capable of penetrating into swards deeply, whereas goats grazed swards from the top downwards. Sheep usually had larger bite dimensions, and hence greater bite weights than goats when grazing legumes (irrespective of maturity stages), and vegetative grasses, leading to greater bite weights on average. However, goats were able to achieve greater bite weight in relation to live weight. On reproductive grasses goats showed a greater willingness to eat, whereas sheep were selective through pushing into the swards to graze leafy components and rejected the rigid components. When swards matured, intake rate of sheep decreased because an increase in bite weight was counterbalanced by a large fall in biting rate. Intake rate of goats increased as a result of a substantial increase in bite weight and a relatively small fall in bite rate, compared with sheep. The variation in bite weight and bite depth between sheep and goats for a given sward may be attributed, at least partially, to the discrepancy in incisor arcade breadth between the two species. Increases in bite weight and bite dimensions with increasing maturity of animals were ascribable to increases in incisor breadth of animals with increasing maturity over time. Experiment 3 (Chapter 7) demonstrated that the results of indoor crate grazing can be reliably extrapolated to field grazing on a short-term basis, and grazing behaviour of animals is not significantly altered by oesophageal fistulation. Recommendations and suggestions for application to grazing management and plant selection programmes were made in practical terms.