Short- and long-term trade-offs in the diet selection of sheep

The purpose of this thesis was to investigate, in a series of 5 experiments, the basis of the diet selection of sheep, with the aim of shedding some light on the mles that ruminants might use when selecting their diet. This was done by investigating the effects of offering foods with rapidly degradable nitrogen and rapidly fermentable carbohydrates, within the rumen, on the food intake and diet selection of sheep.Experiment 1 investigated whether sheep select between foods that differ in their effective rumen degradable protein (eRDP) content to achieve an estimated ‘optimal’ eRDP: fermentable metabolisable energy (fME) ratio. The foods offered were presented in a meal form to prevent chemical changes to the food ingredients that may occur during the pelleting process. However, as the food ingredients separated during the course o f the experiment and the composition of the selected diet could not be sufficiently predicted, the results of the experiment were unclear.Experiments 2 and 3 re-asserted the hypothesis that diets selected by sheep would be those that meet the eRDP requirements of the animals and at the same time avoid excess consumption of rumen degradable protein (RDP). The sheep were given choices between pelleted foods differing in their urea content, which was used as an eRDP source. However, sheep selected a diet in favor of the urea-supplemented food regardless of the basal food to which the urea was added. This resulted in an excess consumption of RDP. The results did not support the hypothesis that sheep would avoid excess RDP when given a choice and suggest that eRDP may not be a relevant dimension in diet selection in the conditions of these experiments. It was thought that urea may have had other properties, such as a high buffering potential, that the sheep were selecting for when selecting a diet.Experiment 4 tested the hypothesis that the preference for an eRDP adequate food supplemented with urea would be reduced by the addition of a buffer (sodium bicarbonate (SB)) to both foods offered as a choice or offering ad libitum access to hay. In both cases urea may no longer be needed as a buffer and the preference for the urea-supplemented food would then be expected to be reduced or even disappear. Insufficient amounts of hay were consumed to reduce the preference for the ureasupplemented food. Supplementing both foods with SB significantly reduced the preference for the urea-supplemented food. It was proposed that the effect of SB addition on the diet selection of sheep was carried though its buffering properties. Therefore it could be hypothesized that the sheep in this experiment, and those in the previous two, were prepared to over consume RDP, when supplied by urea, due to the buffering potential of the urea supplemented food. When this excess consumption was made unnecessary by adding SB, sheep avoided an excess intake of RDP. The idea that sheep might give greater weighting to the benefits of maintaining their rumen within a desired physiological range rather than avoid an excess consumption of RDP, led into the second line of research of this thesis.The final experiment investigated whether a large rather than small change within the rumen environment, due to the consumption of a ‘disruptive’ food (i.e. food that would lower rumen pH), would alter subsequent diet selection. Sheep were offered high or low energy density (ED) foods with or without SB for a period of 2 hours. All sheep were then offered a choice between the high and low ED foods unsupplemented for a further 6 hours. SB supplementation did not alter subsequent diet selection. The magnitude of change in the pH of the rumen may not have been sufficient to alter subsequent diet selection. However, sheep initially offered the high ED food subsequently selected more of the low ED food during the 6 hours when a choice was offered compared to sheep initially offered the low ED food. The results from this experiment agree with the hypothesis that changes within the rumen of large, rather than small, magnitude due to the consumption of a disruptive food alters subsequent diet selection.Taken together, the results from these experiments are in agreement with the hypothesis that one of the objectives of the diet selection of ruminants is to maintain the rumen conditions within a certain physiological range. Therefore, in the absence of long forage, ruminants would be expected to select foods with the greater buffering potential firstly to prevent further disruption to the rumen environment and secondly to return the conditions in the rumen to within the accepted range as soon as possible. This need appeared to override other diet selection objectives such as avoiding an excess intake of RDP. However, the basis of protein selection by ruminants still remains unclear. Further research is required to strengthen the argument that ruminants select a diet to avoid both deficient and abundant concentrations of eRDP