Investigating the role of carbohydrates in the dietary choices of ruminants with an emphasis on dairy cows

Deposited with permission of the author. © 2002 Dr. Sally Amanda FrancisThis thesis investigated the role of carbohydrates in the dietary choices of ruminants with an emphasis on dairy cows. The first two experiments investigated the ability of sheep to select between feeds based on their carbohydrate degradability. A further two indoor experiments using dairy cows were designed to establish whether post-ingestive feedback from rumen fluid propionic acid concentration influenced preference. The final experiment examined the potential of ryegrass bred for high water soluble carbohydrate (WSC) concentrations to increase the long-term (9 days) preferences and intake of grazing dairy cows. Constraints to intake imposed by offering sheep access to only one feed were overcome by offering a choice between two feeds simultaneously. Within each choice, sheep generally selected the more slowly degradable option. However, when overall NDF intake could be maintained at approximately 800g/day, the rapidly degraded feed was preferred. Dairy cows were able to form associations between flavour and postingestive feedback from rumen propionic acid concentration. Although a dose-dependent response was not observed between the concentration of ruminal propionic acid infusion and preference intensity, there was a correlation between ruminal propionic acid concentration and energy status of the cow. In the subsequent experiment, the comparative effect of propionate supplied in the form of salt (instead of acid) on food preference was confounded by a flavour bias. Diurnal WSC monitoring of perennial ryegrass cultivars bred in the U.K. for 'typical' and 'high' WSC concentrations, expressed similar concentrations at different times of the day and year when grown in northern Victoria. Consequently, in a test of preference between the cultivars, cows showed only slight preference that was not based on WSC concentration. In other choices between adjacent monocultures, cows selected a mixed diet of 62% white clover and 38% ryegrass. It was concluded that the ideal diet from the animals' perspective is influenced by the rumen propionic acid concentration and the energy status of the animal. Further, an important priority for the ruminant is to maintain an adequate supply of structural carbohydrates to the rumen. Further work is needed to identify the benefits of feeding pasture with higher WSC, but this might be a difficult objective under Australian field conditions until plant material becomes available that more reliably expresses high WSC