Abstract

The concentration of water soluble carbohydrate (WSC) increases during the day and the difference between morning and afternoon is greater and more consistent than the difference between cultivars. An in vitro batch culture system was used to compare the fermentation of ryegrass cultivars that differed in WSC, after harvesting samples in the morning (08:00 h) and afternoon (16:00 h). Bottles were placed in a shaking incubator at 39°C and removed at 0, 2, 4, 6, 8, 12 and 24 hours to measure pH and concentrations of ammonia and volatile fatty acids (VFA). Ryegrasses harvested in the afternoon had 81-114 g/kg dry matter more WSC, 3-12 g/kg dry matter less neutral detergent fibre and 14-25 g/kg DM less crude protein than morning samples. Compared to morning harvested ryegrass cultivars, afternoon harvested ryegrass cultivars had the lowest yields of VFA. Afternoon-harvested ryegrass cultivars, especially high sugar ryegrass, were rapidly fermented and after four hours the pH of high sugar ryegrass decreased to less than 5.8, which has been reported to cause suboptimal fibre digestion. It may be possible to manipulate the WSC concentration of a pasture diet by altering grazing management practices. Further research is required to characterise the optimum concentration of WSC for efficient rumen fermentation.

D, Burke, JL Pacheco, and GP Cosgrove

Proceedings of the New Zealand Society of Animal Production, Volume 71, Invercargill, 229-233, 2011