Processed grains as a supplement to lactating dairy cows

Keywords: heat treatment, maize, barley, starch, protein, in sacco, in vivo, dairy, perennial ryegrass, grazing, supplementation, ruminal fermentation, VFA, rumen, degradability, synchrony.In this thesis the effect of different ways of thermal processing (pelleting, expanding, toasting) of barley and maize on the degradative behaviour of their starch and protein in the rumen of lactating dairy cows are described. In situ studies showed that all thermal processing methods increased the ruminal starch and protein availability of maize, while all thermal processing methods decreased ruminal starch availability but only pelleting increased ruminal protein availability of barley. Based on in vivo experiments compared to untreated grains, expander treatment increased the apparent rumen and total tract digestibility of maize starch but did not affect the digestibility of barley starch. Supplementing grazing dairy cows with pelleted and pressure toasted maize and barley slightly (not significantly) decreased the dry matter intake of grass in the first grazing event in the morning after milking, and it decreased ruminal clearence of nitrogen. Supplementing pasture grass with pelleted and pressure toasted cereal grains decreased the pH, the NH 3 -N level in the rumen, the ammonia to total VFA (TVFA) ratio, the isobutyrate proportion, the acetate to propionate ratio and the non-glucogenic to glucogenic ratio in the rumen. Simultaneously it increased TVFA concentrations, propionate, butyrate and valerate proportions as a percentage of the TVFA. All processed grains did affect production responses in dairy cows, by elevating milk protein and decreasing milk fat production, and milk urea nitrogen but no significant differences between these two heat treatments were found. It is concluded that the need of synchrony is specially important with diets based on fresh grass, in which markedly asynchronous rates of release of energy and nitrogen occur in the rumen. It appears that to generate a better ruminal N and organic matter synchrony to improve microbial N yield and N utilization feeding different types of cereal grain that differ by nature in rate and extent of ruminal degradation has more effect than using one of this processing methods on the same type of grain. Responses obtained from cereal grain supplementation are very dependent on the quality and degradation characteristics of the pasture consumed, which changes through the grazing season.