Studies with dairy cows (Meijs, 1986) have shown higher grass intakes when grazing animals were supplemented with concentrates based on digestible fibre rather than starch. It has been demonstrated that high inputs of quickly fermentable substrates such as soluble sugars or rapidly fermentable starch can increase the concentrations of volatile fatty acids and lactate in the rumen thereby causing a marked decrease in pH. This can reduce cellulolytic activity by rumen microbes, resulting in a lower rate of forage fibre digestion and an increase in non-fermented residue, which in turn can restrict feed intake. Furthermore, autumn grass has the potential disadvantage of relatively low levels of water-soluble carbohydrates and high levels of rapidly degradable protein. 

The objective of this study was to determine the effects of supplementing cattle grazing autumn grass with concentrates formulated using different carbohydrate types on grass intake and animal performance where grass supply was considered to be either adequate or limiting. 

Eighty-four continental crossbred steers (494kg mean starting liveweight) were assigned to seven treatments in a two (herbage allowances) by three (concentrate types) factorial design, plus a positive control of concentrates offered ad-libitum. The daily herbage allowances were 5.5 and 11.0 kg DM/head, approximating to 1 and 2 % of bodyweight, respectively. Five kg of concentrates were offered individually to each animal daily, in two equal feeds. Three concentrate rations were formulated to be iso-energetic and iso-nitrogenous, but to differ in carbohydrate source (Table 5). Six rumen fistulated steers were simultaneously used to estimate the effect of diet on rumen fermentation and kinetics. The grazing experiment took place between August 22 and November 19. 


Table 5. Ingredient formulation (g/kg) of the concentrates.