Associative effects of mixed feeds for ruminants
The literature on the non-additivity of feedstuffs, their existence, occurence and possible causal mechanisms including factors affecting' fibre and starch digestion and feed energy utilisation by ruminants, was reviewed. Six experiments were designed to provide information on how these effects occur, to conclusively demonstrate the existence of associative effects and to identify possible methods whereby they may be either avoided or alleviated. The reduction in the rumen pH (to 6.0-6.1) in sheep given roughage led to the inhibition of cellulolysis and partial destruction and washout of the rumien microflora. Dry matter (DM) degradation In the rumen and dry matter intake (DMI) were also depressed. Increasing the rumen pH of sheep offered the concentrate diet did not alter greatly the rumen microflora, nor did it increase cellulolysis, DM degradation or DMI. It was concluded that both the type of substrate present in the rumen and the rumen pH influence ruminal cellulolytic activity. It was found that the amount and proportion of barley necessary to depress rumen pH, DM degradation and cellulolysis depended on the availability of the readily fermentable barley attach to the rumen microorganisms and was therefore influenced by the degree of proceeding. A pH threshold of 6.0-6.1 was obtained blow which cellulolysis was totally inhibited. It was observed that the extent of deprecation of fibre degradation was influenced by both the rumen pH and the rate of solubilisation of the concentrate supplement. This effect was further confounded by the degradability of the roughage, since the percentage reduction in the rate of cellulolyais was greatest with those roughage with a low DM degradability. It was concluded that in order to maintain rumen cellulolysis when roughages are supplemented with a readily fermentable carbohydrate, the rumen pH has to be maintained above 6.0-6.1. It was found that the diurnal variation in pH increased with the level of supplementation and degradability of the supplement and decreased as the frequency of feeding increased. Although the rumen pH of the diets supplemented with Harley was reduced to a similar extent to that or diets containing glucose, the pH was depressed over a longer period of time. It was also observed that the extent of fibre degradation was fairly similar for both glucose and barley diets, which confined that there degradation, namely the presence of readily fermentable carbohydrates. In the last two experiments the effect of offering diets containing various proportions of hay and barley, with and without the addition of bicarbonate salts, on the voluntary intake and digestibility of dietary components was examined. In the fifth cameramen fixed amounts of barley were given with hay freely available, while in the final experiment fixed proportions of hay and barley were offered ad libitum. Grinding the hay was found to increase the daily DMI, in both experiments, when the hay was offered alone or supplemented, but decreased the digestibility of the DM, organic matter (OM) and said detergent fibre (ADF) component of the diets. The addition of barley depressed the digestibility of ADF of both the ground and chopped hay based diets but increased DM and OM digestibilities. In the sixth and final experiment, it was found that although DM and OM digestibilities increased linearly with the proportion of barley in the diet, there was found to be an additional quadratic trend - the digestibility values obtained were significantly less than those calculated by the summation of the weighted sum of the separately determined digestibilities of the component feedstuffs, i.e. associative effects or the non-additivity of feedstuffs were shown to exist. A measure of the magnitude of these effects was obtained by considering the extent to which the digestibility of the diets was depressed when compared with calculated values. The maximum deviation from linearity, was obtained when the diet containing ground hay: pelleted barley (1:2) was offered, the \ DM digestibility depressed by over 9%. This represented: a loss in the faeces of about a tenth of the calculated digestible material ingested. This diet also reduced the digestibility of the hay fraction to the greatest extent, to 31.2%, compared with 51.0% when hay was given alone - a depression of 38.0%, i.e. nearly 40% of the digestible material offered as hay was excreted. It was concluded that associative effects could be largely avoided if the rumen pH was maintained above that level inhibitory to cellulolysis. It was suggested that this could be achieved by offering roughage portion of the diet either long or chopped, so as to stimulate rumination and salivation, by offering the concentrate in such a way as to minimise the risk of depressing material, such as bicarbonate salts, in the diets.