The effect of feed type and diet quality on kinetics of digestion : the degradation properties of certain protein supplements, and the effect of concentrate supplementation and basal roughage quality on eating behaviour and particle passage from the rumen of sheep.
The unique anatomical structure of the ruminant animals' digestive tract allows them to convert forages containing high levels of fibre into the valuable products of meat, milk and wool. At the same time, an interdependent relationship between the host animal and the rumen microbial population has evolved and a minimum requirement for dietary fibre by the ruminant was established. Where roughage intakes are high, intake is limited by rumen capacity and the rate of clearance of previously ingested material from the rumen. When animals are fed particularly poor quality roughage diets, high in fibre, as in sub-Saharan Africa, the problem of limited rumen capacity is compounded. The rate of passage of ingested material through the digestive tract will also influence the ability of the host animal to extract nutrients, whilst the degradation that a feed undergoes in the rumen will have a direct effect on the availability of nutrients to the host animal and, conversely, the availability of nutrients to the rumen microbes themselves. In sub-Saharan Africa, where readily available forage tends to be high in fibre and low in soluble nutrients (such as nitrogen) and readily digestible carbohydrates, the potential exists for the development of supplementation strategies that will enhance rumen microbial fermentation and microbial protein synthesis. This study comprised three separate, although interrelated investigations into aspects of rumen clearance and degradation parameters of eight nitrogen supplements. The supplements investigated were canola, copra, cottonseed oilcake, defatted maize germ, luceme meal, lupin seed meal, soya oilcake and sunflower oilcake. The first part of this study investigated the disappearance of dry matter (DM) and nitrogen from eight plant protein sources contained within nitrogen-free polyester bags incubated in the rumen of three fistulated Jersey cows. In addition, the protein sources were force-fed to adult cockerels to obtain an estimate of the intestinal digestion of the available protein by the host animal. The intercept values and time lags for DM degradation were highly significant (P<O .001 ), whilst potential degradability and rate of degradation were significantly different among protein sources (P<0.05). In terms of nitrogen degradation, the intercept values and rates of degradation were significantly different (P<0.001 ), while potential degradability and time lag were not significantly different among feeds. Effective degradability (ED) was calculated using different rates of passage for both solid particles (k) and liquid matter leaving the rumen (klq) for sheep, beef and dairy animals. Lupins had the highest ED for both DM (792 g kg-1 in sheep) and nitrogen (844 g kg-1 in sheep), while canola showed the lowest effective DM degradability (433 g kg-1 in sheep) and copra the lowest effective nitrogen degradability (678 g kg-1 in sheep). Digestible rumen undegradable protein (RUP) was calculated as the difference between total digestible protein and effective nitrogen loss. Calculated in this manner, sunflower oilcake had the highest digestible RUP (126 g kg-1 nitrogen in sheep) while cottonseed oilcake had the lowest (83 g kg-1 nitrogen in sheep}. Further consideration should be given to methods of determination of RUP described in this trial as well as determination of digestibility parameters for complete feeds rather than supplements alone. The effect of increasing the nitrogen supply to the rumen microbes by improving the quality of roughage supplied to the host animal was investigated in the second phase of this work, by substituting the basal roughage (poor quality veld hay) with varying amounts of luceme hay. Five treatments of 100% hay, 75% hay and 25% luceme, 50% hay and 50% luceme, 25% hay and 75% luceme and 100% luceme were used. In this way CP was increased as the luceme proportion of the ration increased while the NDF concentration was decreased. The effect of varying roughage quality on the voluntary feed intake, feeding behaviour and rate of passage of both long and short particles from the rumen of South African Mutton Merino sheep was investigated, using 25 sheep blocked according to weight and randomly assigned to one of five treatments in a completely randomised block design. DM, CP, CF and NDF intakes increased significantly (P<0.001, P<0.001, P<0.05 and P<0.05, respectively) as lucerne inclusion level in the ration was increased. The individual effects of both diet and particle size on the rate constant (k1, indicative of the rate of passage of particles) were highly significant (P<0.001). The particle size and diet interaction was significant (P=0.05). Lucerne inclusion levels in the diet had no significant effect on ruminating time. Increasing the level of luceme in the ration resulted in a significant decrease in total eating time and time spent ruminating per kilogram NDF consumed (P<0.01) and time spent eating per kilogram dry matter consumed (P<0.05). Sheep weight had a highly significant (P<0 .001) effect on time spent eating per kilogram of dry matter consumed, time spent ruminating per kilogram of NDF consumed. There was a significant increase in ruminating time (P<0.05) as sheep weight increased. These results indicate that interactions among feed factors, such as particle size and feed composition, will affect the rate of clearance of ingested roughage particles from the rumen and, consequently, the level of roughage intake. The effect of animal factors and chewing effectiveness warrant further investigation. The third trial conducted investigated the effect of increasing both basal roughage quality and level of concentrate supplementation on the intake of the basal roughage and the rate of passage of particles from the rumen (k1) of South African Mutton Merino sheep. Four roughage qualities were achieved by supplementing the basal roughage (poor quality veld hay) with varying amounts ofluceme hay such that luceme contributed 20, 40, 60 or 80% of the diet. In this way CP, and ash contents were increased as the luceme proportion of the ration increased and NDF and CF concentrations were decreased. A concentrate supplement was offered at levels of 90, 180, 270 or 360 g per sheep per day. Sixty-four sheep were blocked according to weight and sex and randomly assigned to one of the 16 treatments in a completely randomised block design. The interaction of lucerne inclusion level and concentrate supplementation was not significant. The relationship between level of concentrate supplementation and k1 is, inversely quadratic and significant (P<0.05). Lucerne inclusion level had no effect on k1, but the effect of concentrate supplementation level on total dry matter intake (DMI) and total CP intake was highly significant (P<0.001), with DMI and CP intake increasing quadratically as the level of concentrate supplementation increased. NDF intake decreased quadratically (P<0.05) as luceme inclusion level increased whilst CP intake increased quadratically (P <0.001) as lucerne inclusion level increased. These results indicate that below a certain level, the effect of concentrate supplementation on fibre digestion is not severe and that roughage and NDF intake is not affected. Total DM and CP intakes are increased as concentrate supplementation increases. The effect of concentrate supplementation on feeding behaviour warrants further investigation.