Use of inert markers to predict diet composition, forage intake, digestibility and passage rate in sheep.

Abstract

The mechanisms that regulate intake and composition of selected diets in ruminants are complex and vary among animals of the same species and cross species. These are governed by highly variable aspects, which range from animal factors to physio-chemical properties of feeds. Understanding how ruminants select their diets is imperative to improve their utilisation of feed resources regarding the diversity of plant species that can be used as their sources of feed. The objectives of the study were to: (1) determine the effect of group feeding and removal of dietary ingredient (Sorghum bicolor) on diet selection, nutrient and total dry matter intake, and digestibility in choice-fed sheep; (2) asses the effect of animal stocking rate on dry matter and nutrient intake, botanical composition, nutrients selected, total tract digestibility and passage rate of diet consumed by sheep; and (3) predict dry matter and nutrient intake, botanical composition, nutrients selected and total tract digestibility using inert markers. Twelve sheep (mean weight: 29.7 ± 4.63 kg) were assigned to three treatments. In treatment one, five feeds were fed to sheep fed as a group of 9 sheep (G). In the second treatment, five feeds were fed to sheep penned in isolation (I) and in the last treatment, four feeds with sorghum stover (SS) removed were fed to sheep penned in isolation (R). There were five experimental feeds: veld hay (VH), sorghum stover (SS) and maize stover (MS) fed ad-libitum, and Lucerne hay (LH) and bean straw (BS) fed at restriction levels of 0.15 and 0.35 kg/day per sheep, respectively, in a group or individually fed sheep. Diet compositions were similar (p >0.05) between sheep fed individually with or without SS. Similarity in proportion of these dietary ingredients consumed between R and I may be due to less selection of SS; therefore, its removal did not significantly influence consumption and selection of other dietary ingredients. Group feeding of sheep relative to individual feeding with similar dietary ingredients influenced selection of SS. Sheep fed individually had lower intake levels of SS. Establishment of a dominance hierarchy in group-fed sheep may have caused dominant animals to feed on poor quality stovers just to prevent sheep lower in the hierarchy from eating resulting in high consumption of stovers. Fifteen sheep (mean body weight 46.5 ±3.3 kg) were blocked by weight into four groups and each sheep was randomly allocated to four stocking rates (treatments) of 1, 2, 4 and 8 sheep per pen and fed: MS, SS, and VH. All feeds were fed on separate feeding troughs ad-libitum. To evaluate the effect of animal stocking rate (SR) on passage rate of digesta, one sheep each from stocking rates one (SR1) and two (SR2) animals per pen and two sheep each pair from stocking rates of four (SR4) and eight (SR8) animals per pen were randomly selected and dosed with Ytterbium (particulate) and cobalt-ethylenediamine tetraacetic acid (Co-EDTA; liquid) markers. An optimisation procedure was used to predict diet selection by minimising the sum of the squared discrepancies between the proportional concentration of markers (acid insoluble ash: AIA, modified acid detergent fibre: MADF, and acid detergent lignin: ADL) in faeces (A) and their proportional concentration in dietary components (E) (MS, SS and VH), corrected for faecal recoveries of markers. Fractional passage rate (liquid and particulate) from both the rumen and in the hind gut, mean retention time, and total mean retention times across treatments were similar (p >0.05). Similarly, intake of dietary ingredients, nutrients (crude protein: CP, neutral detergent fibre: NDF and acid detergent fibre: ADF), total dry matter intake and composition of diets selected were not different across treatments. Selectivity index factors of diets selected were all within the range of 1.56-3.80, which reflected that animals were able to retain the diets they selected long enough in the gastro intestinal tract (GIT) for efficient digestion. Total tract digestibility and mineral intake (Ash) differed (p <0.05) in relation to animal stocking rate. Sheep in SR2 had the highest digestibility and consequently increased dry matter intake. Predicted dry mater intake and total tract digestibility of a diet selected by sheep were less sensitive to correction of incomplete faecal recovery of the markers and they tended to be similar to observed dietary parameters. Therefore, inert markers can be used to predict several components of a diet selected by grazing sheep and other classes of ruminants.