In vitro fermentation and growth performance of merino lambs fed on umbrella thorn (Vachellia tortilis) leaf meal and sunflower oil.

Abstract

Forage legumes and vegetable oils are supplemented in ruminant diets to improve nutrient quality (energy density and crude protein content) and mitigating rumen gaseous emissions. The effects of both forage legumes and vegetable oils would depend on source, inclusion level and animal species. The broad objective of the study was to determine the effect of Vachellia tortilis leaf meal and sunflower oil on in vitro short-chain fatty acids (SCFAs) production, proportion of methane and growth performance of Merino lambs. The specific objectives of this study were to determine (1) the effect of Vachellia tortilis leaf meal and sunflower oil on in vitro total SCFA production, individual SCFAs composition, proportion of methane, carbon dioxide, and IVDMD; (2) the effect of Vachellia tortilis leaf meal and sunflower oil on growth performance of Merino lambs and (3) the effect of Vachellia tortilis leaf meal and sunflower oil on fractional outflow rate of particulate and liquid fractions of digesta in sheep. Five dietary treatments used were: the control diet (CT), Vachellia tortilis (VT) leaf meal diet (121.5 g/kg DM), sunflower oil (SFO) diet (40.8 g/kg DM), combination of Vachellia tortilis leaf meal and sunflower oil (VSFO) diet (63.4 g/kg + 19.5 g/kg DM) and maize grain – lucerne (ML) (300 g/kg + 180 g/kg DM) diet. Fresh samples were collected, dried in oven, ground and analysed for nutrient composition. Twenty-two duran bottles were incubated, including two blanks for 48 hours. Total SCFAs, acetate and propionate, acetate to propionate ratio, proportion of methane and carbon dioxide were not affected by the inclusion of Vachellia tortilis leaf meal and sunflower oil. Butyrate production and proportion of carbon dioxide were highest in VSFO diet at 16 hours compared to the control. In vitro dry matter digestibility (IVDMD) was higher in VT and VSFO diets compared to the control. The condensed tannin, ether extract and sunflower cake content did not influence production of total SCFAs, individual SCFA, proportion of methane and carbon dioxide. The high ether extract content in SFO diet negatively affected IVDMD. It was evident that inclusion of Vachellia tortilis leaf meal and sunflower oil did not affect production of total SCFAs, acetate to propionate ratio and proportion of methane. The proportion of methane was calculated based on stoichiometric method as observed and the Moss et al. (2000) equation. Both methods displayed a linear relationship with similar results. Ten mixed sex Merino lambs (n= 6) were fed on similar dietary treatments as in vitro fermentation study. An incomplete Latin square design was used where each treatment was represented by a random pair of lambs housed in individual pens for three periods (126 days). Lambs were offered 480 g/kg DM daily of dietary treatments with ad libitum accesses to urea-treated hay (Themeda trianda). For passage rate trial, five lambs were

selected with one from each dietary treatment. The crude protein content was higher in VT and VSFO diets, due to inclusion of Vachellia tortilis leaf meal. Dry matter intake was lower in maize-lucerne diet compared to other diets including the control. Total dry matter intake (TDMI), average daily gain (ADG), feed conversion ratio (FCR) and neutral detergent fibre digestibility (NDFD) were similar across all dietary treatments. The inclusion of sunflower oil in dietary treatments negatively affected apparent digestibility, which was due to high ether extract content. Fractional passage rate, total mean retention time (TMRT) in both reticulorumen (RR) and hindgut (HG) of particulate and liquid fraction of digesta were similar across all dietary treatments. The improvement of roughage with a non-protein nitrogen source (NPN) provided a nutrient balance for lambs. The ether extract content in sunflower oil diets were above the recommended levels, thus possibly explaining the poor digestibility. In conclusion, the inclusion of Vachellia tortilis leaf meal and sunflower oil did not affect growth performance. The combination of Vachellia tortilis leaf meal and sunflower oil has potential to improve average daily gain.