Nutrient content and the effects of feeding four Namibian encroacher bush species on growth, methane production and carcass characteristics of Damara sheep
Date
2022
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Namibia
Abstract
The aim of this study was to evaluate the nutritional and feeding value of four Namibian encroacher species on the intake, digestibility, growth performance and carcass characteristics of Damara sheep. In the first experiment, the nutritional quality of four encroacher bush species (Senegalia mellifera, Dichrostachys cinerea, Terminalia sericea and Rhigozum trichotomum) was evaluated. Leaves and twigs (< 20mm stem diameter) harvested during the late dry, early rainy and late rainy seasons were analysed for proximate content, minerals, amino acids, fatty acids and anti nutrient composition. In the second experiment, in vitro dry matter digestibility, in sacco neutral detergent fibre digestibility and methane production on the same samples above were evaluated. In the third experiment, milled bush biomass was evaluated as alternative roughage source in total mixed rations fed to five castrated 13 month old Damara sheep, with an average initial body weight of 37.2 ± 2.4 kg at 40% inclusion rate in diets, with the main focus on the nutrient intake, in vivo dry matter digestibility and nitrogen retention. In the fourth and fifth experiments, the feed intake, growth performance, slaughter weights and carcass characteristics of Damara sheep lambs (15 males and 15 females) with an average initial weight of 16.7 ± 1.9 kg fed bush-based diets were evaluated. The results of the study indicated that most nutrients analysed were influenced (P < 0.0001) by season x species interaction. The crude protein (CP) contents was moderate (70 to 111 g/kg dry matter (DM)) except for T. sericea which was below 50 g/kg DM. The ash-free neutral detergent fibre (NDFom) and ash-free acid detergent fibre (ADFom) contents were high for all species (594 to 734 g/kg DM and 463 to 580g/kg DM, respectively), across all seasons. The concentration of acid detergent lignin of species ranged from 138 g/kg DM (R. trichotomum) to 223 g/kg DM (D. cinerea) but varied across seasons within species. A moderate proportion (50.9-56.7g/100g CP) of protein in R. trichotomum and S. mellifera was soluble, while the other species had a high proportion (> 70 g/100g CP) of their CP bound to ADF. The levels of condensed tannins (CT) was relatively low (< 55 g CT/kg DM) in all species and within the safe limits. All the bush species had low to moderate concentrations of minerals, while concentration of total amino acids and fatty acids was in the range of 39.4 to 77.7 g/kg DM and 1.17 to 2.84 g/kg DM, vii respectively. In vitro methane gas production of all fourspecies was higher (P < 0.001) during the late dry season compared to the early rainy season (147.6 versus 92.0 mL/g 138 DM). The in vitro organic matter digestibility of the species decreased (P < .001) from late dry to early rainy season, except for S. mellifera. Increase of indigestible neutral detergent fiber (P < .001) was observed from late dry to early rainy season for other species, except for S. mellifera which decreased. Dry matter and CP intakes of the control diet was higher than the bush-based diets. Digestibility coefficients of ≥ 0.70 were obtained on bush-based diets for all nutrients except for NDFom and ADFom which ranged from 0.40 to 0.60. Positive nitrogen retention of 45-58% of N intake was also achieved across diets. The bush-based diets supported average daily gains of up to 156 g/day, feed conversion ratio of 7.6 to 9.6 kg feed/kg weight gain, carcass weights of 11.4 to 12 kg and their carcasses characteristics were similar (P > 0.05) to the control diet. In conclusion, milled bush species could be considered of intermediate nutritional quality, despite their high fibre fraction, high proportion of their CP bound to ADF and indigestible NDF. Milled bush can replace traditional roughage sources such as grass and lucerne hay at 40% inclusion rate in properly formulated and balanced ruminant diets without adverse effects on the dry matter intake, digestibility, growth performance and carcass characteristics of growing lambs. However, there is still a need for further research to improve the utilisation of these bush species
Description
A dissertation submitted in fulfilment of the requirements for the Degree of Doctor of Philosophy in Agriculture (Animal Science)
Keywords
Bush encroachment, Damara sheep, Methane production