Feeding behaviour, plant species selection and in sacco ruminal digestion in Tswana and Boer goats

Abstract

The first experiment of this study was aimed at investigating the factors affecting the feeding behaviour and plant species selection of 3 free ranging Tswana (mean weight ± 37 kg) and 3 Boer (mean weight ± 36 kg) goat does in the False Thornveld of the Eastern Cape Province, South Africa. The feeding behaviour and plant species selection was observed and recorded during winter (cold-dry season) and summer (hot-wet season). Observations included the following categories: browsing, grazing and non-feeding activities. Non-feeding activities (NFA) include separate recordings of walking, standing, lying and drinking. Results showed goats spent 21.9 %, 44.65 % and 33.3 % of their day on browsing, grazing and non-feeding activities. Grass was clearly the preferred feed for goats both in the cold-dry season (54 %) and the hot-wet season (76 %) while browse consumption was higher in the cold-dry season (46 %) and lower in the hot-wet season (24 %). Goats spent more time on NFA in the cold-dry season (68.8 %) than the hot-wet season (31.9 %). Though diurnal variations were observed for both breeds and seasons in terms of browsing and non-feeding activities, no diurnal variations were found between the breeds. Goats selected a wide spectrum of woody-plant species. A total of thirteen woody plant species were encountered and selected by goats in the range. Scutia myrtina was the most frequently consumed (29.9 %) woody plant species in the range followed by Rhus refracta (16.3 %), Grewia occidentalis (14.0 %), Olea africana (10.8 %). These four woody-plant species accounted for 71 % of the woody plant selection by goats. Significant differences (P<0.001) were found in percentage time spent selecting the different woody-plant species by goats. The frequency of woody-plant species selection was higher in the cold-dry season than the hot-wet season. Woodyplant selection was higher in the morning followed by the afternoon and lower during midday. Percentage time spent by goats selecting woody plant species decreased with advancing period of occupation of a grazing/browsing camp. Browse was the preferred feed when feed availability was high, but when feed availability declined, the goats decreased their frequency of woody-plant selection and more grass was eaten. The second experiment was aimed at determining and comparing the nutritive value of forages selected by the same free ranging goats. Three (3) Tswana and 3 Boer goats equipped with remote controlled fistula valves were studied. Fistula valve samples were collected for 15 minutes during the day at 6 different times. All extrusa samples were kept on ice in the field and then stored in a deep freeze prior to drying and milling. Each sample was freeze-dried and hand separated into bush (woody species) and grass components (including forbs). These samples were analysed for nitrogen (N), neutral detergent fibre (NDF), acid detergent fibre (ADF) and acid detergent lignin (ADL). The diets selected by the goats differed in CP (cold-dry season: 5.39 % ± 0.15 and hot-wet season 9.48 % ± 0.15; P=0.001), NDF (cold-dry season 48.09 % ± 1.02 and hot-wet season 53.76 % ± 1.02; P=0.001) and ADF (cold-dry season 34.32 % ± 0.72 and hot-wet season 37.50 % ± 0.72) between the seasons, but the ADF content showed no change (cold-dry season 18.01 % ± 0.52 and hot-wet season 19.49 % ± 0.52 P>0.05). Goats selected diets higher in CP, NDF and ADF during hot-wet season compared to cold-dry season. Generally, there were diurnal variations in the NDF (P=0.04), ADF (P=0.02) and ADL (P=0.01) contents of forages selected by goats. Nutrient content of selected diets decreased in quality with advancing period of occupation of a camp. Knowledge of the nutritive value of the diet as selected by these animals is of primary importance in estimating the productivity of any rangeland. The third experiment was aimed at determining in sacco dry matter (DM) and crude protein degradation of woody plant species incubated in the rumen of Tswana and Boer goats. The ruminal degradability of the three most selected woody plant species by goats during the cold-dry season (G. occodentalis, O. africana and S. myrtina) and the hot-wet season (A. karroo, R. refracta and S. myrtina), were measured in situ, using in sacco digestion techniques. The nylon bag incubation revealed large differences in the rumen degradability of DM of the three woody plant species during both cold-dry season and hot-wet season. Ruminal degradation constants (a, b and a+b and c) varied significantly (P<0.001) between breeds and between goats within a breed. Thus, no breed difference in in sacco degradation of DM and CP was found between Tswana and Boer goats. There was a strong positive correlation between DMD, CPD and time of incubation in the rumen. The crude protein content of the woody plant species positively influenced the rumen digestion of dry matter because when CP increased DMD also increased. Both species preference and degradability of plant species are needed to estimate range suitability for browsing animals. Results from this study will help to develop strategies to optimise range resources for sustainable animal production. In order to improve production, the constraints limiting success should be identified. Tswana goats might be a more favoured breed in semi-arid savannah because of their smaller body size, absolute nutrient requirement and better reproductive performance when compared to Boer goats. However, when forage availability is not a limiting factor, Boer goats might yield better returns for the investment since they are superior in meat production.