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University of Pretoria (2016)

Effect of different nitrogen sources on dry matter intake and digestibility of a low-quality roughage fed to sheep in the subtropical grasslands of South Africa

Hendriks, Karla

Titre : Effect of different nitrogen sources on dry matter intake and digestibility of a low-quality roughage fed to sheep in the subtropical grasslands of South Africa

Auteur : Hendriks, Karla

Université de soutenance : University of Pretoria

Grade : Master of Science (MS) 2016

Résumé partiel
Rainfall variability in and between seasons, lower soil organic matter content, land degradation, and the associated low biomass yield and poor quality of forage are major constraints to livestock production in the subtropical grasslands of South Africa. In this study, the influences of changes in precipitation, grazing management and other land-use types on carbon (C) and nitrogen (N) sequestration potential, herbage yield and nutritive quality were investigated by collecting data from i) the long-term grazing trial site at Grootfontein Agricultural Development Institute (GADI), an arid grassland in Middleburg, Eastern Cape ; ii) various land-use types in a semi-arid area, Pretoria ; and iii) short-term experimental plots set up in semi-arid grassland in Pretoria, South Africa. In the first study, various land-use types (Leucaena plots, exclosures, cultivated pasture and croplands) were compared for soil carbon (C) and nitrogen (N) storage. Leucaena plots and exclosured lands improved C and N stocks, due mainly to high biomass returns to the soil, while frequently cultivated pasture and croplands had generally lower C and N stocks. Although soil bulk density and soil chemical parameters such as C, N, calcium (Ca), potassium (K), magnesium (Mg), sodium (Na), and pH were affected by land-use type, it was only the C and N stocks that showed significant land-use by soil depth interaction. This indicates that C and N are more sensitive to frequent soil disturbances and cultivation, compared with other soil properties, which were less affected by soil depth. In the second study, the long-term impacts of grazing management (namely stocking rate and season of use) on selected soil properties were investigated by collecting soil samples from the long-term grazing experimental site at GADI, Middelburg, Eastern Cape, South Africa. Findings from this study indicated that in this arid region of the country, grazing at contrasting stocking rates (light and heavy) resulted in lower soil organic C and total N concentration compared with the non-grazed control (exclosure). Similarly, both spring and summer grazing resulted in reduced soil C and N stocks, lower water infiltration rate, and higher soil compaction compared with the non-grazed control, due mainly to plant removal from animals feeding on the forage, and animal treading and trampling. Generally, animal exclusion significantly improved C and N stocks and, among others, resulted in higher water infiltration and lower soil compaction. Improved land and livestock management through the establishment of multipurpose tree species and strategic animal exclusion would improve C and N storage in vast potentially restorable grazing areas of South Africa. The last experiment was designed to study the effects of different levels of precipitation (simulated drought) and defoliation intervals on SWC, herbage yield and quality. The study was conducted for two years at Hatfield Experimental Farm, University of Pretoria, using rainout shelters to intercept 0%, 15%, 30% and 60% of ambient rainfall as the main plot treatment and two defoliation intervals (45-day and 60-day) as sub-plot treatments. Both rain interception (RI) treatments and defoliation intervals influenced SWC, herbage yield and quality. Rainfall amount and distribution varied remarkably within and between years. This variation was confirmed by the negative climatic water balance (precipitation-evapotranspiration, PPT-ET) in most months during the study period, suggesting that the trial site was moisture stressed mainly during the active plant-growing period owing to high atmospheric water demand.

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Page publiée le 14 août 2016, mise à jour le 17 juillet 2017