The effect of frame size and stocking rate on diet selection of cattle and range condition in the Camelthorn savanna of eastçentral Namibia

Abstract

Abstract provided by author:

A long-term systems trial set up in 1984 to determine the optimal stocking rate of beef cattle in the camelthorn savanna of east-central Namibia was used to determine diet selection of free-ranging cattle and its effects on animal performance and rangeland condition during the hot-wet seasons of 2001, 2002 and 2003, the cold-dry seasons of 2001 and 2002 and the hot-dry season of 2002. Two cattle frame sizes (CFS), the small-framed purebred Sanga and the large-framed Afrikaner x Simmental crossbreed, and four systematically increasing stocking rates (SR), from low (15 kg cow mass/ha) to high (45 kg cow mass/ha), were combined in a 2 x 4 factorial design. Diet selection of cows was observed directly by bite-counting, replicated for cow, time of day, day and season. Dietary abundance of forage species was calculated and principal forages identified. Rangeland condition was determined by measuring canopy cover of the soil, botanical composition, total herbaceous yield and the tuft vitality of six indicator grasses in every treatment plot before grazing commenced. Dietary preference by the cattle was calculated for every forage species by comparing its dietary to its botanical abundance. The crude protein (CP), energy and mineral content and the digestibility of herbaceous forage was determined from randomly collected samples and compared to samples collected from each forage species individually by hand-plucking in a manner imitating the selectivity of cattle. Assumed dietary nutrient content was calculated from the dietary abundance and nutritive value of each forage species and related toanimal productivity. During statistical analysis using GLM procedures, data was pooled for CFS, SR and season

Of all treatments, SR had the largest effect on the diet selected by cattle and the condition of the rangeland. Season mostly affected the former, while CFS had little effect on either. The principal forage species were the perennial grasses Schmidtia pappophoroides, Anthephora pubescens and Eragrostis lehmanniana. Together, they contributed 59 percent , on average, to cattle diet, but their contribution increased to 74 percent of the diet of large-framed cattle at the lowest SR. These grass species were also preferred forages. Selection of different grass species was severely affected by SR (P 0.01). The utilization of forbs and woody plants increased at higher SR (P 0.01) and during drier seasons (P 0.01). Small-framed cattle were less dependent on the highly preferred grass species than large-framed cattle (P 0.01) and were better able to exploit the available forage resource. This may have contributed to the higher fertility (P 0.05) and systems productivity (P 0.01) of small-framed cattle compared to their large-framed contemporaries The botanical abundance and tuft vitality of the preferred grass species declined with increasing SR, while that of the less preferred grass species increased (P 0.01). Preferred grass species differed (P 0.01) in their tolerance of grazing and only S. pappophoroides was able to maintain a sizeable presence in the sward even at the highest SR. The botanical abundance of woody plants was highest at the highest SR (P 0.05), especially that of the known invasive species (P 0.01). The effects of season and CFS on rangeland condition were much smaller than that of SR, but it appeared that rangeland condition in plots occupied by small-framed cattle was slightly better than in plots occupied by large-framed cattle

The nutritive value of imitated forage samples was much higher (P 0.01) than that of random herbaceous samples, especially in CP content (difference of 71 percent), indicating that cattle were able to select a more nutritious diet than the average of the vegetation on offer. This ability decreased at higher SR (P 0.05) and during drier seasons (P 0.01), whereas CFS had little effect. The assumed dietary nutrient concentration appeared adequate, especially for small-framed cattle, except that the completely inadequate forage phosphorus content and grossly imbalanced dietary calcium : phosphorus ratio required supplementation

In conclusion, cattle could only pursue their dietary preferences at low SR and were forced to select previously less preferred forage species at high SR. This resulted in a less nutritious diet and reduced animal performance; more pronounced during drier seasons and in large-framed cattle. Shifting diet selectivity of the cattle induced changes in the species composition of the grass sward and the vitality of grass tufts at a SR of 25 kg cow mass/ha already. The threshold towards bush-encroachment was approached at a SR of 45 kg cow mass/ha and this SR should thus not be exceeded under any circumstances, to avoid severe rangeland degradation. The perennial grass S. pappophoroides was a good indicator of the change in rangeland condition brought about by foraging cattle in the camelthorn savanna of Namibia. It is recommended that stockbreeders pursuing optimum individual animal production should limit their stocking rate to 25 kg cow mass/ha, whereas this can be increased to 45 kg cow mass/ha if the aim is to optimise beef production on the ranch while maintaining an acceptable rangeland condition