Large herbivore stocking rate effects on plant palatability, forage preference and soil properties in an Alluvium Fynbos-Renosterveld mosaic

[摘要] ENGLISH ABSTRACT: An understanding of the interactions of herbivores, plant nutrients and soil properties iscrucial for grazing management. Of particular interest are plant palatability and herbivoregrazing preferences. These aspects, the focus of this thesis, were studied in a Swartland ShaleRenosterveld and Swartland Alluvium Fynbos mosaic in the Western Cape of South Africa.This vegetation is classified as critically endangered due to the transformation of its landscapein favour of agricultural production and urban settlement.Pre-colonial herbivore vertebrates were hunted out by 1700, leading to a sparse knowledge ofthe early dynamics of the vegetation. However, currently game farmers and landowners arere-introducing selected species into the area. Thus, managers require knowledge on how toimplement practical guidelines for best-practice grazing management. This study investigates,firstly, the relationship between plants and animals. This included seasonal assessment of seedgermination from dung of bontebok and eland; the effect of stocking rates on plantpalatability (crude protein, crude fibre, crude fat, moisture, tannin and mineral) of herbagewas studied with the aim to understand how palatability influences herbivore forage decisions.Secondly, this study looked at how stocking rates influence soil nutrients (nitrogen,phosphorus), and others soil properties such as pH and soil moisture.The distribution of alien grasses was facilitated by grazers. About 58 % of grasses germinatedfrom the dung of eland and Bontebok were alien species. Additionally, there was significantvariation in palatability related to grazing pressure and plant maturity between sites and overseasons. Grasses on the high grazing site had higher carbohydrate (3%-5% higher) and proteincontent (1%-5% higher) than at the least grazed site. Mature grasses contained less water(10%-20%), but no obvious variation in tannin concentration than immature grasses.However, species such as Ficinia sp. showed variations with grazing pressure and maturity.Ficinia sp. on the high grazing site had higher tannin concentration (5mg/ml-22mg/ml) andcrude protein (4%-9%) than at the least grazed site. When the plant ages, tannin concentrationincrease (ranging from 15% to19% higher). Two types of plant were recoreded according totheir chemical response following grazing pressure. These types are 1) plants that are grazingtolerant and which produce more carbohydrate when grazed and 2) plants that are grazingintolerant under high grazing pressure and which produce chemical defence compounds suchas tannin to deter herbivores attacks. The results indicate that in fynbos and renosterveld, therelationship between preference and plant chemical compounds is not consistent since noIVpatterns emerged to explain what compounds drive preference. A combination of chemicalcompounds may be the reason for the selection by the grazers; alternatively, other compoundsnot included in the study may influence the forage selection by an herbivore.The second part of the study showed that stocking rates appeared to have a significant effecton soil properties investigated. Soil moisture was significantly affected by the stocking rate inautumn. The high grazing intensity site had the lowest soil moisture especially in autumn(10% lower than the control site), likely due to heavy trampling and soil compaction. pH wasthe lowest at the high grazing intensity site in all seasons compared to the control site. Likelyreasons were the high deposition of nitrogen through dung and urine deposition, high removalof basic cations and animal exportation through hunting activities. Lastly, grazing maintainsinorganic nitrogen at stable levels regardless of seasonal changes and increases theconcentration of phosphorus especially under high grazing pressure compare to less grazedsites.The grazing dynamics of Swartland Shale Renosterveld - Swartland Alluvium Fynbos mosaicsystem are complex and multifaceted. Farmers need to pay attention to the nutritional status ofplant species grazed to know whether they meet the nutritional requirements of the game inthe area. Overstocking should be avoided in this system as it triggers the production oftanniferous compounds that could decrease the survivorship of herbivores. Moderate grazing0.09 LAU/ha pressure provides the best stocking rates for effective grazing management.

[发布机构] Stellenbosch University