Patterns of plant selection by grazing cattle in two savanna grasslands: A plant's eye view

Abstract

The amount of grazing experienced by individual tufts of the most abundant perennial grasses of two Transvaal savannas was monitored at regular intervals for two growing seasons. In general, Setaria incrassata and Themeda triandra were heavily grazed, Bothriochloa insculpla and Aristida bipartita were lightly grazed, and Heteropogon conlortus and Digitaria eriantha were intermediate. Tufts with a greater amount of moribund material were less heavily grazed. The smallest tufts of all species were the most lightly grazed, although the most heavily‐grazed tufts were on occasion intermediate in size rather than being the largest. A tuft had an increased likelihood of being heavily grazed if it had previously been heavily grazed, both within a growing season and for two successive growing seasons. A spatial pattern of grazing was evident, as individual species were less heavily grazed at different locations within the paddocks. The influence of species identity on the amount a tuft was grazed was not absolute but rather quantitative and conditional on the level of other variables. Tuft size and the amount a tuft had been previously grazed generally had a greater influence on the amount a tuft was grazed during an individual grazing event than species identity, location or moribund material. Species selection is possibly, in part, a consequence of selection for plant structure which optimizes intake.

Die hoeveelheid van beweiding wat ervaar is deur alleenstaande polle van die volopste spesies in twee Transvaal savannes, is gereeld gedurende twee groeiseisoene gemonitor. Oor die algemeen, was die beweiding van Setaria incrassata en Themeda triandra swaar, dié van Bothriochloa insculpla en Aristida bipartita was lig, en dié van Heteropogon contortus en Digitaria eriantha matig. Polle met ‘n groter hoeveelheid sterwende materiaal is minder bewei. Die kleinste polle van alle spesies is die ligste bewei, maar dit was nie noodwendig die grootste polle wat die swaarste bewei is nie. Daar was ‘n groter moontlikheid dat ‘n pol swaar bewei sou word indien dit alreeds tevore swaar bewei was. Hierdie verskynsel het binne ‘n spesifieke groeiseisoen gegeld asook oor twee agtereenvolgende groeiseisoene. Daar was ‘n ruimtelike patroon van beweiding, weens die feit dat individuele spesies nie so swaar bewei is op verskillende plekke in die kampe nie. Die invloed van spesies is voorwaardelik op die vlak van die ander faktore gebaseer. Die hoeveelheid van ‘n pol wat bewei is op ‘n gegewe tydstip was tot ‘n groter mate beïnvloed deur die polgrootte en die hoeveelheid vorige beweiding, as deur die spesies, die hoeveelheid sterwende materiaal, of groei‐plek. Dit mag wees dat spesieseleksie slegs ‘n newe‐effek is van seleksie vir polsttuktuur, wat inname optimiseer.

Additional index words:

• Moribund material
• patch grazing
• plant structure
• species selection
• tuft size