Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1999.Data collected between 1992/93 and 1996/97 from two long-term grazing trials were
used to investigate the interaction between grazing animals and veld grass. In the first
trial, the impacts of stocking rate and time of stocking in spring on both livestock
performance and veld vigour (defined as the ability of a grass plant to regrow after
defoliation) and condition were investigated. In the second trial comparisons were
made, firstly between the impacts of sheep and cattle grazing, and secondly between
various types and frequencies of rest, on veld vigour and condition.
Treatments applied in the first trial comprised four stocking rates, namely 7, 10, 13
and 16 sheep ha¯¹ for the duration of the grazing season, and two times of stocking,
namely as early as possible after spring burning and three weeks later. Sheep grazed
each treatment continuously throughout the growing season. Treatments were applied
to alternate blocks in a two-year cycle with each block resting for a year within a
grazing cycle. Animal performance (mass gains over the season) was measured to
quantify livestock performance. Herbage availability was measured on a species basis
at intervals throughout each season using a dry-weight-rank procedure to determine
grazing patterns. Residual effects of the grazing treatments on vigour were determined
by measuring herbage regrowth on a species basis during the rest season which
followed a season of grazing and comparing these measures to a previously ungrazed
control treatment. Effects of the grazing treatment on proportional species
composition were determined using a nearest plant point technique. Stocking rate had a non-linear effect on livestock performance, with livestock
performance on the lightest stocking rate being less than on the two intermediate
stocking rates. The mass gains on the heaviest stocking rate were generally the
smallest. Delaying the time of stocking in spring resulted in smaller mass gains during
the resultant shorter season. The sheep from both the early and late time of stocking
groups had similar mean masses at the end of the season. The advantage of stocking
early can thus be attributed more to saving the cost of alternative feed for the interim
period than to additional mass gains due to stocking early. Quantification of livestock
performance in terms of selected and available feed quality, quantity and species
availability throughout each season was extremely complex due to multiple thresholds
in the measured variables and no simple cause and effect relations could be
established that would hold for spatial or temporal extrapolation.
The negative impact of grazing on veld vigour was severe. Stocking rate and time of
stocking had a secondary impact with the vigour loss positively related to increasing
grazing pressure. The main factor influencing vigour loss was grazing, irrespective of
time of stocking or stocking rate, as opposed to no grazing. The impact of grazing on
vigour was severely negative in the palatable species, variable in the species of
intermediate palatability and positive in the unpalatable species that were rarely, if
ever, grazed. The stocking rate and time of stocking rate had an impact on the
proportional species composition, with the more palatable species declining in
proportion. There was an observable relation between impact of grazing on vigour and
on species composition. Treatments applied in the second trial involved applying a full growing season rest in
alternate years, half a growing season rest (late season) in alternate years and no rest
to veld grazed by sheep or cattle at similar stocking rates. Residual effects of the
treatments on veld vigour were determined by measuring species regrowth using a
dry-weight-rank technique during the season following treatment application, and
comparing it to controls ungrazed for one and two seasons respectively. Changes in
proportional species composition were determined using a nearest plant point technique.
The vigour of veld grazed by sheep declined rapidly relative to veld grazed by cattle.
The vigour of palatable species was severely impacted, vigour of intermediate species
was variably impacted and vigour of unpalatable species increased dramatically on
veld grazed by sheep compared to the control treatments. Similar trends occurred in
veld grazed by cattle, but to a lesser degree. Resting was beneficial for vigour
recovery in both sheep and cattle treatments but it seems that the grazing treatment
between rests has a greater influence on the veld vigour and condition than the rest
itself. The veld grazed by sheep remained at a substantially lower productivity level
than veld grazed by cattle. This was particularly evident in the change in productivity
balance between palatable and unpalatable species in the sheep treatments, where
palatable species vigour declined and unpalatable species vigour increased relative to
veld grazing by cattle. Species composition of veld grazed by sheep deteriorated over
the trial period in contrast to the veld grazed by cattle, which improved in species
composition. Grazing management recommendations for sourveld should include a bias towards
cattle, optimising stocking rate for improved performance and resting for enhancing vigour of the palatable grasses
