|dc.description.abstract||Data derived from a long-term grazing trial were used to
determine short-term effects of mixed grazing by cattle and sheep
in Highland Sourveld.
Five cattle to sheep ratio treatments (viz. 1 :0, 3:1, 1 :1, 1 :3,
0:1) were applied, each at three stocking rates (viz. 0.5 (low),
0.71 (medium) and 1.0 (high) animal unit equivalents (AUE) ha¯¹).
Ratios were expressed in terms of AUE cattle: AUE sheep. Stocking
rate and ratio treatments were balanced at the start of each
grazing season. Fourteen-month old Hereford steers and 'two-tooth'
Merino wethers were used as experimental animals.
The trial comprised two components, viz. an animal production
component and a simulated component. The animal production
component was conducted only in the medium stocking rate
treatment where the effect cattle to sheep ratio on the
performance of cattle and sheep was determined for each of four
grazing seasons (viz. 1989/90,
four-paddock rotational grazing
The low and high stocking rate
1990/91, 1991/92, 1992/93). A
management system was applied.
treatments were implemented by
simulating a four-paddock rotational grazing system and using a
single paddock for each stocking rate/ratio combination. Grazing
of the simulated treatments coincided with the grazing of a
specific 'test' paddock in each ratio of the medium stocking rate
treatment. These 'test' paddocks and the simulated treatment
paddocks were monitored to determine the impact of mixed grazing
by cattle and sheep on individual grass plants and the sward.
The whole of the experimental area was rested during the growing
season prior to the start of the trial and all paddocks were
burnt in the dormant season just before the start of the first
grazing season. The trial therefore commenced with a with a
uniform sward of immature herbage in all paddocks allocated to
each group of animals. ln the second and subsequent growing
seasons only those paddocks which had been rested in the previous season were burned prior to the onset of growth in early spring.
Rainfall for the first three grazing seasons was similar to the
long-term mean of 790mm whilst the last season was considered
'dry' with 554mm recorded during 1992/93.
In all grazing seasons, as the proportion of cattle in the
species mix increased, sheep performance increased. A decline in
sheep performance was recorded in each ratio treatment from the
first to the third season. This decline was attributed to the
increased maturity and thus lower quality of herbage on offer to
the sheep, and the fact that only one paddock available to the
animals in the second and third grazing seasons had been burned
prior to the start of the season. Sheep performed best during
the 'dry' season where herbage quality was maintained for longer
into the grazing season than in previous seasons. In contrast,
cattle performance was affected by the stocking rate (animals
ha¯¹) of cattle rather than the presence of sheep. As the
quantity of herbage on offer per steer declined steer performance
Animal performance data were also used to predict the effects of
adding cattle to a sheep production enterprise and vice versa.
The general trends were that the introduction of cattle into a
low stocking rate, sheep-only production enterprise would allow
for an increase in the stocking rate of sheep while maintaining
the performance of the sheep. In this way the carrying capacity
of a farm may be improved.
Stocki ng rate and ratio treatments varied from those established
at the start of each grazing season due to the differential
performance of the cattle and sheep in each treatment. As the
proportion of cattle in the species mix increased, stocking rate
increased and the ratio widened in favour of cattle. Stocking
rate (AUE ha¯¹), calculated a posteriori for each season, was the
major influence on the severity of grazing on individual plants
and within patches. As stocking rate of cattle and sheep at the
various ratios increased, the extent and severity of grazing increased. At stocking rates in excess of O.8AUE ha¯¹ however,
sheep-only grazing resulted in a greater proportion of plants,
per species and per area, being grazed more severely than was the
case for an equivalent stocking rate (AUE ha¯¹
) of cattle.
Furthermore, cattle and sheep had similar effects on patch size
distribution when stocked at the same number of AUE ha¯¹.
There were no measurable effects of stocking rate and ratio on
proportional species composition and basal cover over a two year
monitoring period. A technique for estimating basal cover in
tufted grasslands was developed and is presented as an appendix
to the thesis.
Data were also used to evaluate the use of AUE as an integral
part of the grazing capaci ty concept. Results indicated that
cattle and sheep cannot be equated in terms of AUE when referring
to the grazing impact. It is suggested that the definition of
grazing capacity should include the species of livestock and
assume a grazing management system appropriate to the grazing
habit of the animals concerned.
Resul ts of the trial provide strong indications that, in the
long-term, the current recommendations of grazing cattle together
with sheep in order to prevent the degradation or loss of veld
condition which occurs in sheep-only grazing systems, will not
succeed. A four-paddock rotation grazing system does not appear
to be an appropriate veld management system for sustainable sheep
production in sour grassveld. An alternative approach to veld
management is suggested in which the sheep are confined to only
those areas of the farm which were burnt at the start of the
grazing season. Ideally, sheep should not allocated to the same
paddock for two consecutive grazing seasons.||en