|dc.description.abstract||South Africa is still a net importer of beef. Therefore, by increasing off take in the beef sector,
South Africa can move towards self-sufficiency. With fertility being regarded as one of the
main components influencing total beef herd efficiency, it is essential that the quoted calving
percentage of 62% in the commercial beef sector of South Africa must be improved. If the
long calving seasons can be shortened and the calving percentage increased, more and heavier
calves with a more uniform age can be weaned. Cows that calve early also have a better chance
of conceiving in the next breeding season and are generally seen as the more fertile animals
Development, production and quality of replacement heifers is a crucial component in the
extensive beef production system. In general, beef heifers are managed to calve for the first
time at three years of age, but in some cases mating of heifers at one year of age have been
All extensive beef production systems in South Africa are dependent on natural veld and it is
well documented that veld condition have a huge influence on a number of beef production
parameters. Studies conducted on natural veld have concentrated mainly on aspects that affect
herd efficiency, including calving percentage, pre-weaning growth and supplementation of
cows and calves. However, none of the studies focused on the reproduction performance of
beef cattle mated naturally after synchronization, heifer age at breeding and effect of grazing
system on veld condition.
The aim of the study was to evaluate: the effect of estrous synchronization followed by natural
mating on the calving percentage and calving distribution of multiparous beef cows and heifers;
effect of breeding heifers at either 14 months or 26 months of age and the evaluation of a high
utilized grazing system and controlled selective grazing on veld condition and animal
performance. The effects of climate on cow-calf production characteristics over time was also
The study was conducted from 2009 to 2015 at the Roodeplaat experimental farm (REF) of the
ARC-Animal Production Institute (25°34’11.27’’S; 28°22’05.36’’E) on 900 ha of natural
rangeland described as Sourish Mixed Bushveld. The experimental herd (n=92) was divided
in four sub-herds consisting of 23 cows each at the beginning of the project in 2009. It was ensured that the four sub-herds were as uniform as possible at the beginning of the project e.g.
age, weight, previous number of calves.
Within each sub-herd, 50% of the cows and heifers were synchronized prior to the
commencement of the breeding season. Two sub-herds were subjected to high utilized grazing
and two sub-herds were subjected to controlled selective grazing. The two grazing systems
were related to the use of 30% or 60% of the available grass dry matter. Half the heifers were
mated at 14 months and the other half at 26 months.
Results from this study indicated that calving percentage and body condition score did not
differ significantly (P=0.54) between cows that was either synchronized or not synchronized
followed by natural mating. However, estrous synchronization prior to natural mating did
influence the average days to conception with synchronized cows calving earlier, except for
2012 in the calving season. Over the six-year project period 15% more cows from the
synchronized group conceived within 293 days after the onset of the breeding season. Calves
from the synchronized cows weaned on average 5kg heavier than the cows that were not
synchronized although this difference was not significant.
Conception rates of heifers mated at 26 months were significantly (P<0.05) higher than heifers
mated at 14 months of age. It would seem that it may be more viable to breed Bonsmara heifers
in an extensive production system in the Sourish Mixed Bushveld region at 26 months of age
for the first time. Synchronization of 14 month old heifers did not improve conception rate
over 14 month old heifers bred naturally. However, the calving percentage of synchronized
heifers bred at 26 months was 6% higher than the non-synchronized heifers.
Almost no veld condition change was recorded except for veld condition scores for both
controlled selective grazing and high utilization grazing. In addition, the results indicate a
tendency that high utilization grazing improved veld condition score and grass species
composition over that of controlled selective grazing, but the duration of the study is too short
to make a definite conclusion on the effect of grazing strategy on veld condition.
It was also shown that grazing strategy did not have a significant influence on cow weight and
calf growth over the six-year period, indicating that both grazing strategies are sustainable in
the Sourish Mixed Bushveld if carrying capacity is adhered to.
With the significant differences between years (P ≤ 0.05) for calving percentage, cow weight
at calving, cow weight at weaning, calf birth weight, calf weaning weight and body condition
score over the six-year observation period, the effect of seasonal temperature, relative humidity
and rainfall is elucidated. Forward stepwise regression procedures were performed to
determine what climatic data were involved in cow and calf weight at birth and weaning as
well as calving percentage. In spite of the high standard errors (which were probably due to
the small sample size), maximum relative humidity one month prior to the start of the breeding
season, made a major contribution to explain calving percentage and minimum temperature
within the last month of the 3 month breeding season, had a low negative correlation with
calving percentage. It can be speculated that high humidity in the study region (Sourish Mixed
Bushveld) is an indication of warm and wet conditions, negatively impacting cow and bull
comfort, leading to lower conception rates. The negative correlation between minimum
temperature within the last month of the breeding season and calving percentage may indicate
that the cows were unable to cool down at night during the warmer summer months of the year,
leading to lower conception rates and resorptions.
The researcher acknowledge that the available herd size may be a limitation and that a bigger
herd or sub-herds’ size combined with bigger land size could benefit the project outcome,
possibly resulting in more significant differences and/or enhanced interpretation of results.||en_ZA