Browsing by Author "Theron, Pietert Gert"

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    Development of a model for crossbred lamb production
    (Stellenbosch : Stellenbosch University, 2021-12) Theron, Pietert Gert; Brand, T. S.; Cloete, Schalk W. P.; Van Zyl, Johan Hendrik Combrink ; Stellenbosch University. Faculty of AgriSciences. Dept. of Animal Sciences.
    ENGLISH ABSTRACT: Historically the sheep industry has played an important role in the South African agricultural landscape, but due to changing circumstances and growing challenges it may find its continued survival under threat. Producers will therefore need to find ways of improving production output and efficiency to meet these challenges. This study therefore aimed to collect production data of crossbred sheep in order to construct predictive models that could be used as part of a precision livestock farming (PLF) system by producers. Two common wool producing breeds, the Dohne Merino and Merino, were selected as dam lines for this experiment. A mixed flock of these ewes were divided into breed specific groups of 20 and each group then mated to rams of their own breeds or with Dorper, Dormer or Ile de France rams as the most important meat and terminal sire breeds. This resulted in two purebred control lines (Dohne Merino and Merino) and six crossbred trial lines (Dohne x Dorper, Dohne x Dormer, Dohne x Ile de France, Merino x Dorper, Merino x Dormer and Merino x Ile de France) being produced. Four rams and four ewes of each genotype were reared under optimal growth conditions until one year of age. During this time growth, fat deposition and intake data were collected. The data thus generated were assessed with various regression models to predict growth and fat deposition as well as feed intake. The Gompertz growth model provided the best fit for the growth data while a linear regression of age on fat depth was the most accurate in predicting fat deposition rate. An accurate model for cumulative feed intake over the growth period was also constructed. At the end of this period the lambs were shorn and wool traits of economic importance assessed. The remaining lambs were weaned at ~31 kg and reared on pastures with no supplementary feed being provided until they were deemed slaughter ready at 4 mm of backfat depth (A2 carcass classification). The growth of these lambs was also monitored and subsequently modelled, where the Logistic model provided the best fit. Upon achieving slaughter readiness, the lambs were slaughtered and selected slaughter and carcass traits recorded. Overall it was found that rams outperformed ewes for the production traits studied. This is due to sexual dimorphism benefiting rams for these traits. Crossbred lambs also had a significant advantage over their purebred contemporaries for all the traits except feed efficiency and wool quality. This was attributable to a combination of heterosis for these traits as well as breed dimorphism between the smaller dam and larger sire lines. The increased degree of breed dimorphism between Merinos and the sire breeds meant that the improvement in the performance of crossbred Merino lambs relative to the dam line was greater than the corresponding performance improvement in Dohne Merinos. It is concluded that crossbreeding offers a viable method of improving commercial production efficiency in South African Dohne Merino and Merino flocks. Producers who wish to implement a PLF system for a terminal crossbreeding enterprise can now do so using guidelines from this study. It is advised that further work be done on various crossbreeding options to expand the current local knowledge. More crosses could be included in future research while more complete models for pasture-rearing systems would also be of benefit to the industry.