Studies on the genetic parameter estimates and trends to reduce reproductive wastage in the Tygerhoek Merino sheep resource flock
Date
2024-12
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
Reproductive wastage has been and continues to be a major constraint to production in the South African small stock industry. Reproductive wastage is a large contributor to income loss and raises welfare concerns for the sheep industry worldwide. This study evaluated different methods to minimize reproductive wastage by estimating phenotypic and genetic parameters, as well as analyzing genetic trends for ewe reproduction and lamb survival in a well recorded Merino resource flock. Furthermore, the study also considered the effect of the ambient environment on ewe conception to curb reproductive wastage. The Tygerhoek Merino resource flock has detailed records for ewe reproduction and lamb survival trait complexes. The research farm is also close to an official weather station, which allows for long-term climate data to construct a temperature-humidity index (THI) as a descriptor of the ambient environment. This placed the flock in a unique position to allow for a comprehensive analysis on strategies to minimize reproductive wastage. The first aim of the study was to conduct an extensive analysis on the composite trait; number of lambs weaned (NLW) and its individual components (conception (CON), ewe rearing ability (ERA) and litter size (LS)). The genetic relationships of NLW with the components were examined and yielded no unfavorable correlations. It also suggested that component traits (CON, ERA and LS) should be emphasized to increase reproductive efficiency instead of only focusing on NLW. The study then focused on the effect of the ambient environment on CON. Heat stress has had a negative effect on CON in dairy cattle, however, similar studies have not been repeated in sheep. Hence, this study aimed at being one of the first studies to consider the THI and its effect on CON in Merino ewes. Increased levels of heat stress, as reflected by an increased THI, exhibited a clear negative effect of heat and a more prominent effect of accumulative heat on the CON of ewes. Apart from ewe reproduction, another comprehensive genetic analysis was conducted on the early stages of lamb survival. This analysis considered the traits lamb survival at birth and survival until 3 days of age. The aim of this was to minimize reproductive wastage by targeting survival as a trait of the lamb, rather than the more common approach of concentrating on ewe reproductive traits. The thesis conformed with previous literature by reporting low heritability estimates for both ewe reproduction and lamb survival traits. However, despite low heritability estimates, genetic gain can be achieved over numerous generations. It is recommended that reproductive efficiency must be included in selection objectives and, where possible, the early stages of lamb survival must also be considered to minimize reproductive wastage. Derived genetic trends reflected slow change (<0.2% of the overall mean per annum), generally in the desired direction. These results were expected as the experimental protocol followed precluded directed selection for reproduction or lamb survival. These results conformed to previous studies suggesting that reproductive efficiency and lamb survival can be increased while still maintaining a high standard of wool production. It also identified keyways to minimize reproductive wastage through concentrating on traits of the ewe as well as of the lamb.
Description
Thesis (MScAgric)--Stellenbosch University, 2024.