Early life health of breeding bulls
Across both beef and dairy sectors, genomic selection has increased the rate at which we can select young bulls that have superior genetics. High demand from farmers, for these high genetic merit, first-season bulls means that there is a high demand for their semen at a young age.
Recent research by Teagasc has highlighted that earlier puberty and thus earlier semen production can be achieved by improving the plane of nutrition in early life, specifically up to six months of age. Additionally, decreased growth rate during this period cannot be compensated by increasing the plane of nutrition after six months of age.
The period between three and five months of age is critically important for ensuring that necessary hormone changes occur to promote sexual development in bulls and promote normal testicular development of the bull. Many studies report that daily growth rates of >1.2kg/day are required to hasten sexual development in bulls, meaning that high intakes are required in early life.
Despite our knowledge on the effects of improved early life nutrition on sexual development in bulls, there has been little work on the effects of early life health and the potential that underlying infections have on reducing an animal’s reproductive potential; in some cases, the effects may not be realised until later in life.
A six-year observational study at a bull station in Sweden reported that 38% of the 970 bulls observed over the period required treatment at least once for respiratory infection before eight months of age.
Work from an AI station in Finland has reported that calves infected with bovine respiratory syncytial virus (BRSV) during their quarantine period between five and seven months of age had a 1.62% lower non-return rate at 14 months of age, which was associated with a 7.1% decrease in normal spermatozoa. In the study just mentioned, it is also worth noting that, at 14 months of age, there was no difference in semen motility between bulls that had BRSV and bulls that were healthy between five and seven months of age.
Recent work presented by Colin Byrne and David Kenny, Teagasc, at the International Ruminant Reproduction Symposium in Galway again highlights how minor infections in the early life of a bull calf may influence the testicular development of the bull.
The study examined how a short, seven-day infection with BRSV could influence the genes that determine how the development of cells in the testes of five-month-old, Holstein Friesian bulls calves. The effects on calf health are published in the journal, Scientific Reports and confirmed that there was sub-clinical infection in the infected calves.
In the testes, genes associated with connective tissue disorders and immune response were increased in the infected compared to non-infected calves, indicating that the infection was having an influence on the developing cells. Genes associated with development of the cells that are necessary for sperm production were decreased in the infected compared to non-infected calves.
This research indicates that respiratory disease in young calves leads to an immune response that may influence development of the testes. The studies also highlight that clinical symptoms of respiratory infection do not have to be observed in early life to have potential effects on fertility in later life.
Further research is required on whether the BSRV infection has long-term effects on testicular development and sperm production in the bull. This research highlights the importance of rearing bulls, destined to become AI sires or stock bulls, in high health status farms with uninterrupted growth in early life.
This article is by Colin Byrne, a Teagasc Research Officer, based in Teagasc Grange.