Investigation into the relationship between scrotal circumference, body weight, semen characteristics, daughter fertility and genomic breeding values, as well as monitoring behaviour in commercial pasture-based dairy breed natural-service sires

Abstract

Use of natural-service sires in the pasture-based dairy industry of south-eastern Australia is common, however, research into their selection and management is sparse. This investigation included a cross-sectional study of the genetic merit of pasture-raised natural-service dairy breed sires and quantified the association between natural-service sire scrotal circumference and their daughter fertility breeding value. Additionally, investigations into the association within breeds between scrotal circumference (as an explanatory variable) and daughter fertility breeding value as the outcome variable in a linear regression model were conducted. And finally, this investigation aimed to monitor bull behaviour, including mounting and serving behaviour, as well as tracking distanced walked.

Deoxyribose nucleic acid was submitted for genotyping from two groups of Tasmanian Holstein (n=124) and Jersey (n=85) bull calves to get a set of genomic breeding values. Scrotal circumference and body weight measurements and semen characteristics were recorded at 8 weekly intervals from the age of 6 months to 18 months of age. In addition, an observational study was conducted using collar mounted tri-axial accelerometers and global positioning systems on bulls (n=10) on a commercial pasture-based dairy farm in Tasmania, Australia.

The genomic information from natural-service sires was compared to contemporary artificial insemination sires, however a clearly defined association between scrotal circumference and the daughter fertility breeding value could not be elucidated, despite the link in beef breeds. Linear regression quantified the relationship between scrotal circumference and body weight and classification and regression tree analyses were determined the predictive value of scrotal circumference and body weight on semen characteristics. Linear regression showed that 50 kg increase in a Holstein bull’s body weight was associated with a 2.9 (95% CI 2.8 to 3.0) centimetre change in scrotal circumference (P= <0.001). For Jerseys, 50 kg increase in body weight was associated with a 2.4 (95% CI 2.3 to 2.5) centimetre change in scrotal circumference (P= <0.001). Classification and regression tree analysis for Holsteins and Jerseys combined showed that 95% of animals that weighed greater had a scrotal circumference of at least 27 cm had a percent normal sperm score greater than 70%. Using learning algorithms bull behaviour was monitored including grazing (F = 0.86) and walking (F = 0.91) compared with ruminating (F = 0.19) and resting (F = 0.38). However, due to the short, yet explosive nature of the mounting and serving signatures, the algorithm was not able to detect them. Mean distance travelled per day ranged from 12.0km to 6.5km and maximum distance travelled per day of 28.0km to the least maximum distance of 14.6km.

This study provides information about the genomic merit of natural-service sires and the relationship between scrotal circumference and daughter fertility, augments information on scrotal circumference and body weight relationships in pasture-raised Holstein and Jersey breed bulls and supports the use of the bull breeding soundness exam to manage the risk for sub-fertility in the dairy industry. Additionally, it was found that walking long distances may contribute to the lameness in dairy natural-service sires.