Genetics creates potential, management realises the potential and disease can destroy it.
Amazingly, a bull’s mother’s nutrition at the time of conception can have an effect on the bull’s subsequent fertility. Unsurprisingly, insufficient energy in the ration can have a negative effect but so can too much protein. The age of onset of puberty (usually at around 42 weeks) is moderately heritable but is also strongly influenced by nutrition. There is also a genetic link between a bull’s scrotal circumference and heifer fertility so selecting for more fertile females will have a positive impact on any male progeny too.
Reducing the age of onset of puberty in bulls destined for AI stud will increase the rate of genetic gain, particularly when used alongside genomic technology so research into this area is very useful. Nutrition before 6 months of age has the biggest impact on age at puberty and sexual maturation – this is not correlated to body weight either so bigger bulls at any given age aren’t necessarily the most fertile.
Nutrition Of The Mature Bull
The take home message is to avoid major fluctuations or extremes of nutrition; getting bulls in ‘show condition’ for the sale ring then expecting them to run with a herd on a very different ration straight away is not good for fertility. Sadly, fit bulls that are ready to work will not currently achieve their true value so attitudes do need to change. Stock bulls fed a TMR dairy ration are also being overfed which can lead to degenerative joint disease and poor fertility. Acute rumen acidosis can also lead to sperm abnormalities for up to 90 days.
Ideally, stock bulls (for any system) should be bought at least 2 months in advance so their concentrate ration can be gradually reduced down and to get them used to the new ration. This also gives us time to blood test, vaccinate and worm etc before running with the cows.