Barth AD, Brito LFC, Kastelic JP. The effect of nutrition on sexual development of bulls. Theriogenology 70. 2008; 485-494. Barth AD, Brito LFC, Kastelic JP. The effect of nutrition on sexual development of bulls. Theriogenology 70. 2008; 485-494.
Why they did it
It has been shown that over feeding of weaned bull calves results in an increased scrotal circumference (SCC) as yearlings, however this increased SCC is often attributed to a increased scrotal fat and not superior fertility. Bull calves born to heifers often have a smaller SCC as yearlings and may have delayed onset of puberty. As the trend in North America to use bulls younger than 2 years at a stocking rate of 1:25 becomes more popular due to the cost of raising and maintaining bulls, the nutrition in the early part of their life before weaning becomes pivotal in ensuring the production of a well grown out bull for both introduction into the breeding herd and preparation for sale. For this reason they designed a series of experiments to test the hypothesis that calfhood nutrition affects sexual development in bulls.
What they did
Due to the fact that calves are most commonly weaned from their dams at 26 weeks :
In experiment 1: 40 calves received a medium energy diet from 8-26weeks after which they either received a low, medium or high energy diet until 70 weeks.
In experiment 2: 37 calves received a low, medium or high diets from 8 weeks all the way through to 70 weeks.
In experiment 3: 44 calves were fed low energy diets from 8 – 26 weeks after which half were then fed a medium energy ration and the other half a high energy ration until 70 weeks.
In experiment 4: 33 calves received a high energy ration from 8-30 weeks and then a medium ration until 70 weeks.
What they found
Bulls that had a higher plane of nutrition post weaning showed no differences in age of puberty. Those calves on a higher energy diet pre weaning however showed an accelerated age at puberty, they were also heavier and with a larger SCC than those calves fed a low energy diet pre weaning irrespective of the diet fed to them post weaning. Restricted nutrition in calfhood affected steroidogenesis negatively.
Take home message
Reproductive function in bulls can be maximised by providing high nutrition during pre weaning calfhood. This appears to be more important than the post weaning phase where there is little difference shown between bulls raised on a moderate or high nutrition plane in terms of reproductive performance. From the data collected during these experiments the authors suggest a target daily gain of >1.2 kg/day in the pre weaning phase.