Grain fattening seedstock bulls lowers their fertility

by Allan Nation

LETHBRIDGE, Alberta: Grain-fattened seedstock bulls may top the sale ring but they are worth far less for breeding purposes than leaner bulls raised on all-forage diets, according to a Canadian study.

In a study at the Lethbridge Research Centre by G. H. Coulter, R. B. Cook and J. P. Kastelic, bulls that were grown from weaning to maturity on 100 % forage diets had 13% greater efficiency of sperm production, 19% more daily sperm production and 52% greater sperm reserves.

The Canadian study involved straight bred Angus, and Angus and Hereford crossed with Simmental.

The sperm production of the grain fattened bulls were all extremely marginal in both total sperm production and total motility. The fatter the bulls the less motile the sperm became.

The forage fed bulls had much better testicular tone which is generally associated with improved seminal quality.

The negative effects of grain feeding on semen quality were most pronounced in the straight bred Angus with a near doubling in the number of secondary defects.

A previous Canadian study involving only Continental breeds had found a similar decline in fertility with an increase in backfat thickness.

In the Lethbridge study, the seminal quality of all the bulls declined as body condition increased. In other words, the fatter the bull the less fertile he was.

It appears that the grain fattened bulls were unable to cool the scrotum due to excessive fat deposits in the neck of the scrotum.

The thermoregulatory mechanism maintaining the testes at ideal temperatures may be overwhelmed by increased scrotal insulation through increased fat deposits.

The researchers concluded that for the best fertility, seedstock bulls should be grown from weaning to maturity on non-grain, forage diets. This is particularly true for more easily fattened English breed bulls.

Cattle nutritionist, Dr. Dick Diven of Tuscon, Arizona, said that this study confirms the decline in fertility found in bulls on grain-based gain tests in the USA.

He said bull buyers need to extremely skeptical of semen tests of bulls on gain tests as the semen tested may have been produced before the animal was put on the test.

“We have seen instances were bulls were able to pass a semen test but were found to be completely shooting blanks a few days later on the pasture.”

Diven said that fat cells once formed are permanent. Any subsequent attempt to put the bull into breeding condition will again result in fat in the scrotum and a subsequent decline in sperm production and motility.

“The bottom line is that a bull that is ever made fat is for all intents and purposes ruined forever,” he said.

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