The bull charts listing breeding values also state the non-return rate of the bulls. But what are the factors behind that figure and what do we use it for?
The non-return breeding value is indicative of the fertility of a bull’s daughters, while the non-return rate of a bull is a measure of the fertilising capacity of his semen. In other words, the bull’s own fertility or reproductive performance. CRV uses the non-return rate to show the differences between the fertilising capacity of various bulls. And we can intervene where necessary by rejecting bulls with a poor reproductive performance.
So… when does a bull have a poor fertilising capacity? If a higher than average number of cows inseminated with his semen fail to conceive. But imagine that these animals were six-calf cows who all just happened to have fertility issues? Does this mean the bull has a poorer reproductive performance compared with another bull who has only been used to inseminate heifers? The result of the insemination gives insufficient information. Whether or not a cow conceives depends on the semen, but also on the cow herself and environmental and other factors. Se we need a fairer method of comparing bulls.
CHANCE OF CONCEPTION
The non-return rate is the method used to make for fair comparison. At each insemination, various effects are examined that research has shown to be influential in determining whether insemination is successful or unsuccessful. The sum of these effects determines the chance of conception of an insemination. If all the inseminations of a bull over the past 12 months are examined, his average chance of conception can be calculated and this figure can be compared with his insemination results. For example: assume a bull’s semen was used for 100 inseminations, 68 of which resulted in
successful conception. His insemination rate is 68%. Of these 100 cows, the chance of conception was assumed to be an average of 65%. So 65 cows are expected to conceive, but in fact the figure is 68. This means the bull has a +3% non-return rate.
The method above is applied in the non-return calculation. A calculation model corrects the insemination result of all the inseminations using all the known effects that we can measure. The result is the non-return rate a bull would have achieved with a standardised cow and environment. In other words: the bulls are scored with the non-return rate they would have achieved if they were all used to inseminate exactly the same group of cows. This makes for fair and transparent comparison of all the bulls mutually!
The higher the number of inseminations a bull has had in the past year, the easier it is to calculate a more reliable non-return rate. When the non-return rate is listed with a bull’s information, we take the reliability into account. No non-return rate is published for bulls with too few inseminations. A bull is only awarded the ‘BullsEye’ merit (good fertilising performance) if he has been used for enough inseminations and is significantly better at getting cows to conceive (+4 percent) than the average bull.
USING THE NON-RETURN RATE
It certainly makes sense to take the non-return rate of the bull of your choice into consideration with cows that have difficulty conceiving, or with low breeding values for fertility. If the bull has a non-return rate of +2, using his semen for insemination will give a two percent higher chance of conception than using an average bull. The CRV bulls merited BullsEye have the best reproductive performance.