Cow numbers for dairy AI
How many cows do you need to breed to dairy AI to ensure adequate numbers of replacement heifer calves?
It depends on the number of heifer replacements required and whether the semen being used is conventional, sexed, or a combination of both. Reducing the number of cows that need to be bred to dairy AI will increase the genetic merit of your replacement heifers but also reduce the number of dairy male calves bred on farm, and increase the number of cows bred to high Dairy Beef Index (DBI) AI bulls.
See Table 1 for an example of a 100-cow herd where there is a requirement for 22 replacements heifers. All dairy straws used are female sexed semen, and all are used in the first three weeks of breeding. The conception rates used are a guide and should be adjusted to reflect past herd performance. This example assumes a 100% submission rate with the maiden heifers and 90% with the milking herd in the first three weeks of breeding.
Table 1: Example of sexed semen use
| Straws | Conception | No. heifer calves | |
|---|---|---|---|
| Heifers sexed straws | 22 | 55% | 11 |
| Heifers conventional straws | 65% | 0 | |
| Total | 22 | 11 | |
| Straws | Conception | No. heifer calves | |
| Cows sexed straws | 25 | 50% | 11 |
| Cows conventional straws | 60% | 0 | |
| Total | 25 | 11 | |
| Total heifer calves from cows and heifers | 22 | ||
| Total dairy males from cows and heifers | 3 | ||
Looking at the numbers
As seen in the table, 11 heifer calves are produced from the maiden heifers. Assuming a 50% conception rate to dairy sexed semen on the cows, 25 cows must be bred to produce a further 11 heifers. The number of dairy-bred males born reduces to three due to 90% female probability of sexed semen. The number of cows bred to high-DBI beef AI in the first three weeks increases in this example to 65.
Concerns
The obvious concern when using sexed semen is reduced conception rates. Therefore, consideration must be given when selecting the animals for sexed dairy AI, the timing of AI, and the handling of the straws. Reduced conception rate is somewhat offset by the fact that sexed semen is only being used on a percentage of the herd, and allows for a larger proportion to be bred to conventional high-DBI beef AI at normal conception rates. The example in question is for a herd achieving high levels of fertility performance. Each individual farmer in consultation with their advisor should develop a plan that aligns with their herd’s fertility targets.
This article by Dr Joe Patton, Teagasc Head of Dairy Knowledge Transfer, first appeared in the Teagasc Dairy Advisory newsletter for March. Access the publication here.
Also read: How do you know if replacement heifers are on target weight for breeding?
Also read: Optimized milking intervals with Sean Kelly, Tipperary