HOW MANY REPLACEMENTS DO YOU NEED?

The first two installments of this 5-part series on managing herd size focused on animals that leave the herd. Part 1 covered calculating culling metrics and part 2 discussed interpreting those metrics. We started by counting the animals that will leave because under ordinary circumstances we will want to replace all of them. Now we move on to the animals that enter the herd. The right number of replacements is important. Raising replacements is expensive, but not having enough replacements to maintain the milking herd is even more expensive. High value beef bred calves add a completely new variable to the equation. Let’s work through the numbers with a hypothetical 500-cow herd.

Replacing culls

The cull rate math is pretty simple. If we used the math in part 1 of the series to calculate a 30% cull rate, then 150 cows will leave the 500-cow herd each year and need to be replaced. In view of the high cost of raising replacements, and the high value of beef bred calves, progressive dairies have gotten good at creating exactly the number of replacements they need – and some have ended up short. Recent experience with bird flu is a great reminder that it’s a good idea to leave some margin for the unexpected. I suggest raising 10% more replacements than you need, and even more if there is a herd expansion in your future. That margin for error brings the total to 165 for our example herd.

Non-Completion

The 30% cull rate we calculated only includes the milking cows that left the herd. We don’t

like to think about it, and we are likely to underestimate it, but some of our heifers will leave the herd without calving. That’s usually called the non-completion rate because they don’t complete the process of becoming a cow. Some will die from illness, others might be injured, and a few will be infertile. The industry average non-completion rate is about 10%. You can calculate yours by taking the total heifers that leave the herd in a year by the total heifers on the farm. On our example farm, assuming 10% non-completion means we would add 16 more to the 165 for a total of 181 per year.

Calving Age

Age at calving must also be considered. If our heifers calve at 24 months, 181 will be enough. We won’t need as many if they calve younger than 24 months. We will need extra heifers if they calve later than 24 months. Because it takes 2 years for a heifer to enter the milking string, the total heifer inventory will be about 362.

From here, it is useful to calculate how many heifer calves are needed each month. In our example, 181 heifers divided by 12 months means that this herd needs to generate 15 heifers per month to maintain herd size. Keeping the monthly total close to 15 will help to maintain an uniform herd size though the year.

The next articles will follow with strategies to control the number of heifers born each month, and compensate for months with larger or smaller numbers of breeding-eligible cows to produce those heifer calves.