The annual selection of heifer replacements is an important decision for herd development and deserves some serious consideration. In Multiplier herds a key objective is to ensure continued genetic improvement. This can be achieved by using high merit sires and replacing cows with poor figures with heifers that have better ones. There are also the normal cull decisions to make on older cows, normally around feet, udders and fertility– but we can park that one for the moment.
An easy measure of genetic progress in the cow herd is to look at the average MPV of the females that have calved each year (you can do it with any trait or index). For example, if you have a herd of 10 cows as in the table below, by removing the two with the lowest MPV and replacing with heifers of the same MPV as the best, you increase the herd average by 3 MPV points.
Other considerations need to be included in the decisions like the soundness of the heifer and any structural or reproductive defects. Obviously if the animal is not “sound in wind and limb” it does not matter how good the figures are, she is not worth considering. There may also be breeding lines to manage to ensure in-breeding is avoided and best use is made of the available stock bulls.
As a general approach it good to list possible replacements in descending order of MPV (or other trait if that’s your decision criteria) and then knock out the ones that are not suitable for management reasons. When you get to the required number don’t go any further down the list……unless the MPV of the heifers is better than the worst cows in your herd then you could probably go a bit further down the list and increase the genetic merit of the herd more rapidly.
In this example we are already using K and L but we could think hard about using M to replace H because she has a higher genetic merit. N and O are no better than the worst cow so should not be considered at all. Replacing H with M would raise the herd average MPV by a further 0.4 points to give and average MPV of 21.4 in the herd.
In their system if a heifer has better EBVs than a cow in the herd, then the heifer goes in and the cow goes out. The “reject” cow is still a highly sought after animal with a ready market for breeding because she is still of high merit compared to the rest of the population. In practice these herds are turning over one third of their cows each year.
The rate of genetic gain is such that the top 10% cow of today will be average in 5 year from now. How do they achieve this? By using AI and elite sires throughout the herd. AI at pasture in range conditions is something they now do as routine. Now that’s something to think about…………………