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• How Many Replacement Heifers Do You Need?

How Many Replacement Heifers Do You Need?

It is easy to drift along each year with the number of replacement heifers reared being determined purely by how many arrive in the first four to six weeks of calving. In tight calving pattern herds which are using AI for two rounds, up to 80% of the herd may get in calf to AI.  By the time culls are taken out this figure will be closer to 70% in a herd of stable size.  If 45% of these result in a weaned heifer calf, then 31% of the herd size is being reared.

In most cases, less than 25% are reared, mainly because few farms AI for more than two rounds and submission and conception rates are not high enough to achieve a six week, 80% pregnancy rate.  It could be argued that if few culls are needed, then the required replacement rate of heifers is lower.  If herd fertility is excellent, mastitis is well controlled and production and temperament have been heavily selected against, then the number of lactations per cow may be closer to six.

In most herds the average number of lactations is about four years which means that to maintain the herd age profile, 25% of heifers need to be introduced each year.

Factors, which may change this figure, are:

•  Recent introduction of a high number of younger cattle.
•  Expanding herd.
•  Need to cull on mastitis, age or fertility.
  

To ensure that enough replacement heifers are available it is important to plan the likely requirements two to three years ahead since this is the lag time from either mating or rearing to herd entry.

If a poor run of heifers has resulted this year, it is not too late to buy some extras and reach the target required.  Alternatively, the AI program on cows could be extended for an extra round.  This provides more calves but two months behind the first calves.  To increase the number of cows mated in the first three weeks, use of induction and CIDR programs will assist.

It costs between $400 and $1,000 to rear a calf.  The big variation in costs depends on whether or not feed is available at no extra cost for these yearlings until they enter the herd.  If all rearing costs, feed costs and labour are included, then the price approaches $1,000.

To have an adequate number of well grown heifers entering the herd each year is like a release from prison.  All the problems encountered with high cell counts, low fertility, old age and low production can be addressed adequately.
Without these animals, cows will stay in the herd that either pose a risk to herd mating, high ICCC's or perform poorly because of short lactations (late calvers).