Making better bull selection decisions for heifer matings

There are numerous factors which need to be considered when selecting a bull to mate over heifers.

In many cases, the primary aim is to produce a live calf which is born unassisted, with other economic traits (e.g. growth) of secondary importance. Traditionally this has been done by selecting a ‘heifer bull’ to mate over the heifers; that is, a bull with a low birth weight EBV which will produce small calves which are easily calved down by the heifer.

However as birth weight is highly correlated with the other growth traits (e.g. 200, 400 and 600 Day Weight), this has traditionally been done at the expense of later growth.

For those that are breeding replacement females, this has longer term implications for the genetic progress of the herd as a whole. The need for heifer bulls is of more importance for herds which are calving in spring compared to autumn, and for herds which are grazing improved pastures compared to native pastures. This is because calves born from dams which have been on good nutrition during pregnancy are heavier at birth than calves born from dams on poorer nutrition.

This article prepared by Southern Beef Technology Services discusses which Breedplan traits are of particular importance when selecting heifer bulls. In addition, it outline the relationships between these traits, and the trade-offs that need to be considered when making these selection decisions.

While this article focuses on bull selection for heifers having their first calves, where the birth and fertility traits are of particular importance, these concepts are also applicable when considering bull selection for cow matings.

Birth weight

Birth Weight EBVs are estimates of the genetic difference between animals for birth weight, expressed in kilograms. Small or moderate Birth Weight EBVs are more favourable, and indicate lighter birth weights. For example, a bull with a Birth Weight EBV of +1 kg would be expected to produce lighter calves at birth than a bull with a Birth Weight EBV of +7 kg, with a lower risk of a difficult birth.

The importance of considering Birth Weight EBVs when selecting bulls to mate to heifers was highlighted at a recent herd visit. At this particular stud, the producer had used a team of bulls which had Birth Weight EBVs which were about double the breed average. These bulls had some of the heaviest Birth Weight EBVs in the entire breed. This bull team had been mated to the heifers; not surprisingly 50pc of the heifers needed assistance at calving. This had a number of flow-on effects for the business. In particular, this led to:

  • Sleepless nights checking heifers and pulling calves during the calving period.
  • A greater number of vet visits and associated costs during the calving period.
  • Mortalities – both heifers and calves.

While selecting an animal with a low to average Birth Weight EBV to use over heifers should help to reduce calving difficulties, correlations between birth weight and other traits need to be considered. Specifically, lower birth weight sires may cause fewer calving difficulties, but they also tend to produce calves with poorer growth to target market end-points.

The sire at left is a good example of this. This sire has a Birth Weight EBV which is below breed average, being in the tenth percentile of the breed for birth weight. However his EBVs for 200, 400 and 600 Day Growth are all well below breed average.

Fortunately, there are bulls out there that are “curve-benders”; that is, bulls that have below average Birth Weight EBVs but above average Growth EBVs. A good example of this is the bull at left.

Like the first bull, the bull below is also in the tenth percentile of the breed for birth weight. However, unlike the first bull, he has good growth EBVs, being above breed average for 200 Day Weight (10th percentile), 400 Day Weight (10th percentile) and 600 Day Weight (25th percentile).

All other factors being equal (e.g. structure), the second bull would be a better choice to mate over heifers.

Both bulls would be expected to produce calves which are lighter than the breed average for birth weight, thus reducing the chance of calving difficulties, which is an important trait for a heifer bull. However the second bull has better growth EBVs, and thus his calves would be expected to have better growth to target market endpoint than the calves of the first bull.

Gestation length

Another trait to consider when selecting a heifer bull is gestation length. Gestation Length EBVs provide an estimate of genetic differences between animals in gestation length, and are expressed in days.

Lower or more negative Gestation Length EBVs are considered to be more favourable. For example, a bull with a Gestation Length EBV of –2 days would be expected to produce calves that are born earlier than a bull with a Gestation Length EBV of +2 days.

Gestation length is favourably correlated with birth weight and calving ease.

In general:

  • As gestation length decreases, birth weight also decreases. Similarly, as gestation length increases, birth weights also increase.
  • As gestation length decreases, calving difficulties decrease.

The converse is also true; as gestation length increases, calving difficulties also increase. Calves which have had a shorter gestation length are generally smaller, and thus the dam is able to deliver her calf with less difficulty. Therefore, when selecting heifer bulls, consideration should be given to the Gestation Length EBVs of the candidates.

Calving ease

While many large studies have consistently shown birth weight to be the most important genetic factor influencing calving difficulty, there are also other aspects that need to be considered. For example, calf shape, pelvic area and calving “will” all influence calving ease.

BreedPlan Calving Ease EBVs attempt to take all the factors affecting calving difficulty into consideration and allow the best possible genetic improvement to be made for ease of calving. Two Calving Ease EBVs are produced by BreedPlan; these are Calving Ease Direct and Calving Ease Daughters.

Calving Ease Direct EBVs are estimates of genetic differences in the ability of a sires’ calves to be born unassisted from two year old heifers. Calving Ease Direct EBVs are reported as differences in the percentage of unassisted calvings. Higher, more positive, Calving Ease Direct EBVs are more favourable. For example, a bull with an EBV of +5.0pc would be expected, on average, to produce 3pc fewer difficult calvings from two year old heifers than a bull with an EBV of –1.0pc (6pc difference between the sires, then halved as they only contribute half the genetics).

Calving Ease Daughters EBVs are estimates of genetic differences in the ability of a sire’s two year old daughters to calve without assistance. Calving Ease Daughters EBVs are also reported as differences in the percentage of unassisted calvings. Higher, more positive, Calving Ease Daughters EBVs are more favourable. For example, a bull with an EBV of +4.0pc would be expected to on average produce two year old daughters that have 3pc less calving problems than the daughters of a bull with an EBV of –2.0pc. When selecting heifer bulls, more positive Calving Ease Direct EBVs are more favourable. However, Calving Ease Daughters EBVs should also be considered, with more positive Calving Ease Daughters EBVs also desirable for heifer bulls.

The relationship between the two Calving Ease EBVs is generally antagonistic. As Calving Ease Direct increases, Calving Ease Daughters typically decreases. This is because a bull with a high Calving Ease Direct EBV will generally produce smaller calves.

As his daughters grow, they tend to develop into smaller cows, with smaller pelvises. Thus, when they calve as heifers, they are more likely to have calving problems than bigger, more roomy heifers.

Luckily, as is the case with birth weight and growth, curve-bender bulls also exist for calving ease traits. Curve-benders in this situation are those bulls who have above average EBVs for both Calving Ease Direct and Calving Ease Daughters.

For those with self-replacing herds, selecting heifer bulls with above average Calving Ease Direct and Calving Ease Daughters EBVs is especially important, both for reducing calving problems in the current heifers and those in the future.

For those beef producers who are producing male and female calves for a terminal market, Calving Ease Direct will be of importance while Calving Ease Daughters may not be.

Best practice guide to selecting heifer bulls

The traits discussed above are not inherited individually; they are correlated with each other. Relationships also exist with other traits of economic importance. Therefore, placing selection pressure on one trait could push other traits in an unfavourable direction.

To avoid this, it is recommended that heifer bulls are selected using selection indexes. Selection indexes rank animal on profit (dollars per cow mated), within a specific production to market scenario. They have been designed to balance genetic improvement across traits, thus taking the hard work out of deciding how much emphasis to place on each individual trait.

To use selection indexes to select heifer bulls:

  • Identify the selection index of most relevance Identify which selection index available for your breed best fits your breeding objectives.
  • Rank animals on selection index Rank bulls available for selection on the selection index of most relevance.
  • Consider fertility and birth traits Animals with the same selection index value can have very different individual EBVs. Consider the Calving Ease Direct, Calving Ease Daughters, Birth Weight and Gestation Length EBVs and exclude any bulls which don’t have EBVs in acceptable ranges.
  • Consider other traits Consider EBVs for all other traits of economic importance, as per your breeding objective. Exclude any bulls which don’t have EBVs in acceptable ranges.
  • Consider other selection critieria of importance (e.g. horn status, structural and functional soundness and genetic condition status). Exclude any bulls that do not meet requirements.

This approach should allow beef producers to select heifer bulls which, in addition to reducing the risk of calving problems in their heifers, should also improve other economically important traits within the herd.

Take-home messages

When selecting heifer bulls, selection indexes should be used to rank bulls on economic merit.

Particular consideration should then be given to Birth Weight, Gestation Length, Calving Ease and Growth EBVs. Other traits which are of importance to the breeding objectives for the herd, and other information should also be considered before finalising selection decisions.

When selecting heifer bulls, remember:

While lower birth weight is generally associated with lower growth, curve-bender bulls do exist. Where possible, try to select bulls with low to average Birth Weight EBVs and above average Growth EBVs. This should equate to smaller calves at birth, thus reducing the likelihood of calving difficulties, without compromising on growth as the calf gets older.

Shorter gestation lengths are more favourable. Bulls which have shorter Gestation Length EBVs should sire calves who are born earlier than those sired by bulls with longer Gestation Lengths. Thus the progeny of bulls with shorter Gestation Length EBVs are generally smaller at birth, and in turn, more likely to be delivered with less calving difficulties.

Bulls with higher, more positive Calving Ease Direct EBVs are more favourable, as they are more likely to have calves that are born unassisted from two year old heifers. For those in self-replacing herd situations, higher, more positive Calving Ease Daughters EBVS are also desirable in a heifer bull. These two traits need to be carefully managed to avoid decreasing the calving ease ability of future heifers.

SOURCE: Beef Central, 19 July 2022