Page 52 - Hereford Magazine 2021

Figure 1. Most variation in the Days to Calving EBV is due to the time taken for the female to get pregnant once she is out with the bull.

(e.g. the small, young animal). In both situations, the fertility are retained for breeding, both calving ease EBVs are of
traits – days to calving and scrotal size EBVs – are important. importance. It is important to note both traits relate to calving
The days to calving EBV describes the genetic differences difficulty as expressed in two-year-old heifers; a bull that has
between animals in the time from the start of the joining had no calving issues when mated to mature cows may still
period, when the female is introduced to the bull, until cause trouble for heifers.
subsequent calving, and is expressed in days. Most variation in The gestation length EBV describes the genetic differences
this trait occurs in how long it takes the female to get pregnant between animals in gestation length, and is expressed in days.
e.g. did she get pregnant in the first cycle, second cycle, or not Lower, more negative gestation length EBVs indicate a shorter
at all, with only a small amount of variation in this trait being gestation length and are more desirable. In general, a shorter
due to gestation length (Figure 1). Lower, more negative days to gestation length results in a smaller calf, which is usually born
calving EBVs are more desirable, as they indicate shorter days with less difficulty than a larger calf, and also gives the cow a
to calving or females conceiving earlier in the joining period. longer period of time to rebreed without falling later and later
Similarly, scrotal size EBVs are another important in the breeding season each year.
indicator of fertility in the female herd. While this may seem The birth weight EBV describes the genetic differences
counterintuitive (after all, a heifer does not have a scrotum), between animals in calf birth weight, and is expressed in
research has shown bulls with higher scrotal size EBVs tend kilograms. Small or moderate birth weight EBVs are generally
to be more early maturing than those with lower scrotal size more favourable as they indicate lighter birth weights. In
EBVs. In turn, bulls with higher scrotal size EBVs tend to have general, a lighter calf at birth is likely to result in less calving
daughters that mature earlier than the daughters of bulls with difficulty than a heavier calf, although there can be exceptions.
lower scrotal size EBVs. Therefore, the ideal maternal female This is because birth weight, as mentioned above, is not the
should have more negative days to calving EBVs (indicating a only factor influencing calving difficulty.
shorter days to calving) and more positive scrotal size EBVs. The ideal maternal female should have more positive
calving ease direct and calving ease daughters EBVs
2. CALVING (indicating less calving difficulty), more negative gestation
The next important test for a female in the breeding herd is length EBVs (indicating shorter gestation length), and a low to
to give birth to a live calf without assistance. Calving difficulty moderate birth weight EBV (indicating lower birth weight).
has a negative impact on the profitability of the herd due
to increased calf, heifer and sometimes even cow mortality, 3. RAISING THE CALF TO WEANING
slower re-breeding performance, and considerable additional The next test for the Hereford female is to raise her calf to
labour and veterinary expenses. weaning. A good maternal cow should provide adequate
While non-genetic factors can contribute to calving difficulty nutrition to raise the calf to weaning, and wean her calf with an
(it’s still important to keep heifers and cows in optimal adequate weaning weight. The milk EBV provides an estimate
condition in the lead-up to calving), there are a number of of the maternal contribution of a dam to the 200-day weight of
genetic factors that also influence calving difficulty. These her calf. The milk EBV is expressed in kilograms and indicates
include birth weight, gestation length, shape of the calf, pelvic the expected difference in the weight of the calf at 200 days
area, and willingness of the cow to push. These factors are due to the maternal contribution of the cow. However, it is
covered by several Breedplan EBVs, namely calving ease direct, important to note the optimum milk EBV is dependent upon
calving ease daughters, gestation length, and birth weight. the production system and the cows’ environment. Selection
The calving ease direct EBV describes the genetic for increased milk production may be warranted when cows
differences in the ability of a sire’s calves to be born are run under good nutritional conditions such as improved
unassisted from two-year-old heifers, while the calving ease pasture, while other poorer environments such as hill country
daughters EBV describes the genetic differences in the ability may not support cows with higher milk EBVs. In addition, high
of a sire’s two-year-old daughters to calve without assistance. milking cows may not get back in calf as easily as lower milking
Both EBVs are reported as differences in the percentage cows in the following year. While the ideal maternal female
of unassisted calvings, with higher, more positive calving should provide adequate nutrition to raise the calf to weaning,
ease EBVs (indicating less calving difficulty) being more the optimum milk EBV for a maternal female will depend on
desirable. In a self-replacing Hereford system, where daughters the environment in which she is run.

50 HEREFORD MAGAZINE Year 2021