Selecting for early puberty in heifers

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One of the bigger problems we see during times of feed shortage, is the effect on herd fertility. Pregnancy test results after a drought joining are often disappointing.

But one of the more important ways to regain a better financial position after drought is to have lots of calves to sell. It’s a difficult situation.

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Cows in poor body condition, with young calves at foot, can be very difficult to join up. Many won’t be cycling when the bull is put in, and short joining times can lead to low pregnancy rates.

Young heifers in light condition may also have delayed cycling.

In cattle, body weight and body fat affect reproductive performance. Droughts show this.

Droughts also help show the benefits from selection for early puberty in heifers.

Body weight and age at puberty are influenced mostly by nutrition. Heifers fed for optimal growth reach puberty at an earlier age, as well as being heavier than those on poorer diets.

Research has shown that heifers that reach puberty at a younger age also have the ability to continue cycling when feed becomes scarce.

A trial using British crossbred heifers imitated the beginning of drought feed conditions. Heifers that had been cycling for at least three months were put on a ration of only three kilograms of poor quality hay a day – not much feed.

Body weights and condition scores were measured every two weeks. Regular blood samples determined which heifers were still cycling, by measuring ovarian luteal activity.

As expected, heifers gradually lost body weight and condition. Luteal activity also ceased, but at different stages.

Heifers that had reached puberty at a later stage stopped cycling earlier in the trial (at 106 days after the tight feeding began).

Heifers that were moderate in the time to reach puberty took 136 days to stop cycling.

But those heifers that reached puberty early in their lives did not stop cycling until 185 days of poor feed.

And there was no real difference in body weight and condition between the groups.

Selection of heifers for early puberty increases reproductive performance, and the difference is even greater as the feed gets tough.

This does not mean we should be joining these more fertile heifers at a younger age. Management should specify calving at two years of age, and joining nine months before this.

We can identify cycling activity in heifers by spending some quality time with them. But we can also select for it genetically through the bulls we use.

Breedplan presents us with Days to Calving and Scrotal Size EBVs.

Both are related to female fertility.

Bulls with larger scrotal size produce daughters that reach puberty at an earlier age. These daughters will be more fertile as the feed becomes less.

Dry times in the cattle breeders journey are common and may become more so. Setting up the herd genetically to cope usually doesn’t cost much more, but may lessen the frustration.

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