The right bull helps make calving two-year-old cows easier

Meat & Wool NZ beef genetics co-ordinator Russell Priest.

01-05-2009 | Jackie Harrigan

Calving heifers as two-year-olds can significantly improve the efficiency and profitability of a breeding cow enterprise.

Many farmers have mated yearling heifers and calved down two-year-old cows successfully for a number of years; others have never quite cracked it.

Calving problems or later rebreeding issues have caused them to rethink or abandon the practice.

Meat & Wool New Zealand beef genetics co-ordinator Russell Priest says there are two prongs to success-selecting the right bulls to mate with yearling heifers and the correct pre-mating management of the heifers.

Priest says bull selection is one of the most critical areas, as the ease at which calves are born to heifers has a significant effect not only on the number of live calves but also on the rebreeding rate of the heifers.

However he says it is always worth remembering there are two sides to genetics, the sire and the dam.

"When I talk to people who may not be having success calving their heifers, I always ask, what are the birth genes like on the maternal side?

"Invariably they have big birth genes in the cow herd and this has come through to the heifers. Even though they are choosing a low birthweight sire they are still getting big birthweights that their heifers struggle with."

Priest says there are a number of issues to consider:

Bull breed

Choose a breed of bull in a similar or lower mature weight category to that of the heifers you are breeding and in the case of crossbred heifers err towards the lower smaller breed component.

The mature weight categories are:

• High mature weight breeds: Chianina, Blonde d'Aquitaine, South Devon, Friesian, Charolais, Simmental, Maine-Anjou, Limousin.

• Moderate mature weight breeds: British breeds including Angus, Hereford, Shorthorn, Murray Grey, Red Devon, Welsh Black.

• Low mature weight breeds: Lowline, Miniford, Galloway, Dexter, Jersey, Highland, Wagyu.

For example, it is safer for avoiding calving difficulties to mate Hereford-Friesian cross heifers to bulls from the moderate mature weight breeds, as Herefords are in this band.

However Priest says it must be appreciated that there are animals within breeds that do not conform to the category in which they are placed.

Crossbreeding will produce bigger calves due to hybrid vigour, however the closer the mature weights of the two breeds, the smaller will be the hybrid vigour effect.

Estimated Breeding Values

Using EBVs to select a sire for breeding to heifers is very important, as using raw data (actual birthweight of the sire) is dangerous, says Priest.

"Raw data is very misleading as a calf's birthweight is a result of both the parents genes and the feeding level of the cow also has a huge effect," he says.

"So choosing a bull with a low actual birthweight will not guarantee low birthweight progeny-the raw data needs to be adjusted for the age of dam, the length of the gestation and other non-genetic factors."

Priest says a lot of emphasis should be placed on the calving ease (CE) (direct) EBV. The higher the CE (direct) EBV, the more easily the calves will be born.

However CE (direct) EBVs are not always available, particularly for young bulls or if the breeder was unable to collect the calving ease scores. It is not a highly heritable trait because there are many factors unable to be directly measured like the dam's pelvic size and shape.

There is also a trait called the CE (maternal) EBV that is an indicator of the ease with which the bull's daughter's calves will be born. This EBV needs to be positive also.

In the absence of CE EBVs, Priest says the birthweight EBV is the next best genetic predictor of how easily a bull's calves will be born.

The lower the birthweight EBV, the more easily calves will be born.

He says farmers should get hold of the breed average EBVs and target a bull below the average for birthweight, but they should remember that birthweight and growth rate are strongly related so a calf that is small and easily born will likely be smaller and slower growing over its life.

If farmers are targeting low birthweight EBV bulls for mating to their heifers and then retaining replacement females from the heifers' calves, there is a risk that the frame size will be getting smaller, thus introducing later calving problems into the herd.

"If you are retaining heifers from that calving, you are best not to go for the lowest birthweight EBV bulls, but compromise on a below average birthweight EBV bull and choose better growth rate EBVs, to avoid later problems," says Priest.

A relatively new gestation length EBV can also help in selecting an easy calving sire, as bulls with a lower gestation length EBV tend to have smaller calves.

"A bull with low gestation length EBV is always a good indicator of a curve bender, as these bulls do more growing outside the uterus than inside it."

Avoid bulls with heavy shoulders, broad heads, wide hips and an appearance of excessive bone circumference.

Use a mating ratio of 1:25 or 30 for heifers, unless the bull is known to have higher mating potential, then the ratio can go to 1:40.

Yearling or lighter two-year bulls will reduce the chance of injury to the heifers during mating, but yearling bulls do not necessarily leave smaller calves than older bulls. Birthweight depends on the level of feeding of the cow/heifer and the birthweight genes carried by the parents of the calf.

Yearling bulls should be fed generous rations of high quality grass before, during and after mating as they are still growing rapidly at this stage and their fertility is likely to suffer if they are not adequately fed. Subjecting them to a reproductive soundness check by a vet is a cheap form of insurance.

Index Selection

Some of the breed societies have developed an index with which to select bulls for mating with yearling heifers. The Angus breed society has the ease of calving index and the Hereford breed society has the dairy maternal index.

These indexes rank bulls on their ability to generate profit per cow mated in a production system where the main focus is to minimise calving problems, without neglecting subsequent growth.

Priest says when selecting by an index it is still wise to view the component EBVs before making your final selection. Indexes are found on the breed societies websites.

Summary - Mating

• Select bulls of same breed or smaller mature weight category as the heifers.

• Target direct calving ease, or if this is not available, birthweight EBV to reduce calving problems.

• Remember low birthweight animals generally have low growth rates.

• Avoid using bulls with ‘heavy' shoulders, broad heads, wide hips and ‘heavy' bone.

• Yearling bulls do not necessarily leave smaller calves than older bulls.

• Use yearling bulls or lighter two-year-olds to mate with yearling heifers.

• Use bulls with minimum scrotal size of 28cm.

• Mating ratio of 1 bull to 25 heifers.

• Hereford and Angus societies have indexes for heavy emphasis on calving ease.

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