Friday, March 29, 2024

A No Bull policy

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Sharemilkers Paul Gow and Sonia McKerchar, who milk 1200 cows near Culverden, have used short gestation semen to get rid of bulls on the farm.
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“We had a policy of no inductions and no October calving so we were doing most of our culling on calving dates. We felt our bulls weren’t doing the job they should be as we had a high empty rate,” Gow said at a South Island Dairy Event workshop.

“As well we were spending a lot of time dealing with lame cows and lame bulls during the mating period and the bulls were eating grass which the cows should have been getting.”

They decided instead to reduce the mating period by two weeks and do six weeks of normal AB and four weeks using short gestation semen.

“We realised we had to improve our heat detection if we were going to do this so instead of Paul doing it all himself we shared it,” McKerchar said.

“We had the cows in two herds so I would do one and he would do the other on the vet stand during milking. It meant we got to know our own cows.”

They were also noting down heats when getting cows in from the paddock.

“We had a policy of when in doubt put her up.”

They used heat detectors and tail paint, changing the colour after each insemination.

They also began identifying heats five weeks before the planned start of mating.

“We wanted to identify patterns for individual cows and we entered the data into Protrack so we knew which cows should be coming into heat.”

Just before Christmas the entire herd was pregnancy scanned and those not pregnant were drafted into a separate herd to reduce the number of cows they had to detect heats for.

They achieved a 77% six-week in-calf rate.

David Chin of LIC said there were 120 herds nationwide in the 2013-2014 season that did 10 weeks or more of AB. Their average was a 66% six-week in-calf rate.

He said cows often came up twice for AB in these herds and some were inseminated four or five times.

When using short gestation semen care needed to be taken when selecting calves for rearing the following spring.

“Having the cows which are mated to short gestation, low Breeding Worth sires in a separate herd at calving minimises this risk.”

Using longer AB periods and short gestation sires increased the cost of heat detection and inseminations but meant extra days in milk the following season and fewer interventions to get cows back in-calf.

“Every day earlier a cow calves improves her chances of getting in-calf in the first six weeks of mating.”

Buying or leasing bulls and feeding them was another cost eliminated, he said.

The long and the short

The gestation length of a New Zealand dairy cow is 282 days but through natural genetic variation some cows have a shorter gestation length than others.

The trait is highly heritable (44% compared with protein at 32%) and recently sires have been bred which can shorten the gestation length by 10 days.

These sires may not have high Breeding Worth as they were selected on gestation length rather than production. Short gestation Hereford semen is available which gives farmers a marker to identify the resulting calves.

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