Breeding indexes of dairy cattle that are based on parent averages can change considerably after genomic testing is carried out, Richard Miller from National Milk Records (NMR), told farmers at an event in Drumahoe last week.

Speaking at the NMR open day on the farm of Stephen and Mark Montgomery, Miller said that the traditional method of calculating £PLI breeding indexes based on parent averages was not an accurate way of identifying the best breeding animals in a herd.

Farmers were told that genomic testing gives more reliable information to identify the genetic potential of young stock in over 40 traits.

In the Montgomery herd, 10 heifers were randomly selected for genomic testing

It has been widely used on young bulls, but female testing is becoming increasingly popular. The test is based on a tissue sample from an ear tag and costs around £33/head.

In the Montgomery herd, 10 heifers were randomly selected for genomic testing. The £PLI breeding index of all heifers changed afterwards, with one dropping from £445 to £212 and another rising from £429 to £564.

Overall, £PLI ranged from £200 to £620 across the 10 heifers, which were sired by three bulls of similar £PLI.

Miller said that an accurate understanding of genetic potential is important, as it allows the best heifers to be identified for breeding and sires to be selected to correct weaker traits.

For example, if the Montgomerys selected the top 70% of the 10 heifers for breeding replacements based on £PLI from parent averages, then the results of genomic testing show that the fourth best would have been missed and the eighth and ninth worst used.

The rate of progress in sire genetics means the best genetic potential in your herd is in your young stock shed

“Improving herd averages can only be achieved by identifying weaker animals and ensuring the bottom end does not put another heifer on the ground,” Miller said.

He described £PLI from proven cows as “bomb proof”, but pointed out that progress is slow as a cow will be at least four years old before enough data is collected from milk recordings.

“The rate of progress in sire genetics means the best genetic potential in your herd is in your young stock shed,” Miller said.

Selective dry cow therapy is farm-specific

The cut-off thresholds for determining which cows are suitable for selective dry cow therapy should be specific to individual dairy herds, veterinary consultant Norman Beggs said at last week’s NMR open day.

Selective dry cow therapy involves some cows being teat-sealed only at drying off and farmers were advised not to consider implementing it if bulk milk somatic cell count (SCC) has been above 250,000/ml in the last year.

Beggs said that cut-off thresholds for determining which cows are suitable for selective dry cow therapy depends on factors such as bulk milk SCC, dry period mastitis infection rate and early lactation infection rate.

Host farmers Stephen and Mark Montgomery use selective dry cow therapy on cows that have not had mastitis in the past three months and have had a SCC less than 200,000/ml during milk recordings (170,000/ml for heifers). It means around 75% of the herd does not receive antibiotics at drying off.

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