21 July 1995

Lower mastitis rate selection

As dairy margins tighten across the Atlantic, producers see genetic progress as the answer to survival. This is the first in a series on USdairying by

Rebecca Austin

US DAIRY producers can now select bulls on their ability to sire daughters with lower rates of mastitis.

In January 1994 the United States Department of Agricultures (USDA) sire ranking for predicted transmitting ability (PTA) for somatic cell score was introduced (PTASCS).

"Genetic studies of dairy cattle have found that single-trait selection for higher milk production brings with it slightly higher rates of mastitis," says Dr George Shook, chairman of the universitys department of dairy science. "But not all high production bulls sire daughters with high rates of mastitis."

In the US cell counts are recorded on a voluntary basis, with a state dairy herd improvement co-operative average of between 275,000/ml and 300,000/ml. This figure accounts for nearly 3m cows, as only 10% of the herds recording performance data offer somatic information.

In the US only 49% of all herds officially record for yield, fat, protein and cell counts. From these figures it has been possible to rank sires for somatic cell score. Of all the active bulls in the USA, B-Hiddenhills Mark-O-Polo has the lowest PTASCS. A Chief Mark son marketed by Federated Genetics, he scores a PTA of 20kg for protein and 2.78 for somatic cell score, which correlates to 75,000 cells.

When compared with milk yield or protein and fat, somatic cell count heritability only averages 10% to 12%, whereas yield stands at 25% and fat and protein at 50%. "The higher the percentage the better the genetic progress," explains Dr Shook. "But we have found SCS differences between bulls are similar in herds with low SCS and high SCS."

The new selection tool has not been used freely by US dairy men so far, but Dr Shook is more interested in its long-term position. "Selection programmes for milk production are long-term projects, so there is a concern that cases of mastitis are likely to increase," he says.

"We therefore have strong justification for embarking on a genetic improvement programme which will prevent and control mastitis,which is a costly disease."

It reduces milk production, increases culling rates prematurely and adversely affects milk quality."

In the US quality premiums are available at a rate of 0.2p/litre of milk per SCS point.

Dr George Shook, chairman of the department of dairy science, University of Wisconsin, Madison.