Make Sire Selection Part of Your Mastitis Control Strategy

Follow These Recommendations for Using
Genetic Evaluations for Somatic Cell Score

What does genetics have to do with mastitis and milk quality? In the U.S., semen distributors and sire selection specialists in the artificial breeding industry often give little thought to mastitis and milk quality. Milk quality and animal health specialists generally give little thought to genetics. Even producers are often unaware of the genetic tools available for preventing mastitis. As a result, genetic evaluations for somatic cell score (SCS) have not gained the attention they deserve among producers, AI personnel, and other dairy professionals.

Milk quality specialists and veterinarians are naturally focused on short-term management issues, so a genetic approach to controlling mastitis is rarely considered. However, while day-to-day management has large and immediate impacts on udder health, genetics can have important long-term impacts. The genetic impacts can be favorable or unfavorable depending on how the sires are chosen.

The objectives of producers, milk quality specialists, semen suppliers, and yes -- even the cows! -- could be advanced by a well-reasoned approach to bull selection. The following recommendations are provided to enable all segments of the industry to make appropriate use of genetic evaluations for SCS.

Recommendation 1
Include sire selection as a point on your checklist when consulting with herds for mastitis and milk quality. Designate someone on your milk quality teams who will evaluate your clients' mating sire choices in terms of SCS, fertility, and survival. Red-line those bulls with PTA-SCS 3.2 or higher; use them sparingly and only when they are truly exceptional for other economically important traits. Around 5 to 10% of AI bulls have PTA-SCS of 3.2 or higher. Recognize that sire selection is a long-term, not a short-term solution to improvement. For this reason, sire selection will not be an immediate priority for herds facing loss of their milk market, but it should be part of every herd's long-term outlook. Because genetic improvement is a long-term process, even herds with excellent udder health management should avoid sires with high PTA-SCS.

Recommendation 2
Use a selection index, e. g., Net Merit or a production and type index, as an initial screening for selecting AI bulls. This will provide a moderate amount of selection emphasis on PTA-SCS. Selection indexes consolidate the genetic evaluations for all traits into a single quantity for each bull. Producers should create a 'long list' of candidate bulls using their chosen index. Be sure the index chosen is most appropriate for the business goals of the herd.

Recommendation 3
Include PTA-SCS among the criteria for selecting a 'short list' of service sires from the 'long list', but do not overemphasize SCS to the exclusion of other economically important traits. Make only limited use or avoid altogether the 5 to 10% of bulls with the highest (most undesirable) genetic evaluations for SCS. In Canada, the percentile ranking for SCS is provided for every bull by the Canadian Dairy Network (http://www.cdn.ca/query/). Avoid bulls with PTA-SCS or EBV-SCS 3.2 and higher. Other traits to consider in forming the short list include herd life, daughter fertility, calving ease, stillbirth, genetic defects, type, and semen cost.

Recommendation 4
Herds with low average SCC should give the same attention to PTA-SCS in sire selection as herds with high average SCC. Genetic differences among bulls' daughters are expressed to the same extent in both high and low SCC herds. This will promote continued genetic improvement for SCS and enable producers to obtain milk quality premiums even when a herd encounters milk quality problems.

Recommendation 5
Producers who can successfully manage artificial insemination should use AI bulls. A wealth of genetic information and true genetic superiority are available through AI that is not available for non-AI bulls. Producers who cannot manage AI, should purchase bulls from dams with high selection index values and sired by well-qualified AI bulls. The same tools used for selecting AI bulls should be applied to selecting the sires and dams for non-AI bulls.

References: NMC 46th Annual Meeting Proceedings (Shook, 2007) pp 49-67 and NMC 2006 Regional Meeting Proceedings (Shook) pp.25-37. Updated Nov. 2009


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