Selecting for Subclinical Mastitis Resistance

Animals determined to have subclinical mastitis show no clinical signs or visible changes in the milk, but they do have a SCC greater than 200,000. ( Farm Journal )

With milk prices not predicted to see much improvement until the second half of 2019, now is an important time to reduce your bulk tank somatic cell count (SCC) and examine methods to lower the occurrence of subclinical mastitis (SCM) on the farm.

Animals determined to have subclinical mastitis show no clinical signs or visible changes in the milk, but they do have a SCC greater than 200,000. Often times, mastitis in first-lactation heifers is rarely detected before calving, because clinical signs, such as a swollen quarter, can be hard to identify.

A recent study conducted by the University of Guelph, published in the December issue of the Journal of Dairy Science, examined the genetic parameters of SCM in first-lactation animals.


A BAD WAY TO START

Heifers who enter the milking herd with SCM will often have a lower lifetime productivity and profitability in the future, according to Canadian researchers. The prevalence of SCM in first-lactation heifers is usually highest during early lactation.

SCC test-day records were collected monthly on 91 Canadian herds. Taking samples on first-lactation animals during their first 5 to 30 days in milk, scientists compared a total of 8,518 records to six alternative traits that were defined as indicators of SCM.

GENETIC SELECTION

Using both linear and threshold genetic models, the study identified strong correlations among alternative SCC traits. Based on estimated breeding values (EBV) it was determined that genetic variation for SCM resistance existed among sires, making daughters less prone to the disease.

The study found that despite low heritability, genetic selection to improve genetic resistance to SCM in early lactation of heifers is possible. Working to select animals who are genetically less likely to obtain SCM may help reduce economic losses and premature culling of first-lactation heifers.

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