With only 3% of the US dairy cow population certified organic, the majority of data underlying national genetic evaluations come from conventionally managed cows. As a result, if the differences in an environment that an organic cow encounters, such as eating pasture and minimal or no exposure to synthetic hormones and antibiotics causes different genes to be important, bulls selected based on PTA estimated from conventional data may not be best for the organic dairy producers.
Penn State, University of Minnesota, and USDA researchers gathered data from Holstein cows on 16 different organic farms to determine if indeed the health trait PTA published by the Council on Dairy Cattle Breeding (CDCB) should lead to genetic improvement for disease resistance on organic farms. Depending on the trait and lactation, we use PTA from 402 to 1,012 Holstein bulls and 4,558 to 16,465 daughters to determine the odds of disease for daughters of sires with PTA at the 75th (greater disease resistance) and 25th (greater susceptibility to disease) percentile. For the 6 CDCB health traits of displaced abomasum (DA), ketosis, mastitis, metritis, milk fever, and retained placenta we present the odds of disease relative to the daughters of a bull in the 50th percentile for cows in their first and third lactations. Lactation 2 results are intermediate to lactations 1 and 3 so we left those out for simplicity.
Evident in Figures 1 and 2, the main conclusion is that, regardless of trait or age, daughters of sires with poor genetic merit for disease resistance (dark blue bars) had more disease than those with good genetic merit for disease resistance (light blue bars). How much more likely to experience disease depended on the trait. Because the incidence of milk fever is extremely low among first lactation cows, we were unable to provide odds ratios for this age group. Also, we were unable to provide estimates for retained placenta among third lactations cows. As you might have expected, the differences in odds of disease increased with each lactation.
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Categories: Pennsylvania, Livestock, Beef Cattle