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.
Regardless of lactation, some of the biggest increases and decreases from the average cow occurred for DA. First lactation daughters of sires with poor genetic merit were 51% more likely to have a DA than those from the average sire and daughters with a sire with favorable genetic merit were 34% less likely to have a DA. Remember, the PTA are estimated on terms of disease resistance so higher is more resistant, or less likely to encounter disease. Of these health traits, DA are consistently found the most heritable by researchers, likely because their severity leads to the most accurate recording. As a result, PTA estimates are most accurate and better able to predict occurrence of disease.
Mastitis has a much higher incidence than DA, which should improve our ability to see variation in resistance among sires, but unfortunately, it is much harder to consistently record. For some individuals performing mastitis detection, even on the same farm, a few flakes in the milk may constitute a case of mastitis whereas for others it may not, and then cases get recorded differently. Similar challenges occur when declaring cases of metritis and ketosis. However, for all three of these diseases, we can still see that daughters of high genetic merit sires had lower odds of disease than the average sire’s daughters and those with sires with poor genetic merit had greater than average odds of disease.
Selecting for a healthy dairy cow is important for the profitability and sustainability of any farm. Even though some farming practices differ between conventional and organic dairy producers, health trait PTA from national genetic evaluations primarily based on conventional farm data can identify bulls whose daughters will have superior health on organic farms, too. An organic farmer can select bulls with greater than average PTA for health traits and expect on average to have healthier daughters.
Figure 1. Change in likelihood of disease for first lactation daughters of bulls with PTA at the 75th or 25th percentile relative to daughters of bulls with PTA at the 50th percentile.
Figure 2. Change in likelihood of disease for third lactation daughters of bulls with PTA at the 75th or 25th percentile relative to daughters of bulls with PTA at the 50th percentile