by Chad Dechow
The author is an associate professor of dairy cattle genetics at Penn State University.
The Animal Improvement Programs Laboratory (AIPL) at USDA releases DHI statistics every year. Those trends found at http://aipl.arsusda.gov/publish/dhi.htm allow us to track a variety of traits. There are many impressive trends in this data, but the combination of a steady rise in milk, fat and protein yields coupled with lower somatic cell counts stand out to me.
The reason I find this combination impressive is the antagonistic genetic correlation between yield and mastitis. The correlation suggests that we should see a slow but steady rise in the rate of mastitis as milk yields go up over time. We also need larger udders to hold the extra milk, and big udders are likely to be closer to the ground and more easily injured. Today's cows have the same number of teats as lower-yielding cows in the past. Higher yield means more milk exiting through the same four teats, resulting in more potential wear on teat ends.
Despite the potential added stress on a cow's udder, average test-day somatic cell counts have declined from over 300,000 in 1995 to just over 200,000. Meanwhile, milk yield has improved by nearly 8 pounds per day in the same time period.
Credit type-minded breeders
We need udder capacity to hold more milk, but there is more than one way to boost udder volume. Udders can be deeper or they can be longer and wider. Breeders have placed a lot of selection pressure on making sure udders did not become deeper and more loosely attached, even though bulls that transmit the best udder conformation may not have always been at the very top of bull rankings for other traits.
The genetic relationship between udder traits and mastitis is well-documented. The single most involved trait is udder depth. Udders held far from the ground and close to a cow's body wall are less susceptible to mastitis. A strong fore udder attachment also has a favorable relationship with mastitis resistance and is highly correlated with udder depth. The relationships between the other udder traits and mastitis are relatively modest.
A 1994 study helped to document the success that North American Holstein breeders had in limiting upward movements in mastitis cases in response to higher milk yields. Researchers from Sweden evaluated the change in milk yield and mastitis incidence. Sweden has a national database of mastitis treatments and is able to track how mastitis rates have changed over time. Swedish dairy producers imported North American Holstein semen, and researchers noted that this yielded a substantial gain in milk yield. However, mastitis rates did not climb as milk yield rose, which was attributed to the terrific udders of North American Holsteins. As the authors stated:
"The "Holsteinization" of the Black and White cows in Sweden has, on average, led to a marked increase in production without a change in the prevalence of mastitis, despite the generally observed antagonistic relationship. A possible deterioration has probably been counteracted by correlated positive effects of simultaneously improved udder conformation," (Journal of Dairy Science, Volume 77, pages 3252 3261).
I have challenged the direction that we have taken for type traits such as body size and will provide some thoughts on dairy form in the future that not all will agree with. However, it is quite clear that breeders knew what they were doing when it came to udder traits. They deserve credit for placing strong emphasis on udder conformation at a time when many involved with genetics thought that it would shift selection pressure away from more economically important traits.
It wasn't until 2000 that udder composite was included in the Net Merit index. There is some evidence that the relationship between udder traits and herd life is not as strong now as it once was. I think this is largely because of the tremendous progress we've made in udder conformation.
The Swedish researchers were able to pinpoint improved udders as largely responsible for the favorable mastitis outcome because genetic evaluations for SCS (somatic cell score) were not yet available at that time. The researchers noted that even though we had prevented upward trends in mastitis, improvements in genetic selection programs were certainly possible. Indeed, the genetic trend for SCS rose slightly until the last 5 to 10 years for most breeds. The release of SCS and productive life (PL) evaluations in 1994 have provided additional opportunities to select for udder health. As a result, SCS genetic trends are now declining for most breeds. The exception to this is the Jersey breed, where the genetic trend appears to have leveled off more recently.
While we have developed a fairly robust system to select for mastitis resistance with the combination of udder conformation, SCS and productive life, additional improvements are possible. Some bulls have daughters with high rates of mastitis even though they have good udders and low SCS. Such bulls can only be detected by generating direct mastitis PTA (predicted transmitting ability).
The nationalized cow health databases in Sweden and other Nordic countries facilitate estimation of mastitis-resistance PTA. We do not have such a system in the United States. However, our dairy farmers do record a substantial amount of cow health data with various dairy herd management software programs. Researchers have demonstrated that such data can be used to generate disease resistance PTA, so there is potential for us to expand our udder health selection toolkit to include direct mastitis evaluations in the future.
It takes a team effort
The maintenance of high levels of udder health in our dairy population despite substantial gains in milk yield represents a successful collaboration of different industry segments. Much of the credit for the improvement in udder health certainly goes to changes in management, milking routines and housing strategies. Quality bonuses that provide incentive to improve udder health have been a help. Scientists have developed novel traits like SCS for genetic selection, and type breeders have helped to improve udder conformation. Ultimately, it is our dairy producers who implement the genetic selection and management changes that have helped to protect the well-being of dairy cows as we select for higher yields.