Will genomic selection be the key to improving feed efficiency in dairy cattle? That was the question addressed by Dr. Kent Weigel, a geneticist at the Department of Dairy Science at the University of Wisconsin during his presentation at the Four-State Dairy Nutrition and Management Conference held June 11 and 12 in Dubuque, Iowa.

“There is no ‘magic gene’ that will make cows more efficient,” Weigel said, noting that there are genes known to have a small effect on efficiency, and they are still seeking others.

“But we know there are some that have a 1 or 2 percent effect, and it is likely there are a number of them that we can look for that will have even more of an effect.”

Even before the use of genomics, the genetics of dairy cattle have mainly focused on increasing milk production, along with improving milk composition, reducing disease and lameness, and increasing health and longevity.

In the case of the Holstein breed, a lot of focus was also on increasing size. Weigel noted that improvements to health, fertility and longevity have been modest because of the focus on physical traits, such as size.

Now, thanks to improved science in the field of genetics, functional traits such as calf health, early postpartum health and feed efficiency can be given more consideration.

Although the focus of genetics in dairy cattle in the last 20 years has not been on feed efficiency, Weigel said, “It appears that we have already captured most of the gains that can be achieved in feed efficiency simply by increasing the milk production per cow.”

But there are still potential gains that can be made.

Feed efficiency is a complex genetic trait, Weigel explained, noting that with dairy cows it is particularly challenging because of the need to balance lactation, reproduction, health and maintenance.

Weigel and other geneticists believe that using residual feed intake (RFI) may be the best way to use genomic selection for feed efficiency. RFI is a composite trait representing the sum of several other traits and numerous underlying physiological processes.

RFI is the amount a cow overconsumes or underconsumes feed compared to other cows in the same setting, after adjusting for milk production, bodyweight and bodyweight change. RFI is basically the observed dry matter intake of a cow minus the expected dry matter intake.

He explained that “selection for RFI is calculated to be phenotypically independent of milk yield and milk composition, so those traits should be unaffected.”

They are currently studying if RFI is also phenotypically independent of early postpartum health, productive longevity and female fertility.

Weigel said if RFI is found to be independent of those traits as well, calculations suggest that it could “warrant a relative economic weight that would represent 10 to 20 percent of the overall breeding goal.”

There may be other benefits to reducing RFI, Weigel explained. “The underlying biological changes associated with reducing RFI are largely unknown – is it better health, less methane, more efficiency of digestion or reduced heat loss?”

A cow’s efficiency of feed utilization varies throughout her life, but making the animal more profitable at any given time is beneficial to dairy producers. About 72 percent of the total lifetime feed cost for a dairy animal is spent while she is lactating, about 24 percent is spent while she is a growing heifer and 4 percent when she is dry.

Whenever a dairy cow is able to produce more milk on less feed it is valuable, and increasing feed efficiency will help producers to spend less on feed.

If breeding for feed efficiency were to reduce the amount of forage required by 1 pound per day for each of the nine million cows in the U.S., the scope of the impact would be very substantial, Weigel noted.

Although Holsteins have been bred for size for many years, Weigel noted that “it appears that the solution to improving feed efficiency in dairy cattle is not as simple as just selecting for smaller body size. At the same time, these data provide no evidence that selecting for larger body size will enhance feed efficiency.”

It is just as well that size doesn’t seem to matter with feed efficiency, as few bulls that sire below-average stature have semen available to dairy farms, he noted. Larger Holsteins can, however, create problems by outgrowing their housing or milking facilities, and Holsteins selected for large body size have significantly greater health costs, particularly for displaced abomasums.

“I don’t think we’ll make huge gains in feed efficiency by breeding for smaller cows, but we should definitely stop breeding for larger cows,” he said.

A modern dairy cow producing 100 pounds of milk a day needs four times more energy for milk production than she needs for maintenance.

Genetic selection and enhanced management have led to dramatic gains in production in recent decades, Weigel said. “However, it is important to recognize that as milk production per cow continues to increase, the marginal savings in feed costs per unit of milk produced are diminishing.”

As geneticists continue to work on RFI, dairy producers may be able to increase feed efficiency the same way they have been by breeding for milk production, he said. Weigel also noted that moderating the size of the animal may be helpful.

Genomic predictions can help dairy producers to become more efficient and serve as an effective way for a farm to become more profitable, Weigel noted.

The cost of raising replacement heifers represents 20 to 25 percent of the total cost of producing milk, and 60 to 65 percent of the costs are feed-related. Many farms now have an excess number of replacement heifers due to sexed semen and advances in cow comfort and animal husbandry.

“For the first time, many farmers are faced with the decision whether to rear all of the available heifers or to cull some potentially inferior heifers in order to reduce feed costs,” he said.

Studies show that inexpensive low-density genotyping done as young as 2 months old can accurately help a farmer to choose the right candidates for early culling. PD

Kelli Kaderly-Boylen is a freelance writer based in Waterville, Iowa.

PHOTO:Photo by PD staff.