Count him out on RFV

By Mike Rankin

“If you still think relative feed value (RFV) is a good measure of forage quality, I’m not going to be able to help you today.”

That was the opening disclaimer offered by Duarte Diaz, extension dairy specialist with the University of Arizona, to the annual meeting of the Northeast Chapter of the American Registry of Professional Animal Scientists (ARPAS) held in Harrisburg, Pa., last week.

In his position, Diaz often finds himself interfacing between dairy producers and their alfalfa growers. It’s not always an easy conversation when alfalfa pricing is discussed.

“One of the issues is that growers still want to talk in terms of RFV and the dairy nutritionist is more concerned about other fiber digestibility assays,” Diaz said. “Further, the alfalfa grower wants to be compensated for higher quality forage because, if it is cut earlier, yield is being sacrificed.”

Diaz explained that more and more of these conversations are occurring within the same family, where one member is responsible for the dairy while the other oversees the cropping enterprise.

Alfalfa is a big deal in Arizona; in fact, it’s the state’s number one crop. “What makes our state unique is that we feed more greenchopped alfalfa than anywhere else in the U.S.,” Diaz said. “We have many large dairies that feed no dry hay — it’s all greenchop and perhaps a little haylage that is harvested from fields cut 11 or 12 times per year.”

Highly digestible alfalfa

Diaz sees a lot of value in new alfalfa technologies that tout reduced lignin concentrations and higher fiber digestibility.

Duarte Diaz

These new alfalfa varieties offer us better flexibility and the ability to manage risk if a cutting gets delayed,” Diaz said. “Though this forage should provide more energy to the dairy cow, I see the biggest potential advantage in terms of greater dry matter intake.”

Diaz noted that for every 1-pound increase in dry matter intake, a cow has the potential to produce an additional 2.5 pounds of fat-corrected milk. However, he explained that the benefit would hinge on the cow’s stage of lactation, body condition, and rumen fill.

“Our biggest milk responses from feeding highly digestible alfalfa will come with cows in their first 150 days of lactation and in good body condition,” Diaz noted. “The benefits of improved forage quality don’t always translate into improved milk production. For example, some cows that are in poor body condition may partition the additional energy into body weight gain. That doesn’t mean you didn’t capture a benefit, it’s just that it didn’t come in additional milk,” he explained.

Through the advantages of reduced lignin, highly digestible alfalfa varieties to the dairy cow are clear, Diaz believes more feeding research is needed to document the responses and help develop feeding systems.

“One advantage that doesn’t get talked about a lot is the potential for reduced-manure production,” Diaz said. “This could be a big factor in areas where environmental pressures are placed on the dairy.”

Is there a seller benefit?

Diaz said it’s not always easy to convince an alfalfa grower that economic value can be realized from investing in the new alfalfa technologies.

“If a grower is dead set on using RFV, then they probably won’t realize an economic gain because they aren’t measuring for the higher digestibility,” Diaz asserted. “All that’s being measured are neutral detergent fiber (NDF) and acid detergent fiber (ADF) concentrations.”

At a minimum, Diaz believes relative forage quality (RFQ) should be used. “It’s not the best answer, but it’s better than RFV,” he said.

Diaz also pointed to measures such as total tract neutral detergent fiber digestibility (TTNDFD), percent acid detergent lignin (ADL), and various time points of NDF digestibility as improved assays that are being used in the industry to assess forage quality.

The situation gets more complicated because there are regional preferences among both growers and dairy scientists.

“When I talk to my colleagues, there is not often agreement about what is the best single assay to quantify alfalfa quality,” Diaz said. “Often, they want several measurements, so I’m not to a point where I can say ‘this’ is the best method. What I’m saying is that if you don’t have one method that accounts for digestibility, then you’re falling behind,” he added.

Diaz noted that one of our inherent issues is that alfalfa growers are often concerned with different factors than nutritionists are concerned with.

“Those two disciplines are strongly linked and each probably needs to do a better job of understanding the other,” Diaz said. “I require all of my dairy nutrition graduate students to also get a minor in agronomy.”