NUTRITION

More than gain, look to beef production efficiency
By Dr. Jim White

Beef feed efficiency is important; feed cost is usually reported to be 65 to 70 percent of the total cost of beef gain. Given that the principal cost of producing beef is feed, there is tremendous interest in having the most profitable feeding strategies. In the past, we would feed animals, weigh the feed and the animals, then determine how many pounds of feed were offered per pound of gain. This gave us feed-to-gain ratio, and it is a pretty good number when looked at from the perspective of Jim hauling out buckets of feed to the calves.

As handy as feed-to-gain is, there are some concerns. There is interest in selecting animals not so much on average daily gain or feed-to-gain but rather on “residual feed consumption” (RFC), also called “residual feed intake” (RFI). The RFC or RFI of an animal is basically the difference between the animal’s actual performance compared to the animal’s expected or calculated performance. The RFC method looks to select efficient beef animals by comparing their residual feed consumption rather than their feed-to-gain or average daily gain.

A decrease in feed-to-gain or increased ADG, which is better? In the bottom of my heart, I believe that the more feed I can get down a feedlot steer, the higher the animal’s ADG. I know that is not accurate, because I have restrict-fed steers, held ADG and improved feed efficiency. Over a wide range of energy concentrations, the correlation between feed intake and ADG is poor.

Over diets with the same energy concentration, particularly hot feedlot diets, the correlation is better—but you will see substantial scatter in the data. What this means is that while you might see higher feed intake giving higher ADG, you also see higher feed intake with lower ADG. Under most situations, decreasing feed-to-gain by a percent provides better economics than increasing ADG by a percent.

Certainly both are important, but the bigger effect tends to be with declining feed-to-gain ration rather than increasing ADG, while holding the other one constant. A producer would be about $20 a head better off by reducing feed-to-gain by 4 percent rather than improving ADG by 4 percent. Feed to gain is correlated with growth and frequently confounded with growth rate and animal maturity or implant programs.

To sort this all out, RFC consumption is the difference between the metabolizable energy intake and metabolizable energy needed for maintenance and gain: RFC = Intake - (energy for maintenance + energy for gain). Thus, if an animal had a modest gain, yet ate a small amount of feed, the RFC would be negative, because they ate much less than expected. In other words, they are very efficient, so the RFC would be large. You select for negative RFC. RFC is therefore independent of growth and maturity patterns.

When the animal’s energy intake equals the animal’s energy requirement, the energy requirements of the animal are “met” (the difference between the two is zero). Thus, a “positive” residual feed consumption means the animal’s energy intake exceeds its requirement for maintenance and growth. While I was feeding out Holstein steers, I noted that they would eat for fun, between bouts of trying to gnaw down my barn.

When the animal’s energy intake is less than the energy needed for maintenance and growth, the residual feed consumption is negative, which means that the animal either requires less energy than estimated or is eating less to produce the same weight gain (they are more efficient than we expected).

There are several factors that could go into explaining differences in RFC: genetics; diet energy or protein concentration or solubility; characteristics of diet ingredients; diet processing, etc. RFC will allow you to evaluate feeding programs more thoroughly than using the conventional parameters of dry matter intake, ADG and feed-to-gain.

While this is very handy for evaluating beef animals, it does not hold much promise for dairy animals. One study (Kennedy et al., 1993) evaluated the heritability of residual feed intake in dairy animals. They reported that the heritability of residual energy intake based on genotypic regression approached zero—meaning that RFI contributes little additional information beyond that which is provided by milk production.