Have you yet come across the acronym RFI? Of course, you say, as you run and check the internet: Radio France Internationale, request for information, the RFI app for Android smartphones and also RFI Corp., a high-tech electronics company in the aerospace industry. These are all real, but RFI also means something quite important to us in the cattle industry. RFI stands for relative feed intake. (This has nothing to do with your cousin’s turkey consumption at Thanksgiving.) It has been the focus of quite a lot of recent research. It’s a topic of growing importance for feeding, pastures and breeding. Let’s learn more about it.
The concept of RFI is simple: it is the difference between the actual feed intake of an animal and its predicted feed intake. (To be mathematically precise: RFI = actual intake – predicted intake). The actual feed intake is a measured amount. The predicted feed intake is a number derived from equations that include the animal’s bodyweight and level and type of production (weight gain, milk production, etc.). A lot of research has gone into deriving these prediction equations, and they are probably not too bad. RFI is a measure of feed efficiency. It’s more precise and useful than anything used previously.
We are all familiar with the traditional feed-to-gain ratio (F/G ratio). This is the amount of feed per unit of gain. Just measure the amount of feed consumed for any time period and divide it by the number of pounds gained during that period. This is easy to do for a pen of animals, but it gives no information about individual animals within that pen.
Determining values
To get individual values, you have to measure individual intakes, and that presents a serious problem. Since cattle are herd animals, housing them in solo crates to obtain individual intakes causes stress and changes behavior. The isolation affects feed intake and weight gain. The resulting numbers are scientifically suspect, and they are also very cumbersome and time-consuming to obtain.
RFI is quite different. A few years ago, a private company developed the GrowSafe System for feeding animals. This is a sophisticated feeding unit where animals are housed in a group pen and feed is offered in a feed trough. Each animal bears a high-tech radio frequency ear tag. Every time an animal goes to the trough and eats, its information is automatically transmitted to a computer, including individual ID, time of eating, amount consumed, etc.
Computers then distill and process this data, and the resulting numbers represent an accurate measurement of individual feed consumption. The GrowSafe System was an engineering breakthrough for scientists. Although not cheap, this equipment has been installed in research facilities in more than 20 states and many other countries.
Armed with valid individual feed intakes, researchers could now calculate RFI values. Let’s look at the numbers. An animal with an RFI of 0 means that its actual feed intake exactly matched its expected feed intake. But researchers found very few animals like this. In reality, RFI values ranged across an entire spectrum from low to high.
Many animals had RFI values lower than 0, which meant they ate less than expected and thus were more efficient than the average animal. Some animals had RFI values greater than 0, which meant they ate more than expected and thus were less efficient than other animals.
For the first time, researchers could rank animals based on feed efficiency values derived from dependable feed intake amounts. Not only that, researchers found that these RFI values were generally repeatable throughout the animal’s lifetime and were also moderately heritable, which means animals tend to pass this trait to their progeny.
Another thing that nutritionists like, RFI is independent of body size or performance. RFI can be calculated on many different types of animals without introducing a subtle bias to the numbers. In contrast, F/G ratios are not independent of bodyweight. F/G ratios tend to favor larger animals.
The reason is that weight gain in growing calves contains a combination of lean and fat, and it takes less nutritional energy to deposit lean tissue than fat tissue. When measured at the same weight during their growth period, animals with a larger mature body size will tend to be leaner than animals with a smaller mature size. Therefore, animals with a larger mature size will tend to show better feed efficiency at that weight than animals with a smaller mature size. RFI does not have this drawback.
So what’s so great about this RFI number, aside from making nutritional researchers happy? Simply, RFI identifies the most feed efficient and least feed efficient individuals in a herd – an obvious economic trait for genetic selection.
Differences in efficiency
But I must also ask a much broader question about RFI: Why are some individuals more efficient at using feed than others? This is not a simple question. If there are RFI differences between animals, there must be some biochemical and/or anatomical mechanisms to explain those differences. Unfortunately, our current knowledge is not complete, but we do appreciate that a number of factors may influence the variability in RFI. Here are some of them:
Protein turnover rates: Many proteins in the body are not permanent. They periodically must be replaced by freshly made proteins, kind of like when we routinely replace the brake pads in a car. Replacing proteins is called protein turnover, and this comes with an energy cost for synthesizing new proteins, transporting them, degrading old proteins, etc. Although the reasons are unclear, some individual animals have lower protein turnover rates than others, and this lowers their RFI values.
Heat production: Some animals burn more calories than others for the same amount of production. Why? Maybe this is related to the relative amounts of muscle and fat in the animal, or differential rates of mitochondrial respiration (use of oxygen), or whatever. Here’s something I’ll throw in: Two years ago I wrote a column about the existence of brown fat in adults and its possible relation to the “easy-keeper” individuals we all observe on farms. Brown fat specifically burns more calories than other tissues. Hmm. It’s an interesting hypothesis, if I do say so myself.
Feeding behavior: Some individuals eat faster than others, some eat slower. Feed intake has an energy cost associated with it, and differences in feed intake behavior imply differences in energy usage. There is some research data that supports this.
Digestibility: The digestibility of a feed is not a physical absolute like the speed of light. In addition to the obvious factors of fiber and starch composition, there is some variation between animals. (We tend to ignore this during nutrition courses.) For example, some animals will produce more methane from a feed than other animals. This may be related to differences in the microbial populations in the rumen. Using new techniques for DNA analysis, researchers are just beginning to get a handle on the complexities of rumen ecology and methane production. This is an active area of research.
Activity: Some animals are more nervous than others; not exactly earthshaking news. We all know animals who continually walk fencelines (or leap over them), animals with extraordinarily large flight zones, animals that are particularly “flighty.” Every herd has individuals like this. Sometimes we give those individuals names that are unrepeatable in public. Nonetheless, active animals tend to burn more calories than calmer individuals. Like the old song says, “Slow down, you move too fast / You got to make the morning last.”
These factors are all interrelated. For example, higher energy usage due to increased nervousness could stimulate appetite, so the animal wants to eat more. Although it’s usually hard to get cattle to fill out questionnaires, we can postulate about these interactions. But one thing stands out: many of these factors are based on DNA. Although we don’t yet know the mechanism details, there are definitely some genes that control the factors that influence RFI.
So if we wanted to use RFI values to increase the efficiency of our cattle operations, how could we do it? We could, of course, select genetic lines that consistently have low RFI values. Which means, if we carry this to its logical conclusion, that we could save pasture because our cattle would rarely walk out there. They would spend their time quietly standing around, or more likely sitting, and chewing their cud s-l-o-w-l-y. Never running, never getting stressed out. We would breed for cattle with a dreamy, zenlike temperament. A true use of grass.
