Feed Intake by Mike Mehren Ph.D.

Feed Intake
by Mike Mehren Ph.D.

A good friend and I were working over some feed alternatives for the cowherd in case the drought was so severe that he would have to feed much longer than normal.

One of the options was to feed grass seed straw. I had recommended that we should feed a mix of straw and alfalfa while the calves were still on the cows. He had calculated that feeding all the straw the cows needed would meet the needs of the cows for energy and protein. His estimate was that it would take 46 lbs of grass straw/cow/day to meet these needs. This is absolutely correct mathematically. Unfortunately cows don’t do much math, and their bodies cannot eat, digest, absorb, and metabolize that much grass straw in 24 hr.

Feed intake of cattle is controlled by a variety of factors. Any single one of them can make a big difference in what the cattle will eat. Let’s look at some of the factors that effect intake:

1. Odor. A musty or moldy odor will have an adverse effect on intake. Hopefully the person feeding the animals doesn’t force them to eat moldy feed rather than starve. Our sense of smell does not match the cow’s. For instance, ammoniated straw has a very strong smell when it is first removed from the bag. However, cows will dive into that straw while it is still burning our eyes. Some of the finishing rations using a lot of waste products do not smell appetizing to us, yet cattle eat them quite well.
2. Taste. Human taste buds are far-removed from those of cattle. To give that theory a test, just put a little livestock cane or beet molasses on your finger tip and lick it off. Better yet, drink a glass full and you will soon detect a strong bitter flavor that we think of as ‘ sweet to the taste’, and that cattle consume readily. I have many friends that believe they can determine whether their cows will like a mineral just by tasting it themselves. This works about as well as the molasses test. On some feeds, and alkali ground, cattle will eat little, if any, minerals with salt in them. Yet those same cattle will crave salt after they are moved to different feed in a non-alkali area.
3. Gut Fill. After an animal eats a meal, it takes a considerable amount of time for the digestive tract to process the meal. The time it takes varies depending on factors such as fineness of grind, starch content, fiber content, and moisture. A high energy finishing meal moves down the tract more rapidly than does a meal of mostly straw and hay. The hay requires more chewing, mixing in the gut, and follow-up cud chewing before it is ready to move on through the system.
4. Age. Older animals have a greater capacity to consume feed than younger animals when intake is expressed per unit of bodyweight.
5. Weather. Very hot weather and very cold weather have an impact on feed intake. Just before a winter storm cattle will increase their intake very dramatically, and once the storm passes, intake will go back down. During hot weather, feed intake is depressed, and daily eating patterns change, so that feeding very early in the day and at or after dusk may be advantageous.
6. Healthy vs. Sick. One of the earliest signs of some kind of physical problem is reduced feed intake. This occurs several hours before any visual symptom become apparent.
7. Opportunity to eat. Is there enough bunk space, can the animal actually get its mouth into the place where the feed is. For instance: small calves trying to eat out of feed bunks designed for yearlings. Does the animal have to cover 2 miles of range just to find enough forage to survive?
8. Chemical and physical factors in the animal’s body. This brief statement covers many complex reactions in the body that deal with digestion of the feed, absorption into the body, and utilization by the animal. A description of these processes is
beyond the scope of this column. Please don’t take this to mean that they aren’t every bit as important as the other factors mentioned.

We do have a ‘rule of thumb’ that is used to put us in the ballpark of what an animal might eat. The rule is that a ruminant animal will eat about 3% of its bodyweight in feed dry matter daily. Thus a 1200 lb range cow would eat 36 lbs. of dry matter
(1200 lb x 3% = 36 lb).

FEED ID	DRY MATTER, %	LBS OF FEED EATEN DAILY
SPRING GRASS	20	180
ALFALFA HAY 20% PROTEIN	90	40

Each example above provides the same amount of dry feed. To arrive at the lbs. of feed eaten daily, divide 36 lb by feed dry matter percent. (36 lb ? 0.20 = 180 lb) for spring grass.

Notice that the example didn’t show straw or other low quality roughages. Research from experiment stations in the West has shown that cows will consume 1.6% of their bodyweight daily of a low quality forage such as dry mature range grass or straw.
They have also noted, that when the low quality forage is supplemented with protein, that intake will go up to 1.8% of bodyweight. For that same 1200 lb cow, she would now eat 19.2 LBS of grass that had protein of 5% or less, and would eat 21.6 lb of that same grass if a protein supplement was fed.

FEED ID	DRY MATTER, %	LBS OF FEED EATEN DAILY
MATURE GRASS	65	30
MATURE GRASS WITH PROTEIN SUPPLEMENT	65	33

That represents a 10% increase in forage intake in addition to the amount of protein supplement eaten. That can make the difference between maintaining the weight of a cow or her losing weight.

From these two examples we can conclude that cattle don’t eat 3% of their bodyweight of all feeds all of the time. The 3% might be used for a feed budget to allow for waste and an extended feeding season. Cattle waste quite a bit of feed when fed on the ground. More feed is lost when bales of hay or straw are kept outside for any length of time.

One factor not mentioned in those that effect feed intake is individual preference. This is hard to detect when you feed several hundred animals at a time. However, any 4H or FFA youngster that has fed a couple of steers or heifers can tell you about a set of animals where one always ate too much and the other wouldn’t eat enough to gain very well.

Achieving the feed intake to meet a goal is as much art as it is science. In some instances we hope to limit the intake of a feed, while at other times we want to maximize intake. Both circumstances require some degree of management. Since feed is the largest single expense there is in raising livestock, good feeding practices can make a huge difference in money made or lost. Cows don’t display ‘nutritional wisdom’; that’s left up to us!

Michael J. Mehren, PhD. is a livestock nutritionist whining about the wind in Hermiston, Oregon. He may be reached at: mehrens@eotnet.net.