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Leaner beef benefits the health conscious.

Leaner Beef Benefits the Health Conscious

Today's consumers are demanding lean beef, and ARS scientists across the nation are researching methods to help cattle producers meet those demands.

Current goals are to reduce outside carcass fat on meat to a minimum but keep 3 to 7 percent fat in the muscle to ensure good flavor.

Tommy L. Wheeler, a meat scientist at the Roman L. Hruska U.S. Meat Animal Research Center (MARC) at Clay Center, Nebraska, is using a growth-promoting compound known as L-644,969 in studies of muscle growth in beef animals.

Small amounts of this growth promoter have been fed to cattle at MARC, Wheeler says.

"We're not exactly sure how it works, but this compound causes the animals to produce more muscle and less fat," he notes.

As an animal grows, muscle in its body is continually produced and broken down. Levels of natural substances called creatinine and three-methyl histidine in the urine of MARC test animals give researchers an indication of total muscle mass and how much muscle is being degraded each day.

"We look at these rates to find the point in the animal's growth cycle when major changes are occurring," says Wheeler.

The scientists also search the muscle of slaughtered cattle for signs of enzyme activity, specifically the enzyme responsible for the natural breakdown of muscle. Wheeler hopes to delineate the relationship between muscle growth, rate of muscle degradation, and enzyme activity.

"We hope this work will enable us to understand the mechanics of muscle buildup so that we might manipulate the system to make all cattle lean and muscular," says Wheeler.

Across the country at Beltsville, Maryland, animal scientist Christopher K. Reynolds is looking at the impact of diet on the metabolism of steers that have high levels of growth hormone.

In cooperative studies with Agriculture Canada scientist Helene Lapierre and others, steers were injected with a naturally occurring compound that causes an increase in the level of growth hormone in the animal's body.

The steers were then fed one of two levels of a feed concentrate diet. "I wanted to see if the response to elevated growth hormone would differ according to the level of nutrition received by the animal," explains Reynolds.

As a steer grows, its body transforms dietary energy into either protein or fat. Protein deposition--which primarily translates to lean meat--can be estimated from the amount of nitrogen the animal's body has retained from its diet.

Since scientists know the amount of nitrogen in the diet they've fed the animal, they can calculate how much dietary nitrogen remains in the animal by checking the its feces and urine to see how much nitrogen is excreted.

In turn, fat energy deposition is calculated as the difference between the total energy retention and energy retained as protein.

In Reynolds' study, boosting growth hormone levels did not affect total energy retention. But the steers' nitrogen retention doubled, which means protein deposition also doubled. And since total energy retention did not change, the amount of fat deposited decreased.

The researchers concluded from this study that injections of compounds to boost growth hormone levels in the animal will increase body protein gain regardless of whether the animal is being fed a high or low level of nutrition.

Eschewing the Fat

The role of nutritional management in the production of lean beef is taking center stage as increasing attention is focused on finding various methods of producing low-fat meat.

"Nutrition has not been as much a factor in lean beef production in the past as it is now," says Lewis W. Smith, ARS National Program Leader for animal nutrition.

"In the past, producers tried to feed the cattle as much as possible to fatten the animals. Now they try to get growth without as much fat. It's more a question of what's not being done rather than what is done."

For example, producers are not putting cattle on high-energy feedlot diets as often as in the past but are using forage-based systems to reduce the amount of fat tissue deposited on the animals, notes Smith.

Finding the most efficient means of producing palatable lean beef in a range environment is one goal of ARS physiologist Bob Short's research at Fort Keogh in Miles City, Montana. Joining Short in this work are Fort Keogh colleagues Elaine E. Grings, a range nutritionist; range ecologist Rod K. Heitschmidt; and plant physiologist Marshall R. Haferkamp.

"Since consumers want leaner beef, we need to find ways to speed weight gain while minimizing the fat content," says Short.

Traditionally, producers use one of two feeding systems to raise cattle. After weaning, calves are generally sent either to a feedlot for finishing or to pastures where they graze before being sent to the feedlot.

Feeding grain to cattle, as is often the practice during the finishing stage, generally boosts the animal's rate of weight gain and fat deposition. Feeding more forage to finishing beef cattle decreases fat content in carcasses because high rates of gain and fat accumulation are more difficult on grasses.

"The problem is that range forage production is seasonal," Short explains. "High-quality forage may only be available for 2 months in some parts of the United States."

The Fort Keogh research program, dubbed the "lean meat" project, studies the effects of manipulating herd management practices and genetic variables to produce the leanest meat possible using range forage. Scientists will study eight variables, including the animal's genetic make-up, sex, age at weaning, and metabolic rate.

"We believe that most if not all of these eight variables can be manipulated to increase the efficiency of range forage systems," Short says. "Of those factors, the genetics of the animal and the feeding system are the two most important considerations."

Genetic traits have a tremendous impact on an animal's ability to produce lean meat. But within a genetic type, there are opportunities to alter body composition through management techniques, according to Samuel W. Coleman, an animal scientist at ARS' Forage and Livestock Research station at El Reno, Oklahoma.

Several years ago, scientists at the El Reno facility began studying growth rate patterns and body composition in Angus and Charolais steers.

"We wanted to see if body composition, altered by a period of restricted feeding, would influence an animal's ability to gain weight after it was restored to a normal diet," says Coleman.

Researchers found that for both Angus and Charolais, the size of the animal when it began eating a high-energy diet has a great influence on the resulting carcass composition.

For Angus steers, the age of the animal and its previous nutrition had little impact on the percentage of carcass fat if the steers were the same size when they started eating the high-energy feed concentrate. The animals that had weighed at least 800 pounds before changing diets were leaner.

"Results for Charolais steers were similar, but not as marked as those for the Angus," says Coleman.

That data led Coleman and Robert H. Gallavan, an animal physiologist, to their current project, determining how forage-based production affects the palatability and amount of meat produced.

In one study, a group of cattle ate a diet of 65 percent forage and 35 percent feed concentrates. A second group ate a diet of 85 percent feed concentrates and 15 percent forage.

Rates of gain were similar for the two groups until they reached 800 pounds. At that point, all the cattle were fed diets that were 85 percent feed concentrates.

Later, steers from each group were selected for slaughter according to body weight. "We wanted to know if the source of nutrition, independent of rate of gain, had an effect on body composition and organ size," says Coleman.

Of those slaughtered at 800 pounds, only 1 to 16 graded "choice," the quality preferred by most consumers. The carcasses were fairly lean and averaged 23 percent fat.

Seventy-five percent of the steers slaughtered 30 days later graded "choice," but were lean. When the animals were slaughtered at 1,000 pounds live weight, carcass fat was 29 percent; when they were slaughtered at 1,100 pounds, carcass fat was 32 percent.

Research at other locations has indicated that internal organs may enlarge when cattle are fed a forage diet. In the El Reno study, however, the weights of internal organs, such as the liver, rumen, and gastrointestinal tract, were unaffected.

Research data from the El Reno and Miles City studies are being relayed to a team of scientists in the Production Systems Research Unit at MARC. Gary L. Bennett, leader of the MARC research unit, is working with animal scientists John W. Keele and Charles B. Williams to develop a computer model that can predict the body composition of slaughter-weight cattle.

A variety of diet combinations might be fed to cattle, each resulting in different growth patterns. The vast array of diets makes it nearly impossible to comprehensively study the implications of all feeding systems; the model could help fill in the blanks.

"The computer program evaluates the effect of patterns of growth on body composition at slaughter," says Bennett. "It will ultimately enable producers to look at different combinations of feeding systems to find which will produce the best beef."

The model takes into account animal growth rates at each phase of a feeding cycle, as well as the number of days at that rate. The model then calculates fat and nonfat composition of the animal at slaughter.

Researchers hope the model will eventually be able to predict characteristics such as yield and quality grades that affect the value of cattle, says Keele.

Currently, the model is loosely based on the size and fatness of a Hereford-Angus cross. Adjustments to consider breed, gender, and other factors will broaden the program's potential applications.

Before any of these methods results in more lean beef at the local grocery store meat counter, the methods must be passed along to and accepted by cattle producers and the beef marketing system.

"Still, you have to produce what the consumer wants or you might as well forget it," says Lewis Smith.

PHOTO : Food technologist Tommy Wheeler demonstrates the difference in the amount of fat trimmed from lean and fatter beef top rounds. (K-4285-3)

PHOTO : Physiologist Bob Short measures the rib eye steak portion of a beef carcass. (K-4335-6)

PHOTO : Range nutritionist Elaine Grings prepares to collect cattle waste for analysis in studies to measure forage intake. (K-4336-5)

PHOTO : Technician Brooke Balsam prepares ground beef for fat analysis. (K-4332-5)
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Author:Gerrietts, Marcie
Publication:Agricultural Research
Date:Jan 1, 1992
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