Nutrition and Feeding Management to Alter Carcass Composition of Pigs and Cattle

Virgil W. Hays; Rodney L. Preston

I Introduction

Excess fatness in meat produced in the U.S. is largely the result of unrealistic grading standards that encourage the “over-fattening” of lambs and cattle and does not adequately discourage over-fat carcasses in swine (National Research Council, 1988a, p. 100). In addition, live animal ideals have been perpetuated which are contrary to the production of lean, efficient meat animals. As long as carcass quality grade for cattle is driven primarily by fat (marbling), dressing percent is a major determinant in pricing live meat animals, and feed grains continue to be relatively inexpensive; however, U.S. meat animals will continue to have too much fat. The first step in the “War on Fat” depends on improved methods of measuring meat quality (Preston, 1991). The incentive for altering nutrition and management to produce low fat meat will largely depend on how meat is graded and therefore the economic return to the producer, packer, and retailer. In the final analysis economic returns to the various segments of the industry depend on consumer demand for leaner meat. With the relatively low cost of feed grains and the current marketing and pricing systems that discount cattle carcasses not graded as choice and that do not greatly penalize over-fat pork carcasses, it usually is of economic advantage to feed high levels of high grain diets.

Carcasses consist of protein, fat, water, and ash. The major fraction of the ash portion is included in the bone. Though the muscle-to-bone ratio does vary some (Clawson et al., 1991), ash is the least important constituent in terms of edible meat. Normal market animals have a near constant water-to-protein ratio, approximately 3.3–3.5 to 1. This ratio is somewhat wider in young animals, but does not vary appreciably with size at maturity or among breeds and is not affected by feeding regimens. On a fat-free basis, the composition of animals (except for the very young) is rather constant. This is Moulton’s (1923) concept of chemical maturity.

Growth is a coordinated increase in protein, water, and fat, not the growth of lean and then the growth of fat. Fox and Black (1984) presented the following equations to depict the increases in water, protein, and fat with increasing empty body weight of British breed beef cattle (Figure 1),

kg=3.588+0.0671X−0.00034X2proteinkg=−2.418+0.235X−0.00013X2fatkg=−0.610+0.037X+0.00054X2,

where X is the empty body weight of cattle. Coefficients of determination (R2) for these equations ranged from 95.6 to 98.9%. The major determinant of body composition is empty body weight; however, cattle varying in biological type (frame size which reflects mature body size) differ in weights at which they reach a given fat content. In cattle, there is a near linear increase in protein with increasing weight up to about 500 kg after which protein increases at a decreasing rate; whereas, fat increases at an increasing rate.

The introduction of exotic (large-framed) cattle has resulted in changes in body composition because of their differences in size at maturity. Animals will be of similar composition (% fat) at similar proportions of their mature body weight (Figure 2) (Preston, 1971; Owens et al., 1993). As Figures 2 and 3 (Solis et al., 1989) depict, steers will have approximately 22% carcass fat when their slaughter weight is about 68% of their mature weight (average weight of sire and dam). At 78 and 88% of mature weight, their carcasses will contain approximately 26 and 30% fat, respectively (Figure 2). To grade choice, carcasses must have about 31% fat (Fox and Black, 1984). For steers weighing 1000, 1500 or 2000 lb at maturity, their slaughter weight would need to be 880, 1320, and 1760 lb (88% of maturity), respectively, to grade choice. Thus at a given weight large-framed cattle will contain less fat than will lighter mature weight, British-type cattle.

Similar body composition relationships have been reported for pigs (Reid, 1971). The grading system for pigs, however, does not include the factor of marbling (intramuscular fat). Pigs have been selected for less fat at standard market weights. This selection process has likely selected also for larger mature size, thus we are slaughtering pigs at an earlier stage of maturity.

Rate of body weight gain, efficiency of gain, and composition of the gain are genetically controlled, as discussed in Chapter 1; but, each may be influenced markedly by the balance of nutrients in the diet, the rationing of the diet, and, to a lesser extent, other factors such as effective environmental temperature, exercise, and disease. Any essential nutrient may directly or indirectly affect body composition in terms of proportions of lean (muscle), fat, and bones, primarily through its influence on rate and/or pattern of growth. Extended and marked reductions in growth rate due to nutritional deficiencies, followed by ad libitum intake of an adequate diet, may result in excess fat deposition, hence a lower ratio of lean to fat in the carcass. Extended stunting of growth by disease or adverse environmental conditions other than nutrition, again followed by conditions that allow food intake to be resumed at a high level, may have similar effects.

II Impact of Energy Level

The most common and readily demonstrated effects of nutrition on carcass characteristics relate to biologically available energy (e.g., metabolizable energy) intake relative to intake of other essential nutrients. In pigs the metabolizable energy intake, the protein (essential amino acids) intake, and protein-to-energy ratio in the diet have marked effects on carcass composition at a standardized body weight. The related effects of protein and energy are not simple. Excess protein may be utilized almost as efficiently as carbohydrates for energy purposes. This is apparent from the estimated energy values of feedstuffs (National Research Council, 1984, 1988b; Ewan, 1991) and the rates of body weight gain and fat deposition on high protein diets as reported by Wagner et al. (1963). The metabolizable energy values of the high protein ingredient soybean meal are very similar to those of the high-carbohydrate ingredient corn (3.15 vs 3.25 and 3.76 vs 3.84 Mcal/kg for cattle and pigs, respectively). The differences are somewhat greater in terms of net energy values. For beef cattle the net energy values (Mcal/kg) for gain are 1.48 and 1. 55 for soybean meal and corn, respectively; and for pigs the respective net energy values are 1.96 and 2.58.

If adequate energy in the form of carbohydrates or fat is not provided, the animal will utilize protein for energy purposes at the expense of protein accretion. These interrelationships are exemplified in pigs by studies of Cunningham et al. (1962) as summarized in Table I. The low level of feeding (1.60 kg/day) of the low protein diet was inadequate in protein or energy for maximum protein retention. At this level of feeding, protein was being used for energy purposes. Additional protein resulted in some increase in total protein deposition, but a lesser increase in protein intake in combination with increased energy intake (higher feeding level) resulted in an even greater increase in protein retention.

In pigs, the effects of energy intake on performance and carcass composition may be illustrated by varying the total feed intake (restricted vs ad libitum feeding) or by varying the metabolizable energy density of the diet (by substituting fat for carbohydrate or highly digestible carbohydrates for more fibrous...