Feeds Used And Conversion Rates

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The feeds utilized by food-producing animals include forages from nonarable lands, cultivated forages, crop residues such as straw, and a wide range of food- and fiber-processing by-products. Some of the feeds used and estimated global amounts (million metric tons or MMT) fed in a recent year include straws from wheat, maize, rice, and barley (652); oil meals (116); brans (106); and bagasse (80). Information on these and many other byproduct feeds are summarized in Ref. [3]. Forages, crop residues, and many of the by-products are human-inedible materials, while some of the oilmeals are potentially human-edible. These products vary widely in feeding value. Crop residues and by-products such as sugar cane bagasse provide energy, but are deficient in protein, while oilmeals are high in both energy and protein.

The conversion of human-inedible materials to human food by animals clearly adds to food supply. However, livestock are also fed cereal grains that could be consumed directly as human food. Currently, 600 700 MMT per year approximately one-third of total global cereal grain production is fed to animals. The proportion is higher in countries such as the United States and lower in developing countries. Feeding grain can reduce human food supply. The net effect of food animal production on human food supply therefore depends on the relative amounts of human-inedible and human-edible materials consumed, as well as on actual conversion rates. It has been estimated that, globally, 74 MMT of human-edible protein is fed to livestock, which produce 54 MMT of human food protein.[4] This gives an input/output ratio of 1.4:1, or an efficiency of about 70%. The biological value of animal protein, a measure reflecting digestibility and essential amino acid content and balance, is on average 1.4 times that of plant protein, suggesting no loss of human food protein value from livestock production.

Actual conversion rates are affected by product produced and level of technology involved in the production system, and particularly by the two related factors of type of digestive system and type of diet. Ruminant species, including cattle, sheep, goats, and buffaloes, typically have lower conversion rates, i.e., produce less product per unit of total feed intake than monogastric species such as pigs and poultry, but the diets of ruminant species on average contain a much higher proportion (often 100%) of human-inedible feeds. This is possible because of the four-compartment ruminant stomach, including the large rumen where microbial digestion takes place. Rumen microorganisms break down plant materials with high fiber content that monogastric animals (including humans) cannot digest, providing energy for the ruminant host. The microorganisms also synthesize essential amino acids from nonessential amino acids and nonprotein nitrogen, contributing to the protein nutrition of the host animal and of consumers of foods from these animals. Additionally, the rumen microflora synthesize some essential B vitamins and detoxify compounds such as gossypol in cottonseed, which are toxic to monogastric animals. As a result of these functions, ruminant animals are able to convert large amounts of human-inedible materials into nutritionally valuable human food.

Ruminant animals such as dairy cows and feedlot steers are often fed cereal grains and protein supplements as well, with the proportions of these in the diet determined largely by relative prices of different feeds. The inclusion of feeds that are higher in energy and protein and more digestible than many forages and by-products can markedly increase animal performance such as milk yield or growth rate. Thus, their inclusion in ruminant diets reduces the number of animals required for a given amount of meat or milk, with economic and environmental benefits. Also, the inclusion of some of these feeds in ruminant rations can lead to increased utilization of the nutrients in forages and by-products in those rations, by providing a more nutritionally balanced diet.

Pigs and poultry also consume some human-inedible materials such as milling by-products and damaged grains, but on average, their diets include much higher proportions of human-edible grains and protein supplements than ruminant diets. Pigs and poultry also have higher reproduction rates than ruminants, i.e., they produce more progeny per breeding animal and thus, fewer breeding animals are required for a given number of offspring marketed. This, along with the higher nutrient density diets, results in pigs and poultry having much better conversion rates than ruminants, based on total feed intake. However, when comparisons of output/input ratios are based on human-edible inputs, the differences between ruminants and nonruminants largely disappear and their ranking may be reversed.

Estimated conversion rates based on total and human-edible feed inputs for different species of animals, products, and production systems are summarized in Table 1.[5] These results show that, on the basis of total feed inputs, ruminant meat production is relatively inefficient, but those returns per unit of human-edible input can be higher for ruminants than for pigs or poultry. In some cases, the return is more than one unit of human food per unit of human-edible input. This is notably true for dairy production, where output/human-edible input ratios exceed 1.0 for all systems studied. This reflects the fact that milk production is a relatively efficient process as well as that much of the diet of these ruminants is human-inedible material. For beef production, the values of 7:1 to 12: 1 (conversion rates of 8 to 14%) sometimes cited as grain/meat ratios[6] are based on total feed rather than grain inputs, and on only the feedlot period, and thus are incorrect.

These data indicate that conversion rates in general are higher for U.S. livestock production than for other countries, reflecting use of more grain in rations and greater application of technology in the United States. The very high conversion rates for human edible inputs for ruminants in other countries reflect the fact that little grain or protein supplement is fed. One consequence is lower productivity, e.g., slower growth rates or lower milk production, resulting in lower output/total input ratios and larger numbers of animals kept for a given amount of product.

Another relevant point is that the primary food grains, wheat and rice, have lower yields than the primary feed grain, maize. Estimates of the numbers of people that could be fed with the grain now fed to livestock involve the implicit assumption that for each unit of grain not fed to animals, one unit would be available as human food. However, a systematic reduction over time in feeding of grain to livestock would undoubtedly result in a shift in grain crops grown, primarily from maize to wheat, resulting in a substantial reduction in total cereal grain

Table 1 Conversion rates of dietary energy and protein to human food energy and protein

Energy Protein

Table 1 Conversion rates of dietary energy and protein to human food energy and protein

Energy Protein

Product

Countries

Total input

Human-edible input

Total input

Human-edible input

Beef

U.S.A.

.07

.65

.08

1.19

Beef

3 countriesa

.05

7.63

.03

5.69

Pig meat

U.S.A.

.21

.31

.19

.29

Pig meat

5 countries1"

.16

.40

.10

.33

Poultry meat

U.S.A.

.19

.28

.31

.62

Poultry meat

5 countriesb

.19

.50

.32

1.29

Eggs

U.S.A.

.17

.24

.24

.36

Eggs

5 countriesb

.13

.26

.16

.45

Milk

U.S.A.

.25

1.07

.21

2.08

Milk

3 countriesa

.19

3.05

.15

5.67

"Argentina, Mexico, South Korea. bArgentina, Mexico, South Korea, Egypt, Kenya. (Data from Ref. [5].)

"Argentina, Mexico, South Korea. bArgentina, Mexico, South Korea, Egypt, Kenya. (Data from Ref. [5].)

supply.[5] Also, feed grains represent an important buffer for temporary food grain shortages.

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