Introduction

The variety of domesticated animal breeds within any given species existing worldwide indicates great diversity of body size and composition. In domesticated animals, intense genetic selection for body composition was not applied until the 20th century. In developed countries, genetic selection produced remarkable changes in the body composition of the common species raised for meat production, particularly in the latter decades of the century. Currently, cattle, pigs, and sheep are considerably more muscular and less fat than in earlier times. The impetus for these changes was to increase the economic return for the producer by increasing the marketable product, muscle, and concomitantly reducing the fat mass, a product whose value declined with the replacement of by-products made from fat by oil-derived moieties. During the last quarter of the 20th century, consumers began to demand that retail meat products have less fat, so fat was trimmed as a waste product and animals were bred for less fat mass.

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