THE MERCK VETERINARY MANUAL
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Performance Modifiers in Beef Cattle

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Also see Growth Promotants and Production Enhancers.

There are three groups of performance modifiers used in beef cattle production: hormone-like growth enhancers; ruminal chemistry modifiers, which alter volatile fatty acid production in the rumen; and β-agonists, which increase muscle.

Hormone-like growth enhancers for finishing beef cattle have been used for almost 70 yr. There are many implant choices, and the specific implant protocol needs to be tailored to the cattle genetics, nutrition, and marketing plan. The average increase in daily gain due to the use of effective implants is ~0.23 kg (0.5 lb)/day, and the improvement in the feed to gain ratio is 0.56 kg feed/kg gain.

Ionophores are antibiotics that alter the chemistry of the rumen by altering the rumen microflora to produce increased proportions of propionic acid and decreased proportions of acetic and butyric acids. These three acids, called volatile fatty acids (VFAs), are the products of ruminal fermentation and can be absorbed from the digestive tract of the cow and used as energy sources. Because propionic acid releases more energy per unit weight to the host cow upon oxidation than do the other two VFAs, it is important in beef cattle production.

In modern feedlot diets, ionophores improve feed efficiency by 2.5%–3.5%, decrease dry matter intake 3%, and increase average daily gain (ADG) 2.5%. A more dramatic improvement in ADG is seen in cattle fed ionophores on high-roughage diets than in cattle fed high-energy diets. This is explained by energy availability between the two systems. On high-energy diets, cattle eat until they meet energy requirements; thus, ionophores help them derive more energy per unit of ingested feed, and they eat less. On high-roughage diets, which have less energy per unit weight, cattle consume feed until the rumen will hold no more. In grazing or high forage feeding systems, if ionophores are fed, these cattle derive more energy per unit of feed consumed, and thus gain more. Additional benefits of feeding ionophores include improving the consistency of feed intake in cattle fed high-grain diets, thus reducing the incidence of ruminal acidosis along with control of coccidiosis.

β-Agonists are the newest of the performance modifiers used in feedlot operations. These feed additives are absorbed in the blood and distributed to muscle tissue, where they bind to specific β receptors. Their action results in increased protein synthesis, which results in increased muscle fiber size. The β-agonist ractopamine is fed for the final 28–42 days before slaughter, whereas zilpaterol is fed for the final 20-40 days before slaughter. Performance improvements of β-agonists include live slaughter wt 18–22 lbs, hot carcass wt 20–30 lbs, ADG (during β-agonist feeding period of 20–42 days) 8.5%–30%, feed efficiency (during β-agonist feeding period of 20–42 days) 12.5%–31%, and dressing percentage 0–1.5 units.

β-agonists can increase muscle shear force and reduce the percentage of cattle that grade prime and high choice, so use in cattle with low genetic potential for tenderness and/or marbling may be less advantageous.

Last full review/revision October 2014 by W. Mark Hilton, DVM, DABVP

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