A beef cow requires energy, protein, minerals and vitamins in its diet.
What determines how much of these nutrients are required?
What determines if they need to be supplemented in the diet?
Many factors affect the amounts of required nutrients. A female performs many functions – body maintenance, activity, weight gain, reproduction, and milk production – that all require nutrients. The amount of nutrients required depends on body size, environmental conditions, how far an animal travels, desired rate of gain, stage of gestation and level of milk production.
The nutritional value and quantity of available forage determine if nutrients need to be supplemented in the diet. During most of the year, warm-season forages are likely to be deficient in some minerals, especially phosphorus and certain trace elements like copper and zinc.
In most situations, supplementation should include at least year-round provision of salt and a mineral with 8-12 % phosphorus and a similar level of calcium.
Vitamin A, which usually is low in dry or weathered forages, should be injected or fed in mineral or other supplements if it is suspected to be deficient. Mineral and vitamin supplementation should be a high priority because deficiencies can be corrected for relatively little cost.
After addressing mineral and vitamin needs, protein and energy deficiencies must be considered. Forage protein and energy vary seasonally. Warm-season forage typically becomes deficient in protein in mid-summer and again in winter. Forage lacks adequate energy content primarily in winter, but energy available to the animal is restricted more often by a limited supply of forage rather than by deficiencies in plant composition. There are six critical factors that affect supplementation needs.
The amount of available forage obviously affects the need for supplemental feed. If grazing or hay will be limited, take immediate action. Reduce the number of animals in order to lessen the need for supplemental feeding of the remaining cows. As forage supply declines, the opportunity for animals to selectively graze decreases, and so does diet quality. Then, supplementation may become necessary even if animal numbers are reduced.
Poor-quality forage has less than 6-7% crude protein (CP) and is low in digestibility, with less than 50% total digestible nutrients (TDN). These deficiencies limit the amount of such forage that an animal can eat. Because both consumption and nutrient content of poor-quality forage are low, supplemental needs are high. Medium-quality forage (7-11% CP, 50-57% TDN) eliminates or significantly reduces the need for supplementation. High-quality forage (above 12-14% CP and 57% TDN) can be consumed in the largest amounts and usually removes any need for supplementation, except possibly for high-milking cows in low body condition. However, forage that is high in quality but low in quantity, a common situation in early spring, increases the need for supplementation of dietary bulk and energy. The amount a cow can eat in a day ranges from as little as 1.5% of body weight for very low-quality forage to near 3% for very high-quality forage. The typical amount is 2-2.5%.
The level of body condition (amount of fat) affects supplemental requirements. Low body condition markedly increases the need for supplemental nutrients, and meeting such needs often is cost-prohibitive. Moderate body condition significantly reduces or eliminates the need for supplements. Fleshy cows generally need little if any supplement, and the daily amount of forage required often can be reduced. If forage consumption is not reduced, higher production is possible or reserves of stored body energy can be maintained.
The potential for forage consumption is related to body size, so larger animals may not require more supplement than smaller ones. Adjustments in stocking rate, to allow adequate amounts of forage per cow, may offset differences in size but will increase the cost per cow. But if forage is sparse or limited, larger cows require proportionately more supplement.
Higher-milking cows can consume somewhat more forage but not enough to completely satisfy extra needs. When forage quality is inadequate, higher-milking cows need more supplement; 50-100% more may be required for high versus low milk production in cows of the same body size.
Young animals are still growing and require extra nutrients, but their body size is not as large as mature animals. Because of their smaller body size, growing heifers cannot consume as much forage as mature cows. For these reasons, young females require higher-quality diets than mature cows and often require more and different supplements.
Factors and Feeds for Supplementing Beef Cows; Stephen P. Hammack and Ronald J. Gill, Extension Beef Cattle Specialist and Extension Livestock Specialist, The Texas A&M University System.