The essential nutrients required by grazing animals are water, energy, protein, minerals, and vitamins. These nutrients are needed to maintain body weight, growth, reproduction, lactation, and health. There are other factors that affect nutritional requirements.
Water is essential for all livestock, and producers should plan for an adequate supply of clean water when designing any type of livestock enterprise. Dirty, stagnant water can lead to inadequate water consumption, which will reduce feed and forage intake and compromise livestock performance. The amount of water required depends on the physiological stage of the animal and the climate. Lactating animals require more water, and the amount of water required increases as atmospheric temperature increases. For example, at temperatures above 35°C (95°F), cattle require about 8 to 15 liters (2.1 to 4 gallons) of water per kg (2.2 lb) of dry matter intake. Generally, cattle require ~2.6% of their body weight in dry matter (DM) intake per day. Therefore, a 1000 lb cow could require as much as 175 liters or 45 gallons of water a day! Daily water consumption of ewes will vary from 0.75 to 1.5 gallons depending on climate and stage of gestation. Water availability should be closely monitored because a deficiency in water will result in death much faster than a deficiency of any other nutrient.
In most situations, the amount of protein supplied in the diet is more critical than the quality of the protein. Ruminants have the ability to convert low-quality protein sources to high-quality proteins through bacterial action. Microbial protein synthesis is sufficient to supply the protein needs as long as adequate precursors are supplied, except during lactation for high milk producing animals. Protein is required by all grazing animals for tissue growth and repair. Protein required for a 1000 lb nonlactating cow is around 1.6 lb/day or 7% crude protein in the diet. When the cow is lactating, 2.0 lb or 9.6% dietary crude protein is required. If protein is deficient in the diet, grazing animals must break down body tissue to obtain sufficient protein. A protein-deficient animal must break down 6.7 lb of lean body tissue to supply 1 lb of protein, resulting in severe weight loss.
Insufficient energy probably limits performance of livestock more than any other nutritional deficiency. Energy requirements vary greatly with stage of production, and adequate amounts of energy are extremely important during late gestation and early lactation. Energy deficiencies can cause reduced growth rate, loss of weight, reduced fertility, lowered milk production, and reduced wool quantity and quality. Energy is obtained from carbohydrates in the plant material and can be stored in the form of body lipids. However, heavy demands against fat stores as an energy source to meet daily needs may delay estrus and reduce conception in breeding females. Live weight gain can only occur after the animal’s energy requirements for maintenance and lactation are met.
Vitamins and Minerals
Ruminants require all the fat-soluble vitamins (A, D, E, and K), but they can synthesize the B vitamins in their rumen. Normally, the forage and feed supply contain all essential vitamins in adequate amounts, except vitamin A which is obtained as carotene from green plants and is often deficient in dormant forage. However, vitamin A can be stored in the liver in amounts sufficient to last considerable periods of time, such as winter dormancy or prolonged drought. Salt is essential for many body functions and important to maintain intake of feeds and water. Calcium and phosphorus are needed to maintain growth, feed consumption, normal bone development, and reproductive efficiency. Other nutrients and minerals such as vitamin E and selenium are important for maintenance of healthy bodies and reproduction.