What Does a Cow Need? A Review of Nutrient Requirements Emalee K. B. Buttrey, M.S. Extension Beef Cattle Assistant Texas AgriLife Extension Service, Amarillo Introduction Profitability and production efficiency are common objectives for many cattle producers. Feed represents the majority of total costs associated with livestock production. As fuel and feed prices continue to rise many producers are trying to lower costs wherever possible. An adequate nutrition program for cow‐calf producers should meet the cow’s nutrient requirements at a minimum cost. It is important to understand the biological priority for nutrients by beef cows. Nutrients will first be used for maintenance of the animal. The second priority is growth, followed by milk production, and finally, reproduction. A review of nutrient requirements and factors affecting requirements may help producers make informed decisions about supplementation and feeding practices, potentially optimizing production while reducing costs. Annual Cow Production Cycle The annual production cycle of a beef cow can be divided into four periods or phases. Because each period is physiologically distinct, nutrient requirements (energy, protein, etc.; Table 1) change as the cow goes from one period to the next. 1) Post‐partum, Open and Lactating This 80‐85 day phase is the most nutritionally demanding. During this time the cow must nurse a calf, repair her reproductive tract, resume normal heat cycles, breed, graze, and if young, grow. Failure to meet nutritional demands during this phase may result in lower milk production and thus lighter calf weaning weights, and lower conception and pregnancy rates. 2) Pregnant and Lactating Nutrient requirements during this 120‐130 day period remain relatively high, however energy and protein needs are lower as compared to period 1. Milk production will begin to decline during this phase. Early fetal growth has little effect on requirements; however, young cows are still growing. Some weight loss during this phase is common. 3) Mid‐gestation, Dry This phase, lasting about 70 days, immediately follows weaning and is when nutrient requirements are at their lowest. Milk production has ceased, fetal growth demands are still low, and activity has decreased. This is a good time to put weight on thin cows and increase body condition. 4) Late‐gestation, Dry This is the 90 days prior to calving during which the majority of fetal growth occurs. Cows should reach or maintain a BCS of 5 or 6 to ensure healthy calves and the ability to breed back more quickly. Additionally, cows are preparing for lactation. Intake may decline at this time due to the fetus and reproductive organs taking up space normally occupied by the rumen. Table 1. Daily nutrient requirements for an 1,100 pound beef cow producing 15 pounds of milk per day (adapted from NRC, 1984) Item TDN, lb/d NEm, Mcal/d CP, lb/d Ca, g/d P, g/d 1 14.5 14.9 2.3 33 25 Nutritional Periods 2 3 11.5 9.5 12.2 9.2 1.9 1.4 27 17 22 17 4 11.2 10.3 1.6 25 20 Factors Affecting Nutrient Requirements In addition to the stage of production of the cow, there are other factors that can affect nutrient requirements throughout the year. Primary factors include: Weather‐ Cattle have a “thermoneutral zone” in which the temperature is neither too hot nor too cold. Nutrient requirements increase with temperatures above (heat stress) or below (cold stress) this zone. Body Size‐ Total nutrient requirements increase as body size increases. Milk Production Level‐ Greater milk production increases nutrient needs. Milking Potential‐ Cows with higher lactation potential have greater maintenance requirements that persist year‐round. Growth Potential‐ Cattle with increased growth potential have greater maintenance requirements that persist year‐round. Age‐ Young females that are still growing have higher nutrient requirements than older cows. Activity‐ Grazing cows will have higher maintenance requirements than cows in a drylot. Energy Requirements Two systems are commonly used to express energy requirements: total digestible nutrients (TDN) or net energy (NE). TDN is a measure of the digestible portion of a feed, reflecting the relative energy value of the feedstuff. It is usually expressed as a percentage (%) or pounds per day (lb/d). For example, #2 corn has a TDN value of 90% according to the NRC (2004), which means in 100 pounds of corn, there are 90 pounds of digestible nutrients. The NE system assigns energy values to a feed according to how the feed will meet energy needs for maintenance, gain, and lactation. NE values are expressed as mega‐calories (millions of calories) per pound (Mcal/lb) or mega‐calories per day (Mcal/d). As previously mentioned, energy requirements will change throughout the year and are affected by several factors. As a general rule of thumb, for every 100 pound increase or decrease in body weight, TDN requirements increase or decrease approximately 0.5 pounds per day. Examples are presented in Tables 2 and 3. Protein Requirements Protein requirements are generally expressed in terms of crude protein (CP), either as a percentage (%) or pounds per day (lb/d). Crude protein is calculated as the amount of nitrogen in the feed times 6.25. There are two sources of protein for the cow: that in the feed they consume and from microorganisms in the rumen. Protein in feedstuffs may be further divided into two fractions. Degradable intake protein (DIP) refers to the fraction that is digested in the rumen. Undegradable intake protein (UIP) is the fraction not digested in the rumen but rather in the small intestine. UIP is often referred to as “escape” or “bypass” protein. Quite often, cattle performance is limited by energy intake. Many times, however, energy intake is limited by protein content of the diet. Forages containing less than 7% CP are associated with decreased dry matter intake because microbial fermentation of the forage is limited by protein. Protein supplementation on these types of forages will generally improve intake. As with energy, protein requirements will change (Tables 2 and 3). In general, CP requirements increase or decrease 0.1 pounds per day for every 100 pound increase or decrease in body weight. Table 2. Relationship between cow weight and nutrient requirements* Requirement Cow Weight, lb 1000 1100 1200 TDN, lb/d 8.8 9.5 10.1 NEm, Mcal/d CP, lb/d
7.57 1.3 8.13 1.4 8.68 1.5 *Dry, mature cow (body condition score 5) in middle‐third of pregnancy; NRC (1984) Table 3. Relationship between milk production and nutrient requirements of an 1,100 pound cow (body condition score 5)* Daily milk production, lb 10 15 20 Requirement TDN, lb/d 12.1 13.3 14.5 *Adapted from NRC (1984) NEm, Mcal/d CP, lb/d
11.54 2.0 13.24 2.3 14.94 2.6 Water, Minerals, and Vitamins Water is a very important, yet often overlooked nutrient that is required for many vital functions in the body. Cattle get some water through feedstuffs, however most water is consumed through drinking. A clean, readily accessible water source should be available at all times. Mineral and vitamin requirements will be covered in another presentation. Summary When developing a nutritional program for your herd, it is important that you assess your feed resources and production objectives. A basic understanding of the annual cow cycle and nutrient requirements will help guide your decisions. References NRC. 1984. Nutritional Requirements of Beef Cattle (6th Ed.). National Academy Press, Washington, DC.