By: Levi Trubenbach, Ph.D
Nutritionist, Livestock Nutrition Center
It’s a question we are asked regularly: “How much should I be feeding my cows?” It is a valid question with meaningful tradeoffs – on one hand, we want to minimize the largest variable cost in a cow-calf business, while on the other, we yearn for maximum reproductive efficiency. The truth is: finding yourself on either side of this delicate balance can have profound impacts on financial performance. Our goal for this article is to describe our approach in building optimal supplementation strategies for commercial cow herds.
Defining Nutrient Requirements – The first step in designing any cow supplementation program is to define nutrient requirements. The first question we will ask is, “What stage of production are they in?” Generally, what we need to know is an average timeframe during which cows are expected to calve, rebreed and wean a calf so we can estimate nutrient requirements through the current or future months. Immediately after calving, milk production increases demands for nutrients beyond basic maintenance needs. Approximately eight weeks post-calving, milk production peaks, declining slowly until calves are weaned. In a typical production system, cows rebreed approximately 80-90 days after calving, resulting in additional nutrient demands for fetal and maternal support tissues. While gestational nutrient requirements are relatively low during the first two trimesters, these requirements increase exponentially during the last two months of pregnancy. Figure 1 illustrates the dramatic differences in protein requirements throughout the reproductive cycle; a curve describing energy requirements would follow a similar pattern. At peak lactation, protein and energy requirements can be 80-100% greater than the period immediately following weaning.
Figure 1. Total crude protein requirements during each month post-calving for a 1,200-lb cow with average genetic potential for milk production
While the stage of production is the primary variable we use for estimating nutrient requirements, several additional factors can affect nutrient requirements, including cow age, weight, breed and, genetic potential for milk production. Under special circumstances (first-calf heifers, non-beef breed types, extreme body size, poor previous plane of nutrition, etc.) we will adjust nutrient requirements, but under most conditions, using average values is an appropriate approach. For additional information on estimating nutrient requirements, we would refer you to one of two extension articles produced by the University of Arkansas and Oklahoma State University:
https://www.uaex.uada.edu/publications/pdf/MP391.pdf
Estimating Nutrient Intake – Step two is determining nutrient intake, which is determined primarily by forage quality in two ways: 1) its effects on dry matter intake (Figure 2), and 2) its relationship with nutrient concentrations (Table 1). In general, the higher the forage quality, the more cows will eat of it, and the more nutrients are supplied per unit of intake.
Often the second question we ask when designing a supplementation program is, “What is forage quality and availability, and are there other supplemental forage sources available to the cattle (hay, silage, cotton burrs, etc.)?” Understanding the answer to the question will help us estimate total nutrient intake.
For example, a lactating cow on average-quality forage would be expected to eat approximately 30 lb of dry matter. If we assume (or measure) that forage contains 7.5% crude protein, then we would estimate forage crude protein intake to be: 30 x 7.5% = 2.25 lb per day.
Figure 2. Effects of forage quality on dry matter intakes for a 1,200-lb cow during dry and lactation production stages.
Table 1. Protein and energy concentrations in forages of various quality.
Nutrient Balance – Once we have defined nutrient requirements and estimated supplies, the final step in our process is to calculate nutrient balance, which is nutrient supply less demand, with a negative balance suggesting deficiency and a positive value representing excess. In our example above, we assumed a lactating cow on average-quality forage would consume 2.25 lb of crude protein daily. For the first few months after calving, average crude protein requirements are approximately 2.80 lb/d, leaving a balance of -0.55lb/d. In this scenario, our objective would be to correct this deficiency with supplemental protein in the most cost-effective method we can logistically accomplish. With average-quality forage, we would also expect a deficiency in net energy during early lactation (Table 2).
Table 2. Estimating nutrient balance during different production phases on low- and average-quality forage.
Importance of Body Condition– The most basic, yet important, assumption in all our programs is that we have attained adequate body condition. This leads us to the next question we must ask to be successful, “What is the right body condition (BCS) for my herd to be the most profitable?” There is a delicate balance between under-feeding resulting in lower profitability due to reduced reproductive performance and maintaining sufficient body condition for the resumption of estrus and conception.
Although we do our best to estimate nutrient balance as accurately as possible, the reality is that we are not always right; sometimes, we must adjust based on subjective observation. Once a “baseline” supplementation strategy is identified, we suggest managing that program such that the average BCS is at least 5.0 at the start of calving. Figure 3 illustrates the need for adequate body condition: in well-managed herds with healthy, reproductively sound cows, well over 90% resume estrus 80 days post-calving when BCS is at 5 or better at calving.
Figure 3. Effects of body condition score at calving on estrus resumption.
Therefore, when BCS is below 5.0 at calving, we lose profit potential in a cow/calf program by lengthening the time to rebreed cows and reducing overall reproductive efficiency. Conversely, when BCS is greater than 6.0 at calving, research would indicate that overall profitability may be reduced if additional money was spent on feed resources to achieve this level of body condition. A well-managed cow/calf program will invest in supplemental feed resources to maintain cow BCS between 5.0 – 6.0 at calving in order to maximize both short and long term profitability.
The dynamics of nutrient supply and demand, and their effects on reproduction, can certainly be complicated; in practice, we use a fairly complex model to refine our calculations with various adjustments for effects of ionophores, metabolizable energy adjustments, etc. However, regardless of how specifically we define our assumptions, the basic mathematics of deducting nutrient supplies from demands is at the root of our supplementation programs.
To help answer the original question, “How much should I be feeding my cows?”, Table 3 provides general guidelines for feeding rates during each stage of production for cows consuming low- or average-quality forages. In order to better understand the correct answer to this question for your specific situation, a member of our Sales or Nutrition staff would be eager to walk through this process with you to help determine the best way to provide for your cows nutritional requirements.
Table 3. Guidelines for supplementing cows on low- and average-quality forage during various production stages.