Cattle Rations, Percentage or Pounds?

Rory Lewandowski, Extension Educator, Athens County and Buckeye Hills EERA

Generally when we talk about the nutrient requirements of cattle and the ration that they need, it is common to hear those requirements expressed in terms of percentages. For example, this cow requires 12% crude protein (CP), or 55% TDN (Total Digestible Nutrients). Cattle, of course, eat pounds of feedstuffs. We talk about a cow eating 2.5 % of her body weight in dry matter per day. So what is the relationship between percentage and pounds and how does it work in putting together a ration or figuring out if the hay we have is going to meet the nutrient requirements of the cow?

Let’s begin with the National Research Council (NRC) nutrient requirement tables for cattle. These tables list the nutrient requirements for cows according to their stage of production, level of production and body weight. Nutrient requirements for crude protein (CP), energy and minerals are listed. Energy requirements are listed as either TDN or by mega calories (Mcal) in the net energy system. Livestock classes and stages of production include young growing heifers, bred heifers according to gestation trimester, 2-year old lactating heifers according to milk production, mature beef cows according to stage of gestation, and mature beef cows according to stage of lactation and milk production. Weight of the animal is another factor that affects the nutrient requirements. The nutrient requirements in the tables are listed as a percentage value and as a quantity, for example, pounds, kilograms or grams. It is important to recognize that the requirements are really minimum requirements. Environmental conditions such as cold temperatures or walking in hilly conditions can increase the nutrient requirements.

Here is an example of the CP and TDN nutrient requirements of a 1300 pound cow in mid gestation and late gestation:

Production stage CP% CP (lbs) TDN% TDN (lbs)
Mid-gestation 7.0 1.5 49 10.8
Late-gestation 7.7 1.8 53 12.5

From: Nutrient Requirements for Beef Cattle, Edward Rayburn, University of West Virginia Extension, Adapted from NRC for Beef Cattle 2000

Now let’s consider a couple of hay quality analysis test results and see how they match up to the cow’s needs. The first sample is a first cutting grass hay with a CP% of 9.5 and a TDN level of 52%. The second sample is a second cut grass hay with a CP% of 13.9 and a TDN level of 63%. It looks pretty obvious that the first cutting hay will meet the nutrient requirements through mid-gestation and maybe even into the first part of the late gestation trimester. It will not be adequate for most of the late gestation trimester based on the nutrient percentages.

Remember though that the cow eats pounds, not percentages. What kind of nutrient levels are we providing if hay is pretty much fed free choice? We often talk about a cow eating about 2.5% of her body weight in dry matter. If our cow weighs 1300 pounds, that equals about 32 lbs of DM per day. How many pounds of nutrients is she consuming at this level? With the first cutting hay this equals 3.04 lbs of CP (32 x .095) and 16.64 lbs of TDN (32 x .52). This cow is consuming more than what she really needs. Even if this cow consumes at only 2.0% of her body weight in DM (26.0 lbs) she will still eat 2.47 lbs of CP and 13.5 lbs of TDN/day. When the ration is considered from a pounds of hay eaten perspective, this first cutting hay could meet the nutrient needs through late gestation.

A few points can be made:

* Forage intake will vary with forage quality. Poor quality forages have lower intake levels because the rate of passage through the rumen is slow. As forage quality increases, and rate of passage through the rumen increases, forage intake will increase to a point. Just as some rough reference points think about very poor quality forage with an intake of about 1.7% of body weight in DM and very high quality forage with a intake of around 2.8% of body weight in DM.

* Cows will luxury consume in excess of what they need. This is more likely with forage of average or better quality.

* In years when hay is in short supply and/or expensive, knowing forage quality and the nutrient requirements in pounds may allow the manager to limit feed hay while meeting nutrient requirements and saving some dollars.

* In all cases, monitor body condition of cows. They will let you know if you are figuring the ration correctly.