When expected results aren’t achieved from your cow feed formulation program, this generally indicates that either some aspect of feed preparation on the farm went awry, or one or more of the input parameters used to describe either the feed, farm or cows were off. When predictions go sideways, production of milk or components are unfortunately not where they should be. Once farm-related issues and mistakes in feed analyses have been ruled out, if we assume that the models are reasonable, then there might be issues with how the diet was formulated.

Numerous studies have been conducted to compare the accuracy of predictions from the models upon which feed formulation software programs are based. Most of the studies suggest that the models are reasonably accurate. However, in some cases, predicted results vary from the targeted results.

Once extraneous factors have been ruled out, there are two very simple things that nutritionists can do to improve their cow feed formulation success. One, make certain they have reasonably accurate cow weights, and two, verify feed intake.

Importance of body weights  

It is not unusual to see formulators use an average value for body weights of all of their herds. In the August 2011 issue of Journal of Dairy Science, average weights for mature Holstein cows used in lactation studies varied from 565 to 725 kg (1,245 to 1,600 lbs.). Herds vary in average weight with location and genetics. In addition, groups of cows have differing average weights: heifers versus mature cows, and early lactation versus mid-lactation.

Why is weight important? Weight is used to establish many of the nutrient requirements needed for formulation. Maintenance, pregnancy and growth requirements are all functions of body weight. Endogenous losses and rumen turnover rates also depend on body weight for accurate calculation.

When program users adjust their estimation of nutrition requirements for activity (walking, standing, etc.) or for the effects of the environment, maintenance requirements, calculated from body weight, are also adjusted.

Similarly, many individuals like to express their formulation guidelines in relation to body weight. For example, diets might be formulated to a forage NDF level of 1% of body weight. It should be becoming clear why accurate body weights are a key component to successful ration development.


To illustrate this, here are some examples. In this case, the Cornell-Penn-Miner Dairy ration analyzer program was used. A ration was formulated for a herd with a mature weight of 1,450 lbs. The actual weight was set at 1,425 lbs. The herd was producing 100 lbs. of milk, with 3.7% fat and 2.95% true protein (Figure 1). This appears to be an acceptable ration.

The next figure (Figure 2) shows the metabolizable energy and metabolizable protein  requirements for this ration. It is easy to see that the maintenance ME requirement is equal to 16.88 Mcal/day, or 25% of the total requirement. The maintenance MP requirement is 800 g (28% of the total).

The next screen shot (Figure 3), from CPM Dairy, shows a ration formulated for a 1,450 lb. herd, but received by a 1,250 lb. herd. These cows have a maintenance ME requirement of 15 Mcal, which is approximately 23% of the calculated total energy requirement. Similarly, the maintenance MP requirement is 28% of the total requirement. As the bottom right quadrant shows, the needs for both energy and protein were met. In fact, the ME provided exceeded needs by 7%. In most cases, there is a cost associated with an over supply, both in dollar terms and in excess body fat gain. The number of days needed to change one body condition score decreased from 277 to 160.

The final screen shot in this series (Figure 4) shows what happens when the same diet is offered to a herd with a 1,700 lb. average mature weight. Maintenance ME needs for these animals is now 19 Mcal/day, or 28% of the total requirement. The number of days needed to gain one body condition score is a whopping 888! This could conceivably (pun intended) have an impact on the herd’s ability to reproduce.

You may have noticed that the MP requirement for the 1,200 lb. cows is quite similar to the body weight from the 1,700 lb. cows. That is because the MP requirement is a function of both body weight and dry matter intake. Because the diet intakes did not change in the examples, the intakes as a percentage of body weight differ for the three groups of cows.

This is nicely broken down in another program, the Consulting Nutritionist, provided by Dalex Livestock Solutions. Metabolic fecal requirements change with intake as a percent of body weight. Thus, requirements increase slightly with the lighter cows, and decrease with the heavier cows, offsetting some of the other requirements that are based solely on weight.

While weight is the most important variable used to determine requirements, dry matter intake is the most important variable for the assessment of nutrient availability. When 54.2 lbs. of dry matter were supplied to cows with a mature weight of 1,450 lbs., MP available was found to be 2,883 g (Figure 2). This value increased to 2,921 g for the 1,250 lb. cows (Figure 3), but dropped to 2,843 g with the heavier 1,700 lb. mature weight cows (Figure 4). Such differences translate into more or less milk protein.

In dynamic feed formulation programs, passage of feed through the rumen depends on feed intake. As intake increases, feed moves through the rumen more quickly. This provides greater opportunity for microbial growth and increases escape protein, but reduces the amount of time available to digest cell wall.

Here are two examples from the recent Cornell Net Carbohydrate and Protein System (CNCPS6.1). In the first example (Figure 6), dry matter intake is 55.4 lbs./day. Rumen degraded protein is 9.175% of intake and total fermented carbohydrate is 55.2% of the carbohydrate consumed. When the diet recipe is increased by 10% (Figure 7), rumen degraded protein slides to 8.94%, while total carbohydrate fermented is reduced to 54%.

Intake likewise influences the requirements for nutrients that are not dynamic. If, for example, cows are determined to need 150 g of calcium, this requirement may not be met if intakes are under predicted. Similarly, nutrients will be wasted if cows eat more than assumed. It is therefore helpful to get a handle on intake in order to provide the best possible formula.

Two simple items: body weight and dry matter intake can be the difference between a successful formulation or missing the target.