by John Paterson, MSU Extension Beef Specialist (retired)


As an Extension beef specialist, I often recommend that a producer get his/her forages analyzed by a commercial laboratory so we can design a feeding program that will economically meet the requirements of the cow herd. However, I often don't have the opportunity to explain what the analyses indicate and if supplementation is warranted. This month's column shows ranchers how to interpret a forage analyses for minerals and make an informed decision on whether supplementation is necessary.

How do I interpret an analysis sheet that has been sent to me from the laboratory?

Table 1 below is an example of laboratory analysis of grass hay from central Montana. The chart also includes the requirements for a beef cow and if the hay is defi cient in minerals.

The hay analysis indicates that both copper and zinc are defi cient in this sample. To make supplementation recommendations, we also need to know the cow's nutrient requirements.

Often, based on the protein and total digestible nutrient content of the forage, we can estimate how much she will consume each day. The following table provides a rule of thumb for expected hay intakes.


Table 1. Nutrient Analysis of Grass Hay and Recommended Requirements​
Grass Hay analyzed for the following: Dry weight analysis of the grass hay National research requirements for minerals in diet Is this forage adequate or deficient?



Actual  Recommended Deficient? 
Protein, %  11.0   11.0    
TDN, %  60.0   60.0    
Sulfur (%)  .16 .15 .16 .15 = No
Phosphorus (%)  .21 .15 .21 .15 = No
Potassium (%) 2.37 .60 to .70 2.37 .60 = No
Magnesium (%)  .19  .10 to .20 .19 .10 = No
Iron (ppm)  111 .50 111 .50 = No
Manganese (ppm) 59 .20 to .40 59 .20 = No
Copper (ppm) 6 10 6 10 = YES
Zinc (ppm) 15 30 15 30 = YES


Table 2. General rule of thumb for estimating hay intake by beef cows​
 Forage Quality  Protein,%    TDN,% Intake, %BW (body wt) 



Dry cow

Excellent 14 62 2.7 3.0
Good 13 58 2.5 2.7
Medium 8 51 2.0 2.5
Poor 4 38 1.5 2.0


Determining if a supplement is necessary

  1. Based on the forage analysis for protein (11%) and TDN (60%) Table 1 suggests that this is GOOD quality grass hay and, we could expect a dry cow consume about 2.5% of her body weight as hay. if she were a 1,400 lb cow, we would expect that could eat 35 lbs of hay/day (1400 x .025 = 35 lbs)
  2. The next thing we want to do is to determine many milligrams of copper she needs to consume each day. To do this we must convert from pounds to kilograms; 35 lbs x .454 conversion factor = 15. kg of hay.
  3. To determine what her requirement for copper is, we multiply 15 kg of hay x the requirement for cop - per (10 ppm) and we get a requirement of 150 mg of copper/day for this cow (15 kg x 10 ppm = 150 mg of copper required/day).
  4. Next , see if the hay will provide this amount of cop - per; 15 kg fed each day x 6 ppm copper in the hay = 90 mg/day provided. This means that this cows needs an additional 60 mg of supplemental copper/ day (150 mg required 90 mg provided in forage = 60 mg of copper deficient.)
  5. Most of the mineral supplements sold in Montana have approximately 1,500 ppm copper, and cows will consume approximately 3 ounces/day (85 grams/day) (3 oz x 28.3 g/oz = 85 g).
  6. Is this supplement providing enough copper for your cows? If the cow consumes 3 oz/day (85 or .085 kg) then she will be consuming 128 of supplemental copper (.085 kg x 1500 ppm in supplement = 128 mg of supplemental copper.

Why would there be more copper in the supplement than is necessary?

Three reasons: cows may not eat the supplement every day, there may be an antagonist in the diet (sulfate, molybdenum or iron) that requires additional copper or the quality of the hay is poorer than the example that is presented here. If have questions about your supplementation program, ask your county agent, feed nutritionist or beef specialist (406-994-5562) to evaluate your supplement and feeding conditions.


For more information, contact the Montana Beef Council at (406) 442-5111 or at Beef