Forage Analysis: Sampling and Interpretation of Results

Rory Lewandowski, Extension Educator, Athens County

This past summer’s weather, while good for forage growth, was not conducive to making high quality hay. Farmers had to work around frequent rain showers and sometimes it was a question of delaying cutting while the forage continued to mature and lose quality, or cut and then watch as the crop was rained on. Either way, there is some poor quality hay that will get fed this winter.

The economics of feeding livestock are improved by matching the nutrient needs of the animal to forage quality. When forage quality comes up short of the animal’s nutrient needs then the ration must be supplemented. The only way to really have a good handle on forage quality is through forage testing. Reliable results depend upon submitting a good sample. Here are some tips for collecting a hay sample:

* Use a hay probe to collect the sample. Reaching into a bale of hay with your hand and pulling out a sample is not accurate. A hay probe allows you to take a good cross section of the bale, getting a representative sample of both leaves and stems. Check with your county extension office about a hay probe. Many offices have one that can be signed out and used to collect a forage sample.

* Take cores from 15-20 bales within a lot of hay to get a more accurate average of hay quality. On large round bales, if the outer layer is weathered and not going to be eaten by livestock then pull away the weathered layer and sample from that point going in towards the core of the bale. It is important to try to mimic in your sampling what the animal is actually going to consume.

* A lot of hay can be determined by species, cutting date and location. For example, 1st cutting fescue hay vs. 1st cutting orchardgrass hay would be two different lots of hay, requiring two different samples submitted for testing. A first cutting orchardgrass hay baled on May 20th vs. a 1st cutting orchardgrass hay baled on June 20th are far enough apart in quality that separate samples should be submitted even though both are 1st cutting. A 1st cutting hay baled from a high fertility field vs. a 1st cutting hay purchased from a neighbor’s rarely fertilized field should be considered as two different samples even if the hay was cut and baled about the same time and has similar species.

* As you sample, empty the forage cores into a plastic bag. Usually a 1 quart ziplock bag works well. Make sure the bag is clearly labeled. If you are sampling silage or baleage and there is going to be a time lag between sampling and delivery to whatever office may be sending off the sample for analysis, then refrigerate or freeze the sample.

What kind of forage analysis do you need? A basic forage analysis that I recommend for most livestock owners includes moisture content, crude protein (CP), Acid Detergent Fiber (ADF), Total Digestible Nutrient (TDN) content, and Net Energy (NE). For dairy producers who may want to pay more attention to Neutral Detergent Fiber (NDF) when balancing a ration, or for anyone who wants more forage quality information, then the next level of forage analysis provides all the information in the basic forage analysis plus Neutral Detergent Fiber (NDF), Digestible Dry Matter (DDM), Dry Matter Intake (DMI) and Relative Feed Value (RFV).

Once the results of the forage analysis come back you need to be able to interpret the results and make some sense of the numbers. We’ll start with a simple definition and explanation of some basic forage quality terminology.

* Moisture content: No forage is 100% dry matter, even though animal intake, nutrient requirements, forage nutrient contents and rations are based on 100% dry matter so that forages can be compared equally with one another. The moisture content can also tell you something about the quality of that forage. For example, if the moisture content of a large round bale is above 15%, or a small square bale above 18%, there is likely to be some storage and mold problems.

* Crude Protein (CP): This figure is the nitrogen content x 6.25. Protein is needed for amino acid production and growth. Livestock nutrient requirements are based on a percentage of the diet composed of CP, or a given weight of CP that needs to be consumed each day.

* Total Digestible Nutrients (TDN): TDN is used as a measure of energy. It is calculated from the ADF percentage. Often livestock energy requirements are stated in terms of percent of the diet that should be TDN or in pound of TDN that should be consumed per day.

* Neutral Detergent Fiber (NDF): This is a measure of the amount of hemi cellulose, cellulose and lignin contained in the forage. NDF is inversely correlated with intake, that is, as NDF levels increase, intake decreases.

* Acid Detergent Fiber (ADF): This is a measure of the amount of cellulose and lignin contained in forage. ADF is used in digestibility calculations and is inversely correlated with digestibility.

* Relative Feed Value (RFV): This is a calculated value, using the percent digestible dry matter (DDM) and dry matter intakes (DMI), which themselves are calculated values that use ADF and NDF. RFV is generally used to compare forage quality between lots of hay. Full bloom alfalfa and grass hay at the seed head stage will have a RFV around 100.

Now on to the actual numbers themselves. The following tables have been excerpted from an OSU Extension Fact Sheet entitled “Forages for Dairy Cattle”, and will provide some typical forage analysis ranges for various quality terms.

1. Legumes

Maturity Quality Measure
Stage Crude Protein ADF NDF RFV
Pre-bloom >19 <31 <40 >140
Early bloom 17-19 31-35 40-46 124-140
Mid-bloom 13-16 36-41 47-51 101-123
Full-bloom <13 >41 >51 <100

2. Grasses and Grass/Legume mixtures

Maturity Quality Measure
Stage Crude Protein ADF NDF RFV
Pre-head >18 <33 <55 124-140
Early head 13-18 33-38 55-60 101-123
Head (until seed is in milk stage) 8-12 39-41 61-65 83-100
Post head <8 >41 >65 <83

Notice that while the ADF figures for quality are similar between legumes and grasses of equal maturity, the NDF content of grasses is significantly higher than the NDF content of legumes of a similar maturity. When I see grass hay NDF levels that are above 70 I know that intake of that forage will be limited and rate of passage through the rumen will be slow.

Of course, while a forage analysis may be interesting in and of itself, the real value is using the results to balance the livestock ration. Guessing about the need for supplementation can be expensive. For more information about forage sampling, testing and interpretation of results, contact your county extension office, or a member of the OSU Extension Beef Team.