Estimating Alfalfa Neutral Detergent Fiber in the Field

Dr. Mark Sulc, Forage Specialist, Ohio State University 

First harvest of alfalfa is fast approaching. Timely cutting is critical for obtaining high quality forage. For lactating dairy cows, the optimal range for neutral detergent fiber (NDF) of alfalfa is around 40 to 42%. You can quickly and easily estimate the quality of alfalfa in your fields using the method outlined below. This method, developed at the University of Wisconsin, has been referred to as PEAQ, for Predictive Equations for Alfalfa Quality.

This method has been thoroughly tested in Ohio and provides reasonable accuracy for timing harvest operations. It can be used during the entire growing season, not just on the first crop. Use it to monitor alfalfa NDF as the crop develops. If the goal is 40% NDF in stored alfalfa, then cutting must begin before the standing crop reaches 40% NDF because of the effect of harvest and storage losses. In our experiences, NDF concentration of well-made silage is about 2 percentage units higher than the standing crop. Changes in NDF during a hay harvest will be higher. One must also adjust for the time it takes to harvest all acres. During the spring, NDF will increase approximately 5% each week.

The PEAQ method is designed for pure alfalfa stands and will not accurately reflect the NDF concentration of mixed grass-alfalfa stands or weedy stands. If grass is present in the alfalfa stand, begin harvesting earlier. As a reference point, pure grass stands should be cut in late vegetative to very early boot stage for dairy quality feed and by early heading for other classes of livestock.

The PEAQ procedure is NOT intended to replace laboratory analyses for balancing rations once the forage is stored. It should only be used to give a rapid first estimate of quality of the standing alfalfa for making informed harvest decisions. Alfalfa NDF sticks for field use have been developed which simplify the procedure. If you would like to purchase one, contact your county Extension agent.

Although rainy weather can foul up the best plans, using PEAQ in conjunction with weather forecasts should help you come closer to your desired forage quality goal for alfalfa this spring and summer.

Estimating Alfalfa NDF using PEAQ

Step 1: Choose a representative 2-square-foot area in the field area to be harvested.

Step 2: Determine the most mature stem in the 2-square-foot sampling area using the criteria shown in the table at right.

Step 3: Measure the length of the tallest stem in the 2-square-foot area. Measure it from the soil surface (next to plant crown) to the tip of the stem (NOT to the tip of the highest leaf blade). Straighten the stem for an accurate measure of its length. The tallest stem may not be the most mature stem.

Step 4: Based on the most mature stem and length of the tallest stem, use the chart below to determine estimated NDF of the standing alfalfa forage. Example: tallest stem is 28 inches, most mature stem has buds, but no open flowers; NDF = 38.0%.

Step 5: Repeat steps 1 through 4 in four or five representative areas across the field. Sample more times for fields larger than 30 acres. Average all estimates for a field average.

NOTE: This procedure estimates alfalfa NDF content of the standing crop. It does not account for changes in quality due to wilting, harvesting, and storage. These factors may further raise NDF content by 3 to 6%, assuming good wilting and harvesting conditions. This procedure is most accurate for good stands of pure alfalfa with healthy growth.