Estimate Alfalfa First Crop Harvest with PEAQ

May 10, 2019
ICM News

Every spring, alfalfa growth and development is different due to variations in climatic, variety, stand age and other crop production factors. With the 2019 growing season being off to a cooler than normal start, this has slowed alfalfa growth this spring. This is a good reminder that while calendar date may be one method used to determine when to harvest first crop alfalfa, this method is not the best method to use. Instead, the PEAQ method (Predictive Equations for Alfalfa Quality) developed by the University of Wisconsin does a better job.

The PEAQ method uses alfalfa stand height and maturity stage (vegetative, bud, or bloom) to estimate the relative feed value (RFV). In general, it is recommended to harvest alfalfa at about 150 RFV for milking dairy herds and 125 RFV for heifers, stocker cattle, and lactating beef cattle. First crop alfalfa standing in the field can drop three to five points of RFV per day. The steps on how to use the PEAQ method to determine when to make the first crop of alfalfa are listed below.

Note that while PEAQ does a good job of estimating the RFV, under the best harvest conditions, 10 to 20 percent of the forage dry matter will be lost at harvest. This amounts to approximately 15 RFV points for haylage and 25 RFV points for hay. Therefore, if you are trying to end up with 150 RFV alfalfa, you should consider harvesting the crop when PEAQ measurements estimate a RFV of 165 to 175 for the standing alfalfa crop in the field.

Steps for using PEAQ to determine when to take first crop alfalfa

Step 1. Choose a representative two square foot area in the field.

Step 2. Determine the stage of the most mature stem in the area by using the definitions at the top of Table 1.

Step 3. Measure the tallest stem in the area. The tallest stem may not be the most mature stem. Measure the stem from the soil surface to the tip of the stem; not to the tip of the leaf. Straighten the stem for an accurate measure of height. Based on stem maturity and stem height, use Table 1 to estimate the RFV of standing alfalfa crop.

Step 4. Repeat steps 1-3 in five representative areas across the field.

Step 5. To estimate harvest quality, subtract 15-25 RFV units to account for harvest losses during the haylage or hay harvest process, respectively.

Step 6. Determine your optimum harvest time using the PEAQ estimate, your livestock forage

quality needs, considerations of upcoming weather forecasts favorable for harvest or not, and the general assumption that RFV drops three to five points per day.

ISU Extension maintains a PEAQ website that includes a fact sheet to explain how to use the PEAQ method to determine when to harvest first crop alfalfa. This website also includes postings of PEAQ values from some alfalfa fields across Iowa being monitored by ISU Extension and Outreach staff. Follow the progress along in these reports, but conditions can vary field-by-field. Therefore, it is recommended to take PEAQ measurements in your own fields to best estimate first crop harvest to help target the quality of forage you wish to achieve.

predictive equations for alfalfa quality table

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Rebecca Vittetoe Field Agronomist in EC Iowa

Rebecca Vittetoe is an extension field agronomist in east central Iowa. Educational programs are available for farmers, agribusiness, pesticide applicators, and certified crop advisors.

Areas of expertise include agronomy, field crop production and management of corn, soybeans, and...

Brian Lang Field Agronomist in NE Iowa

Brian Lang conducts Iowa State University Extension and Outreach programs in crop production and protection in northeast Iowa. Frequent clients include farmers, ag chemical and fertilizer dealers, seed dealers, crop consultants, and farm managers.  Provide timely in-season crop management inform...