Online Tool Helps Farmers Harvest Alfalfa at the Right Time

Alfalfa growth has been slow this spring due to cooler than normal weather. This is a good reminder that using a calendar date to determine when to harvest the first crop of alfalfa may not be the best method. A better way to make harvest decisions is the PEAQ method (Predictive Equations for Alfalfa Quality) which takes several factors into account to roughly estimate the relative feed value (RFV) of standing alfalfa in the field.

PEAQ uses alfalfa height and maturity stage (vegetative, bud, or open flower) to estimate 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. The RFV of first crop alfalfa can drop three to five RFV points per day after it reaches peak value.

While PEAQ provides an estimate of RFV, adjustments also need to be made for harvest loss. Even under the best harvest conditions, 10-20% of the forage dry matter can be lost at harvest. This amounts to approximately 15 RFV points for haylage and 25 RFV points for dry hay. Therefore, to end up with 150 RFV alfalfa, harvest the crop when PEAQ measurements estimate a RFV of 165 for a haylage harvest and 175 for a hay harvest. Additionally, consider weather forecasts and allowing for proper drying time when deciding when to harvest alfalfa.

Steps for using PEAQ to determine when to harvest 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 as defined in 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 height measurement. 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 optimum harvest time using the PEAQ estimate, livestock forage quality needs, considerations of upcoming weather forecasts favorable for harvest and drying, and the general assumption that RFV drops three to five points per day.

Using the PEAQ method

Iowa State University Extension and Outreach 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 alfalfa fields across Iowa that are monitored by Iowa State University Extension and Outreach staff. Following these reports is helpful in tracking alfalfa development, but remember that crop conditions vary field-by-field. Therefore, it is recommended to take PEAQ measurements in your own field for the best assessment of when to harvest alfalfa for the optimum forage quality for your operation.

Table 1. Predictive equations for alfalfa quality (PEAQ).