With alfalfa growth running well ahead of normal this year, growers should consider using Predictive Equations for Alfalfa Quality (PEAQ) to determine when to harvest, says Brian Lang, Iowa State University Extension field agronomist.

“PEAQ provides an estimate of alfalfa quality based on stage of plant development and stem height,” says Lang. “This allows you to estimate standing-crop forage quality with a yardstick and your observation of the stand’s stage of development – vegetative, bud or bloom.”

He says to choose a representative area in each field and determine the development stage – late-vegetative, bud or flower – of the most mature stems. Then measure the tallest stems. Measure from the soil surface to the stem tip, straightening the stem for an accurate height measurement.

Next, go to Table 1 in Lang’s PEAQ fact sheet to estimate the relative feed value (RFV). An alternative is to use a PEAQ stick, which makes the RFV estimate in the field. PEAQ sticks are available from the Midwest Forage Association for $15 ($10 for MFA members) plus shipping and handling.

Under the best conditions, 10-20% of the forage dry matter will be lost at harvest, says Lang. That amounts to approximately 15 RFV points for haylage and 25 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 an RFV of 165-175 for the standing alfalfa crop in the field.”

To help monitor the progress of Iowa alfalfa, Iowa State University Extension created a PEAQ Web site where RFV scores are posted for various fields across the state. “However, these postings are no substitute for monitoring your own fields,” says Lang. “As you see in these postings, there are considerable differences from field to field, as expected, due to field location in the state, variety, harvest management, soil fertility, soil type and other factors that affect alfalfa growth and development.”