Timing First Crop Harvest of Alfalfa with PEAQ

Timing first crop alfalfa harvest by calendar date does not usually work well. Spring climates vary from year to year, and fields managed differently also affect spring regrowth. Different varieties, age of stand, fertility, last season’s cutting schedules, fall harvest or not, all influence the rate of regrowth in spring.

Since the first cutting is usually has the highest yield with 35-40 percent of the year’s total crop, it is important that it is the quality forage your operation needs. If the first cutting is taken at a very immature stage it can be difficult to feed because its fiber level is too low for most high producing cows plus it can lower the life of alfalfa stands. Timely cutting permits aftermath growth to begin when when temperature and soil moisture are favorable for plant growth and generally increases total yield per acre.

PEAQ, which stands for predictive equation for alfalfa quality, is a quick and easy method to assess when individual alfalfa fields are ready for harvest based on a forage quality estimate. All you need is a yard stick and Table 1 in ISU Extension publication CROP 3141, which is available from your county ISU Extension office or download at: https://store.extension.iastate.edu/Product/15234

PEAQ provides a RFV estimate for the standing crop. It is critical that you do not forget to subtract from your standing crop RFV reading in Table 1 by either 15 RFV units for a haylage harvest or 25 RFV units for a hay harvest to account for anticipated forage quality loss from harvest losses that typically occur. So, if you are targeting alfalfa haylage for 150 RFV, you would harvest when Table 1 for PEAQ reads 165 RFV (bud stage alfalfa, stem height 27-28 inches). Typical alfalfa quality targets are 150 RFV for milking dairy herds, and 125 RFV for heifers, stocker cattle, and lactating beef cattle. Additionally, weather forecasts and allowing proper drying time should also be factors when deciding when to harvest alfalfa.

This spring, ISU Extension Agronomist Joel DeJong and Dairy Specialist Fred Hall will be providing some PEAQ readings from alfalfa fields in Plymouth and Sioux counties and posting them on a website at: https://www.extension.iastate.edu/dairyteam/peaq .  You will be able to track these postings over time, but we strongly encourage that you take PEAQ readings from your own alfalfa fields for best assessment of harvest time.

If you are interested in monitoring your own fields, here are the steps to make the evaluations yourself.

Step 1: Choose a representative area in the field and mark it so you can come back to it for each reading.

Step 2: Determine the most mature alfalfa stems in the area. Determine if the most mature stems are vegetative, bud or flower stage.

Step 3: Measure the tallest stems in the area. The tallest stems may not be the most mature stems. Measure from the soil surface to the tip of the stem. Straighten the stem for an accurate measure of height.

Step 4: Based on stem maturity and stem height, estimate the RFV of standing alfalfa crop using the PEAQ Fact Sheet at: http://www.extension.iastate.edu/dairyteam/files/page/files/PEAQ.pdf

Step 5: Subtract 15 to 25 RFV units to account for harvest losses during the haylage or hay harvest process, respectively to estimate harvested quality.

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 about four points per day.

For more information in Northwest Iowa, contact ISU Agronomist Joel DeJong at 712.546.7835 or ISU Extension Dairy Specialist Fred Hall at 712.737.4230.



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