Alfalfa growers wanting to harvest alfalfa according to forage quality should keep a close eye on the growing degree days (GDD), suggests Phil Kaatz, Michigan State University Extension forage educator.

“Many dairy producers have a goal of 40% neutral detergent fiber (NDF),” says Kaatz. “Data collected over a period of years suggests that an upright silo should begin harvesting at 750 GDD for alfalfa with 40% NDF plus-or-minus 3% most years. The current recommendation for producers using bunk silos is to begin cutting at 680 GDD corresponding to a value of about 38% NDF. Using GDD is an important tool that should only be used for first cutting.”

GDD is calculated by averaging the high and low temperatures for a 24-hour day, dividing by two and then subtracting the base of 41°F. The season total is the sum of daily growing degrees beginning March 1. Anything less than 41° gets zero GDD for the day. For example, if the high and low temperatures for one day are 60° and 40°, the average is 50°. Subtracting 41° (base) from that figure equals 9 GDD.

Current GDD information is available at weather stations throughout Michigan. To get information for a weather station near you, go to Michigan State’s Enviro-weather Web site.

Growers with alfalfa-grass mixed stands should not use the GDD method to establish the cutting schedule, says Kaatz. Consider harvesting those fields before harvesting pure alfalfa stands.

“Grasses have higher NDF content than alfalfa cut at the same age. Spring harvest of alfalfa-grass mixtures is based on alfalfa maximum height and the proportion of grass in the stand. So consider harvesting fields that have the most grass first so purer alfalfa stands can be harvested at the appropriate neutral detergent fiber levels.”

If wet weather delays harvest, you may want to harvest the purest alfalfa stands first since fields with grass will likely be past the optimum NDF levels and may be better used in rations that require higher fiber or for dry cows and heifers.

Another method for evaluating stage of maturity is the Predictive Equations for Alfalfa Quality (PEAQ) stick method. Kaatz says that method takes more time because you need to walk through your alfalfa fields and select several spots to get an accurate reading. The PEAQ stick uses stem height and maturity to predict forage quality. You must be able to identify the different stages of alfalfa: early bud, mid-bud, late bud, early bloom and full bloom. In a six-state study across a wide range of environments, PEAQ sticks estimated NDF within three units of actual wet chemistry in 77% of samples collected.

“PEAQ sticks should only be used for first and second cuttings of pure alfalfa fields that receive adequate moisture and growing conditions,” says Kaatz.

“These methods are not foolproof,” he adds. “Other factors can affect the fiber levels of forages, but these are better than cutting by the calendar date.”