Growers test growing degree day formulas.

Growers must develop their own GDD formulas.

An increasing number of alfalfa growers are experimenting with heat units or growing degree days (GDD) to determine the best cutting date for quality.

At least one has had good success. A Nevada grower says he gets higher-quality hay using GDD than by cutting by the calendar or by stage of maturity.

This irrigated alfalfa grower uses GDD to time all his cuttings. But the method may not work as well for dryland growers. Michigan State University researchers concluded that counting heat units works well for the first cutting. But in later cuttings, moisture has more effect on alfalfa growth than does temperature.

Whether you want to use GDD to set the date of one cutting or all of them, you'll have to develop your own system. There's no standard formula that will work from Stephenville, TX, to Moses Lake, WA. That's because the number of GDD needed to reach a specific quality level is affected by geographic location, variety and fall dormancy, and photoperiod (hours of daylight).

Similarly, there is no hard and fast formula for figuring GDD for a given day. Because base temperatures are affected by photoperiod, four or more formulas are used from Southern Canada to the Southern U.S.

A commonly accepted one is: Maximum daily temperature + minimum daily temperature — 41 = GDD. Heat units or GDD aren't counted until the high temperature hits 41° for five consecutive days.

How many GDD are needed to bring alfalfa to maturity? A general starting point at which a hay grower may develop a learning curve is: 700 GDD — bud stage; 880 GDD — first flower; and 1,075 GDD — full bloom.

Remember that these values vary, depending on location, time of year and the fall dormancy of your variety. A central Nevada grower may use 600 GDD to set a cutting date for his market's quality standard while a western Nevada grower may use 700 GDD.

Some growers use a standard number of heat units for each cutting while others claim that the required GDD changes up or down as the growing season progresses. One can develop a formula over time only to learn that a frost will ruin all formulas and that a formula may change from year to year.

A general rule is that the farther north the location, the more GDD are needed.

How can it be that 700 heat units may be required in a May cutting and 700 units may also be required for a July cutting? Everyone knows that alfalfa grows faster in mid-summer than in spring. That's one of several reasons why a summer cutting can get away from you and fall short in quality.

The answer lies in the fact that all crops, including alfalfa, have optimal growth rates. Tom Griggs, a Utah State University forage agronomist, says alfalfa's optimal growth rate is variety-dependent. But a general synopsis is about 65° for nighttime growth, 86-91° during the day for first-week regrowth and 70-80° for older shoots, says Griggs.

Growth begins to slow at about 90°, so the extra accumulation of heat units in July may not add to alfalfa's growth.

So where does a grower start if he wants to try this GDD method?

You can accumulate heat units per day courtesy of your computer weather reporting service. Or, buy a heat unit accumulator. The GDD formula is built into the hand-held unit and is reset by punching a button after each cutting.

Don't worry much about which temperature base is incorporated into the accumulator. Just get used to a given formula and use it consistently. The GDD accumulation is automatically done for you daily.

Locate the accumulator near an alfalfa field 5' above ground in an enclosure with good ventilation. One grower inserts the unit inside a dense spruce tree, and it has performed quite well.

We're at the beginning of a learning curve on the use of GDD to set alfalfa-cutting dates. At the very least, a grower could use GDD as a trigger to begin watching the crop for optimal maturity.

For more information, call the author at 800-622-6837.