The measure of fiber most commonly used to balance diets of lactating dairy cows is neutral detergent fiber (NDF). The optimum concentration of NDF for alfalfa is 40 percent.

Alfalfa containing 40 percent NDF allows reasonable grain concentrations in the diet while maintaining adequate diet NDF concentrations. The protein concentration of alfalfa with 40 percent NDF is usually moderate (approximately 20 percent of dry matter [DM]), and additions of low-protein grains such as corn, allows flexibility in diet formulation for rumen-undegraded protein while avoiding excessive protein concentrations.

Delaying alfalfa harvest increases NDF percentage and reduces protein concentration. More grain will be required to increase energy density and decrease the NDF concentration (and filling effect) of the diet. In addition, more supplemental protein will be required to meet the cows’ protein requirements, and dry matter intake and milk production will be reduced.

There are now better ways to determine the best time to cut first- and second-crop alfalfa for optimal quality than looking at stage of maturity or the calendar.

Using an alfalfa quality “PEAQ” (Predictive Equation for Alfalfa Quality) stick or calculating heat units, known as growing degree days (GDDs), is more accurate. Visually analyzing the alfalfa for the best time to cut is subjective. What’s early bud stage to one grower isn’t necessarily early bud stage to another.

The PEAQ method was developed in the 1990s by University of Wisconsin-Madison researchers. It uses plant height and maturity stage to estimate the alfalfa’s quality. Alfalfa growers can track GDDs on the alfalfa crop themselves as soon as plants break dormancy and actively start growing.

We recommend growers take their first cutting when GDDs hit 750 (base 41°F).That will give you 40 percent NDF, which is ideal for high-producing dairy cows.

A few years ago, MSU Extension educators tested alfalfa quality at many sites on Michigan farms. Alfalfa was harvested based on the information from tracking GDDs, using the PEAQ stick, looking at the calendar and at physiological indicators like bud formation. Samples were harvested and analyzed using wet chemistry.

In the first cutting, the PEAQ and GDDs were equally effective at predicting the 40 percent NDF target. In the second cutting, the PEAQ stick narrowly edged out GDDs as the more accurate method.

Both were better than the other two. However, GDDs and the PEAQ sticks didn’t work as well with subsequent cuttings because we tend to get into drier weather in July and August. We recommend once you get to later cuttings, use either hand clippings or growth stage for analysis.

Whether to use the PEAQ stick or track GDD is a matter of personal preference. GDD can be tracked on a computer, but using a PEAQ stick in the field gives you the opportunity to scout for insects and monitor plant health at the same time.

Growing degree days and using PEAQ does not work for fields containing grass.If grass is present, then harvesting in the late boot stage of maturity will result in better forage quality.  FG

Rich Leep, Forage Agronomist, Michigan State University

—Excerpts from Michigan State University Field CAT Alert, Vol. 23, No. 7