Accumulated growing-degree days (GDD) may not be an accurate tool for timing the first alfalfa cutting this year, warn Michigan State University forage and dairy specialists.

The neutral detergent fiber (NDF) of Michigan alfalfa, as predicted by GDD, is running lower than the level measured by wet-chemistry lab analysis, they say.

To evaluate the impact of last month’s multiple frosts on the alfalfa crop, they’re collecting scissor-cut samples at several locations across the state. Lab-test results are being compared with the GDD and PEAQ (Predictive Equations for Forage Quality) stick methods of timing the first cutting.

Normally, alfalfa tests 40% NDF, the level preferred by most dairy producers and nutritionists, when 750 GDD have accumulated. Producers with bunker silos and large acreages are encouraged to start cutting at 680 GDD.

“However, the hard frosts that occurred after substantial growth this spring might alter this relationship,” they say.

For plant samples collected May 1 or 2, the PEAQ method predicted NDF well; but NDF predicted by GDD was 2.8 percentage units higher than was measured in the lab. By May 7, when the NDFs estimated by GDD were corrected for differences observed with wet chemistry a week earlier, they were several percentage units lower than what was expected.

“To compensate for the differences, producers should consider delaying cutting instead of basing the first cutting on GDD,” say the specialists.

They’ll continue to collect and test samples weekly until first harvest.