Delay alfalfa harvest and you’ll likely increase neutral detergent fiber (NDF) and lower protein in the crop.

That means your animals may need more grain to increase energy density and decrease the NDF content of their diets, says North Dakota State University Extension Service dairy specialist J.W. Schroeder. More supplemental protein will probably be needed to meet the cows’ protein requirements, and dry-matter intake and milk production will be reduced.

He suggests using one or more methods to predict when to cut first-crop alfalfa based on NDF concentration. Here is a short description of each:

Scissors-cut samples: They provide a direct measurement of NDF in collected plant material. Sampling technique is critical, with a sample representative from across the field and handling that will minimize respiration losses before the sample reaches an analytical lab. Also, NIRS analyses of scissors-cut samples can be in error because equations for fresh alfalfa are not generally available. Scissors-cut programs have not been used in North Dakota.

Growing-degree days (GDD): The GDD calculation for alfalfa is based on the minimum and maximum daily temperatures beginning March 1, using a base of 41ºF. The daily calculation is [(maximum temp + minimum temp)/2] - 41. The total GDD is the sum of the positive daily GDD values across days beginning March 1. Predicting NDF concentration using GDD cannot be done when soil moisture is inadequate, because GDD accumulate with little or no response in plant growth. They have been used only for first cuts.

PEAQ (predictive equations for alfalfa quality): This method is based on an equation that uses the length of the tallest alfalfa stem and the stage of the most mature alfalfa plant (these likely will be two different plants) in the area sampled. The current modified PEAQ method uses a scale of late vegetative, bud and flower plant maturities. Measuring sticks, calibrated for those stages, are used to obtain estimates of NDF. Also see PEAQ: Predicting When To Harvest Alfalfa.

The PEAQ and GDD methods were developed for pure stands of alfalfa. The NDF estimates from PEAQ will not account for weeds or grasses in the stands, Schroeder says. Also, PEAQ isn’t reliable for estimating NDF when alfalfa is very short (longest stem is less than 16”) or very tall (longest stem is more than 40”).

Schroeder suggests these research-based recommendations:

• Use a PEAQ stick or GDD to predict NDF for first-cut alfalfa. Only a PEAQ stick should be used for second cutting. Don’t use the PEAQ or GDD methods for third cutting.
• Begin to cut alfalfa at 40% NDF (750 GDD, base 41ºF) for upright silos and 38% NDF (680 GDD, base 41ºF) for horizontal silos. Start even earlier for horizontal silos if harvest takes more than a week.
• Don’t use the GDD and PEAQ methods for fields containing grass.
• Harvest fields with grass first. Start with fields with the most grass and finish with the purest alfalfa fields.
• Consider using the scissors-cut method for fields with grass and for third-cut alfalfa. Ship samples to labs by next-day delivery to help minimize deterioration in sample quality. Wet chemistry analysis is most appropriate for scissors-cut samples.