Alfalfa First Harvest Management

Producing forage crops and attaining forage quality goals can sometimes be very difficult.  Being able to stay on top of the weather, crop maturity and other conditions and actually get your forage harvested in a timely manner takes a bit of experience and luck.  However, there are some new tools that are being tested to help you make decisions about when to cut to obtain your forage quality goals.

In alfalfa production, the management factor that has the greatest affect on quality is timing of cutting.  Alfalfa in the vegetative stage has the highest feed value.  As the plants mature, this value declines.  So timing the cutting at the proper stage of growth is the critical factor.  If you cut early, you can harvest some very high quality feed, but you will also be loosing yield by not letting it grow.  If you cut late, usually you will have increased yield, but reduced quality.

Accumulation of growing degree days (GDD) and use of the predictive equation for alfalfa quality (PEAQ) are the two new tools being studied.  These methods use % neutral detergent fiber (NDF) as the quality factor for conducting research on.  For dairy cows, 40% NDF is generally the target for alfalfa forages because they will generally eat more at that number and it makes balancing the ration easy or less costly to do in most cases.  The more the cows will eat with a balanced ration, the more milk they will produce.

GDD are calculated on the base 41 scale by measuring the high and low temperature for each day after March 1 of the current growing season.  You find the average temperature for the day by adding the high and low and divide by 2.  Next you subtract 41 from the average temperature to obtain the GDD’s for any given day.  For example, if the high temp is 80°F and the low is 60°F, the GDD’s for this day is 80 + 60 = 140 ¸ 2 = 70 – 41 = 29.  In this example, 29 heat units or GDD’s were accumulated.  This calculation is performed for each day starting with March 1 and accumulated until first cutting.  Previous research conducted by Dr. Mike Allen, Department of Animal Science, Michigan State University indicate that this accumulation of GDD’s are very well correlated with % NDF.  From his research, once 750 GDD’s have been accumulated, the estimated NDF is 40%, which is what the original target was.

These heat units are reported through DTN and MSU Extension.  Make sure you don’t get this confused with the GDD’s for corn.  The formula for corn is different and will give a different number.

PEAQ is a method that estimates % NDF by measuring plant height and maturity in the field.  An equation has been developed by Dr. Ken Albrect, University of Wisconsin that measures plant height and stage of maturity.  It is like a yardstick that measures NDF rather than inches.  The stick is square with each side representing a stage of maturity (vegetative, bud, flower).  The fourth side has instructions.  Simply walk out in the field and randomly select a spot, find the most mature stem and then measure the height of the tallest stem using the appropriate side of the stick.  Note that the most mature and tallest may not be the same stem.  The reading you get off the stick is the estimated % NDF.  Repeat this procedure in 4-5 locations throughout the field to get a good average of the whole field.  These sticks are available from the MSU Extension office for $5.  I would be happy to demonstrate it for you, just give me a call.

Last summer, there were 5 farms that participated in a multi-state research project to evaluate the accuracy of these two methods.  Temperature recorders were placed in one field at each of the 5 farms.  GDD’s were calculated at each site.  PEAQ and fresh cut samples were also collected.  All the data and samples were submitted to Dr. Marc Sulc, Ohio State University for lab analysis.  Michigan, Ohio, Wisconsin and South Dakota were the 4 states involved in the study.  All samples and procedures were identical for all locations across all states.

Figure 1.
Figure 1 shows variance of PEAQ and GDD as compared to the actual % NDF as tested in the lab.  From the chart, you can see that in Michigan, both methods underestimated the actual % NDF by nearly 3 points and were the least accurate as compared to the other states.  Overall, both methods were accurate predictors of NDF to within 1.6 percentage points.  Either of these methods would be reasonable for use on your farm to help in deciding when to take first cutting.  Remember though, that these are estimates only.  Continue to conduct feed sampling and analysis to balance rations.