Fine Tuning When to Begin Harvesting First Cutting

Kevin H. Ganoe

For the past 3 years we have had a Forage Quality Project where we posted first cutting forage quality values at our web site to give an indication when to start harvesting. We would sample selected fields through out our counties every 2-3 days in May and with a quick turn around time on the forage testing could usually have the results posted to our web site the next day.

We will be doing less actual sampling this year and relying more on field measurements because the field measurements work very well for predicting the percent NDF (Neutral Detergent Fiber). As we come close to harvest we will do actual forage testing. But the measuring we are going to do in the field is something anyone can do, so grab a yard stick and note a few of the details below.

Alfalfa

Let’s start with alfalfa, pure or close to pure stands because we have a very good field measurement for predicting alfalfa NDF in the field called PEAQ

(Predictive Equations for Alfalfa Quality). By measuring the tallest alfalfa stem and knowing the stage of the most mature stem we get a pretty good handle on what the NDF of the alfalfa field actually is. Table 1 gives the NDF for the measured height and maturity of alfalfa. An NDF of 40% is the goal so on most farms we will need to start earlier than 40% NDF if we are to get all of the alfalfa harvested on time.

Table 1. Alfalfa Stem Height to Predict NDF for Alfalfa

	Stage of Most Mature Stem
Length of longest stem from soil to stem tip (in inches)	Late Vegetative	Bud Stage	Flower Stage
16	28.5	29.7	31.4
17	29.2	30.4	32.0
18	29.9	31.1	32.7
19	30.6	31.8	33.4
20	31.3	32.5	34.1
21	32.0	33.2	34.8
22	32.7	33.9	35.5
23	33.4	34.6	36.2
24	34.0	35.3	36.9
25	34.7	35.9	37.6
26	35.4	36.6	38.3
27	36.1	37.3	38.9
28	36.8	38.0	39.6
29	37.5	38.7	40.3
30	38.2	39.4	41.7
32	39.6	40.8	42.4
33	40.3	41.5	43.1
34	40.9	42.2	43.8
35	41.6	42.8	44.5
36	42.3	43.5	45.2
37	43.0	44.9	46.5
38	43.7	44.9	46.5
39	44.4	45.6	47.2
40	45.1	46.3	47.9

If you are measuring alfalfa that is on the short-side looking to predict when to begin harvest remember alfalfa NDF increases about 0.5 to 0.7/day. Expect the lower end of that range in cooler weather and the higher end in warm. We have certainly seen these differences play out depending on the year.

Grasses

When most of the acreage in this area is in grass or legume/grass mixes it may seem strange then to begin with alfalfa. Like it or not alfalfa height seems to predict NDF content well, very well, even for grass or alfalfa/grass mixes. David Parsons and Jerry Cherney at Cornell sampled many fields though out the state in 2004 and 2005 to come up with prediction equations for grass or alfalfa/grass NDF that work as well as the ones do for alfalfa NDF. Take a look at Table 2 to see the relationships.

Table 2. Alfalfa Stem height and Grass NDF

For a pure grass field you would find a nearby alfalfa field or part of the field that is pure alfalfa and measure the alfalfa height. If the alfalfa is 15 to 16 inches tall you would begin to harvest nearby grass fields. Typically NDF increases about 0.8 to 1.2/day for grass. Again expect the lower end of that range in cooler weather and the higher end in warm.

Going strictly by alfalfa height there is no difference made for grass maturity, height or species. The research indicated none of these predicted the grass NDF as well as alfalfa height. This may be hard to understand given the emphasis that has been placed on harvesting grass by maturity, hoping to harvest when the grass is in the “boot stage”. We have a plot at SUNY Cobleskill with alfalfa alone, 4 different grasses alone, and each grass in combination with alfalfa, and will be sampling in particular to see how grass NDF compares across species at a given alfalfa height.

In mixed stands make a visual assessment of the amount of grass. If the field is 50/50 grass and legume then begin harvest when alfalfa in that field is 22 to 23 inches tall. At this alfalfa height the grass component will be higher in NDF than would be desired if it was harvested by itself, but the alfalfa in the mixed stand will be of extremely low NDF to compensate. Yield from the alfalfa given its low height when cut will be less than a pure stand but the grass component will be higher yielding and make up for it.

There are certainly some practical aspects to this that may make it difficult to implement on all farms. Many times the alfalfa ground is in the lower well drained valley soils and the grass is being grown on the hill side at a higher elevation giving different growing conditions enough to make prediction with alfalfa inaccurate. So if you don’t have any pure alfalfa areas on the hills you may be able to work with alfalfa height if you adjust for the fact that it may be taller in the valley to give the correct NDF for the hill grass than would be expected at the same elevation. Also in many fields alfalfa will be more concentrated in better drained parts of the field with grasses dominating the poorer drained areas. Although the prediction equations were based on perfect mixes of alfalfa grass these prediction equations still hold some validity in fields where there is not a perfect mix. These fields will still need to be harvested with the alfalfa shorter favoring the chance to cut when total quality of the hay crop is high.

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