How many times have you heard the statements: Timely harvest is critical for achieving optimal quality of alfalfa forage and high forage quality is critical to good performance in high producing lactating cows? But how do you know when alfalfa is at the optimal quality for cutting? How often have you harvested and stored alfalfa without any knowledge of its chemical composition? If you are like most of us you make a best guess, watch the weather forecast, and then decide when to start cutting. Having lab results of forage quality on the standing crop would be nice, but getting them is not practical. Now for the good newsyou can quickly and easily estimate the forage quality of alfalfa in your fields, and you can do so with reasonable accuracy. Sounds too good to be true? Read on.
A method was developed recently at the University of Wisconsin to estimate alfalfa fiber composition of the standing crop. The Wisconsin method has been dubbed PEAQ, for Predictive Equations for Alfalfa Quality. The PEAQ method uses the longest stem and the most mature stem in an alfalfa sample to estimate neutral detergent fiber (NDF) concentration. The accompanying chart describes how to use this method. The charted values are calculated from a regression equation of the relationship between NDF concentration and stem length and maturity stage of alfalfa. The original method included a more detailed maturity staging scale, but we have found that a simple 3-stage scale (vegetative, bud, and flower) works just as well as the more detailed staging system. This simplification was agreed to in cooperation with the How reliable and accurate is PEAQ? We have tested the PEAQ method in production fields across Ohio and over multiple years, locations, and cuttings. It has also been validated in New York, Pennsylvania, Wisconsin, South Dakota, and California. After collecting 545 samples across these six states, we can conclude that the PEAQ method does indeed estimate NDF with reasonable accuracy. The estimated NDF (using PEAQ) was within 3 units of the actual wet chemistry NDF in 77% of the samples we collected. In 58% of the samples, the PEAQ NDF was within 2 units of the actual value, and in 88% of the samples the estimated NDF was within 4 units of the actual NDF. The PEAQ method performed consistently across a wide range of environments. It worked as well in other states as it did in Wisconsin, where it was originally developed. An important advantage of PEAQ over other prediction methods (such as the GDD method) is that it can be used during the entire growing season, not just on the first crop. So it provides a reasonably accurate guide for timing harvests according to your forage quality goals all season long.
What should my forage quality goal be? The NDF content of pure alfalfa forage should be near 40% in order to optimize profitability in a 50% concentrate ration fed to dairy cows. This will usually maximize cow performance and minimize supplementation costs. Optimal NDF concentration for pure grass forage fed to lactating dairy cows is between 50 and 55%. For mixed grass-legume stands (50:50 mix), the forage at feeding should contain 46 to 48% NDF.
When should I begin harvesting? The PEAQ method can be used to monitor the NDF content as the alfalfa crop develops. Use your judgement of how often this needs to be done, but you should make a first estimate at least one week before your expected date of harvest. If the goal is 40% NDF in alfalfa at feeding time, then cutting must begin well before the standing crop reaches 40% NDF. Harvest and storage losses can raise the NDF concentration by 2 to 6 units under good conditions. In our experiences, NDF concentration of well-made silage is about 2 percentage units higher than in fresh forage. Losses with hay harvest will be higher. You also need to adjust for the time it takes to harvest all your acres. During the spring, NDF increases about 5 points per week. So a reasonable goal is to begin harvesting alfalfa when it reaches 35 to 36% NDF as estimated by PEAQ. Harvesting should be completed by the time the standing crop reaches an estimated 40% NDF, assuming the forage is to be fed to lactating dairy cows.
If grass is present in the alfalfa stand, begin harvesting earlier. How much earlier will have to be your judgement, based on the grass maturity and amount present. There are currently no methods for estimating NDF concentration of grasses or grass-legume stands. But in pure grass stands, harvesting should occur in late vegetative to very early boot stage in the spring (head still enclosed in the flag leaf), and at 25- to 28-day intervals thereafter. If the stand is over 50% alfalfa, then the first harvest date and intervals between cuttings should be intermediate between those for a pure grass stand and a pure alfalfa stand.
Precautions when using PEAQ. Be sure to pay attention to technique. Measure the LONGEST stem in the sampling area from the soil surface near the base of the plant (as described in the chart), and correctly identify the MOST ADVANCED stage of maturity. The PEAQ method is designed for pure alfalfa stands, and will not accurately reflect the NDF concentration of mixed grass:alfalfa stands or weedy stands. The method is not reliable for alfalfa shorter than 16 inches or taller than 40 inches. The PEAQ procedure is NOT intended to replace laboratory analyses once the forage is stored. It should only be used to give a rapid first estimate of quality of the standing alfalfa forage for purposes of making informed harvest management decisions. Decisions regarding ration balancing should be based on laboratory analyses after the crop is harvested and stored.
Summary. The PEAQ method provides a reliable guide for optimizing quality of harvested alfalfa. It is more reliable than timing harvests by calendar date, age, or by maturity stage alone. Although rainy weather can foul up the best laid plans, using PEAQ in conjunction with weather forecasts should help you come closer to your desired forage quality goal. It can help you decide the order in which to harvest different fields based on forage quality. The estimated NDF content may also be useful when deciding where to store the forage, such as putting lots of similar quality together