Timing Spring Alfalfa Harvest – The Final Word?
Mike Allen, Rich Leep, and Jeff Andresen
Departments of Animal Science, Crop and Soil Science, and Geography
No doubt, in most dairy farmers’ minds, forage quality is an important determinant of farm profitability. Poor quality forages increase feed costs and limit milk production. The most important factor affecting quality of alfalfa is maturity at harvest. As alfalfa matures digestibility of dry matter and fiber decrease and fiber content increases. However, harvesting too early reduces the yield of alfalfa and results in alfalfa that is difficult to feed because its quality is too high. In addition, harvesting too early can reduce stand life of alfalfa. Although it is impossible to always harvest alfalfa at the optimum maturity because Mother Nature doesn’t always cooperate, there are new methods to help you come closer to your goal.
What is your goal? It is important to have a specific quality goal in mind for alfalfa harvest. Alfalfa to be fed to lactating dairy cows normally should be harvested at 40% neutral detergent fiber (NDF). Why use NDF as a criterion? Because NDF is an important factor for formulating diets. Diets with excess NDF content are more filling and limit intake whereas diets with inadequate NDF content increase health problems such as acidosis, displaced abomasums, and laminitis. Alfalfa should be harvested at the lowest fiber content that will provide adequate levels of fiber in the diet and allow a reasonable amount of grain to be added to maximize energy intake and milk production. When diets are formulated with an overly mature alfalfa (high NDF content), the NDF content of the forage must be diluted with a higher concentration of grain to increase energy density and reduce the filling effect of the diet. Because protein content is usually lower for mature alfalfa, more protein supplement is required as well. Besides being costly to supplement, lower NDF digestibility of diets formulated using overly mature alfalfa might limit feed intake and milk production, particularly for the highest producing cows. When diets are formulated with an immature alfalfa, little grain can be included because the NDF content of the alfalfa is close to the NDF required in the diet. This will result in a lower energy density diet and might result in lower energy intake. Diets containing immature alfalfa also tend to have excess protein content because the protein content of immature alfalfa tends to be quite high. Not only is feeding excess protein wasteful, but it costs energy for the animal to excrete, might be an environmental contaminant in the form of leached soil nitrates, and might reduce reproductive performance. This is a greater problem when immature alfalfa is the only forage fed; it is easier to formulate diets with immature alfalfa when a high NDF, low protein forage is included. A goal of 40% NDF is often chosen for alfalfa because it is felt that this optimizes profitability from a combination of factors influencing yield of alfalfa, supplement cost, and milk production. Some circumstances might justify harvesting alfalfa earlier such as when the immature alfalfa will be blended in a diet with a higher NDF, low protein forage. However, harvesting immature alfalfa risks health of the plants and therefore the productive life span of the alfalfa stand, particularly for spring harvest.
How do I attain my goal? There are several methods that can be used for timing first alfalfa harvest to come closer to your quality goal than by harvesting by calendar date. Cumulative temperature during spring growth may vary greatly from year to year, resulting in large differences in quality on the same date. In an experiment conducted in 1990 and 1991, we found that the NDF content of spring harvested alfalfa varied up to 10 percentage units when harvested on the same date from one year to the next. However, when the difference in cumulative temperature between the years was accounted for using growing degree-days (GDD), little difference in NDF content of alfalfa was observed. Harvesting alfalfa by GDD is one of the methods that can be used to get closer to your goal. Other methods include the scissors cut, harvesting by stage of maturity (such as late bud stage), or by a combination of maturity and height using the PEAQ (Predictive Equations for Alfalfa Quality) method (see below).
What are GDD and how are they calculated? Growing degree-days are a temperature-derived index representative of the amount of heat that the plants are exposed to, which in turn is directly related to the rate of growth and development of the plant. GDD are calculated several ways, so it is important to make sure GDD are calculated using the same formula that was used to establish the relationship you will use to harvest alfalfa. For instance, different base temperatures are used for different crops, and corn uses a different method of calculation entirely. Alfalfa GDD used for recommendations in this article are calculated by averaging the maximum and minimum temperature (° F) for a given day (24-h period) then subtracting the base temperature of 41°F to get the number of GDDs for that day. Daily GDD for days with an average temperature less than 41°F are set equal to 0. Finally, a seasonal total is then obtained by summing the daily growing degrees from March 1st through the current day. Example: if the maximum and minimum temperatures for one day are 75° F and 39° F, respectively, the average is 57 and the growing degrees for this day are 16 (average 57 minus base 41). Agricultural Meteorology Staff at Michigan State University generate a table of GDD using this calculation for alfalfa on Mondays and Thursdays through mid-June of the growing season using data from weather stations throughout Michigan. The alfalfa GDD results are broadcast on DTN and the MSU Agricultural Weather Web Site (at www.agweather.geo.msu.edu) as well as emailed to all county extension offices in Michigan twice weekly. Note that because they are calculated with a different method, the GDD published in the weekly MSU Extension Crop Advisory Team Alert cannot be used for alfalfa harvest at this time.
How close to my goal can I get? It is important to realize that no method will predict the NDF content of the alfalfa harvested perfectly. There are many factors that affect the accuracy of prediction including: the amount of grass and (or) weeds in the field, proximity to the weather station used, proximity to a large body of water, difference in elevation from the weather station, adequacy of soil moisture, length of time wilting, exposure to rain between cutting and chopping or baling, and quality of fermentation for silage. Using GDD (base 41° F), we can predict NDF content within ± 3 percentage units about 70% of the time and within 6 percentage units 90% of the time. Thus, if you are using GDD (base 41° F) to achieve 40% NDF, in 7 out of 10 years the alfalfa will be between 37 and 43% NDF and in 9 out of 10 years it will be between 34 and 46% NDF. Although GDD cannot be expected to predict alfalfa NDF perfectly, it is certainly better than harvesting by calendar date and it seems to work as well or better than other methods in Michigan and it is a very simple method.
When should I begin harvesting? According to data collected in the upper Midwest over several years, alfalfa averages 40% NDF at 750 GDD (base 41° F). It takes about 220 additional GDD to reach 45% NDF. Beginning harvest at 40% NDF will give about a 7 day window to complete harvesting before the alfalfa reaches 45% NDF. If you are storing the alfalfa in a horizontal silo or feeding it in a diet with a high NDF, low protein forage, start cutting at 680 GDD (base 41° F), which corresponds to 38% NDF. Filling horizontal silos with layers of increasing maturity will allow harvest to begin a little earlier because the layers of alfalfa are blended as the alfalfa is removed from the silo. If a higher fiber, lower protein forage is included in the diet it will provide the necessary fiber and dilute the high protein content of the less mature alfalfa. What about alfalfa stands containing grass? Grasses have higher NDF content than alfalfa harvested at the same age, and, therefore, older alfalfa stands containing grass should be harvested before purer alfalfa stands. Begin harvesting the fields with the most grass first so that purer alfalfa stands can be harvested at the appropriate NDF content. If wet weather delays harvest and the fields with grass can be segregated for storage, consider harvesting the purest stands of alfalfa first and harvesting the fields containing grass for feeding to cows in the early stages of the dry period and (or) heifers.
What about harvesting second and later cuttings of alfalfa? Predicting alfalfa NDF content using GDD cannot be done when there is inadequate soil moisture during the growing season because GDD accumulate with little or no response in plant growth. Because of this, GDD is highly related to quality only for spring harvest alfalfa with adequate rainfall and not for subsequent cuttings, which nearly always have periods of inadequate soil moisture during growth. How much does it cost me to delay harvest? A lot! It costs as much as $0.10 per cow per day for each percentage unit of NDF over 40%. This cost is associated with increased energy and protein supplement and lost production of milk from the effect of lower NDF digestibility on dry matter intake. The cost is even higher as production per cow increases because rumen fill becomes a greater constraint on feed intake. Although there will be some increase in yield of alfalfa by delaying first harvest, the cost of lower milk production and supplemental energy and protein easily justify harvesting on time.
Should I continue planting corn or harvest alfalfa? When wet weather delays planting of corn it will usually pay to stop planting corn to harvest alfalfa, particularly for high producing herds. Corn grain yield is reduced approximately 1 bushel / acre per day when planting is delayed in late May whereas alfalfa NDF content increases about 0.6 units per day. For a 100-cow dairy farm feeding alfalfa as the only forage, with 130 acres of corn grain and corn valued at $2.50 per bushel, a 1-day planting delay will cost $325 per year. However, assuming first cutting alfalfa is 40% of the total annual yield of forage, the cost of delaying alfalfa harvest because of the need for more supplemental feed and decreased milk production is over twice this amount. The cost would be less if first cut alfalfa were not the only forage used.
What about measuring GDD on my farm? Although we can use data from nearest weather stations for determining GDD, for increased accuracy it is best to measure the minimum and maximum temperatures as close to the alfalfa field as possible. You can use an inexpensive minimum / maximum thermometer and read it every day from March 1 on, or you can purchase an electronic data logger that records temperature automatically. Whichever you use, it is very important to place the temperature sensor correctly. The database that was used to develop the NDF prediction equations from GDD used temperature data from weather stations in which the temperature sensors are located in a ventilated enclosure 5 feet above the ground. If an enclosure is not provided with your sensor, a good, cheap alternative for a field enclosure would be a small wooden box (interior grade plywood works well) mounted on a fence post. The box should be open on one side and on the bottom, painted white with the open side facing north and the open bottom towards the ground. Try to mount the thermometer off of the surface of the enclosure (at least an inch) to facilitate free airflow around the sensor. Also, never allow a thermometer to be exposed to direct sunlight. Temperatures and resulting GDD sums taken from instruments that are biased because of site or exposure problems may be erroneous or misleading.
What about other methods? Several alternative methods are sometimes used to predict alfalfa quality: scissors cut programs, maturity, and the PEAQ method. Scissors cut programs involve taking samples of alfalfa directly from the field and sending the samples to a feed testing laboratory for analysis. This method is thought by some to be the “gold standard” to which all other methods should be compared because quality is actually measured and not predicted from a formula. However, this method is subject to other errors and might be no more accurate than the predictive methods. The most significant potential error is that the NDF content of the sample begins to increase once the plant is clipped because sugars are converted to carbon dioxide and water through respiration. Fresh alfalfa is very unstable, and there is no way to prevent respiration. Although cooling can slow respiration, the sample should not be frozen because this also will increase the NDF content. In addition, wet chemistry should be used because NIRS (near infrared reflectance spectroscopy) equations for fresh alfalfa are not generally available and NIRS predictions likely are inaccurate. This method is time consuming and costly, takes longer to get results (results may be received after harvest should have occurred), and is not necessarily more accurate for alfalfa than using GDD. However, it can be more accurate for determining NDF for a particular field containing alfalfa and grass. If this method is used, we recommend taking multiple samples representative of a field, submitting more than one sample for analysis, and have the sample tested by wet chemistry.
Harvesting alfalfa by stage of maturity such as late bud or early flower is often used but it is difficult to determine the average maturity of a field of alfalfa, particularly if there is insect damage. This method takes some time and is no more accurate than using GDD. The PEAQ method requires sampling alfalfa stems from several locations in an alfalfa field and determining length of the longest stem and stage of maturity of the most mature stem. These values can be read directly from an alfalfa quality stick to predict alfalfa NDF. This is an easy method, it takes little time to perform, but has been shown as good at predicting first harvest and better for second harvest alfalfa NDF than GDD in 3 years of on-farm comparisons in several locations in Michigan. However, this method or GDD is not as reliable to predict quality for 3rd cutting of alfalfa. To find out how to get Alfalfa Quality Sticks, you can email Richard Leep at firstname.lastname@example.org or call your local extension agent.
Is this really the final word? Although we currently recommend using GDD and PEAQ over other methods for spring harvest alfalfa, definitive studies comparing the methods still need to be conducted over several years evaluating the effects of sample preparation (hay versus silage), analysis method (wet chemistry versus NIRS), and measurement of GDD on site. Because of this, we might change our recommendations as more information becomes available. For the time being, remember these points about using GDD or PEAQ to harvest alfalfa.
Neither the GDD or the PEAQ method are perfect, but will help get you closer to your forage quality goal.
Make sure you use the correct GDD equation for alfalfa.
Use the GDD method for first harvest alfalfa only or PEAQ for cut one and two or better yet, use both methods.
Begin cutting alfalfa at 750 GDD (base 41° F) for upright silos and 680 GDD (base 41° F) for horizontal silos. Start even earlier for horizontal silos if it takes more than a week to finish harvesting.
The GDD or PEAQ method cannot be used for fields containing grass.
Fields containing grass should usually be harvested first. Start with the fields with the most grass first and finish with the purest alfalfa fields (still begin cutting pure alfalfa at 680 to 750 GDD).
Consider using the scissors cut method for fields containing grass.
Harvest second and later cuttings