Alfalfa is arguably one of the most variable feeds on many dairies. This is due to field-by-field variations in the age of stand (grass content), harvest maturity/moisture, fiber digestibility affected by the growing environment and issues around fermentation and palatability. It is well documented that environmental factors have a smaller effect on quality than on yield and that most factors that limit plant development (e.g. lack of water, cold weather, plant diseases) tend to promote higher quality because of their effect on altering leaf:stem ratios (Van Soest, 1996). Many nutritionists would rather that producers delay alfalfa silage harvest and deal with lowered digestibility than suffer with feeding rained-on, poorly fermented silages. Field experience has also conditioned producers to target ideal moisture levels at around 60% to reduce protein degradation and the potential for clostridial alfalfa silages. This does bring up a dietary issue regarding need for supplemental soluble protein (SP) in many high corn silage diets due to the fact that less SP is being supplied in the diet from reduced levels of drier (hence less SP) alfalfa silages.
Nothing influences the nutritional quality of alfalfa more than growing environment and harvest maturity. Fiber digestibility is higher under cooler temperatures (Figure 4) with 1st and 4th cuttings having the highest NDFD; and 2nd and 3rd cuttings typically grown under higher heat units, displaying the lowest NDFD. The biggest environmental factors influencing alfalfa quality are temperature, water deficiency, solar radiation and a distant fourth, soil fertility. Growing conditions that promote the highest alfalfa quality exhibit long day lengths, cool nights and moderately dry weather. Warm, wet weather results in the poorest-quality alfalfa. Cool, wet growing conditions produce high-quality alfalfa due to low neutral detergent fiber (NDF) and low lignification (Van Soest, 1996). However, getting a hay crop harvested in these conditions can be problematic, with harvest delays resulting in maturity issues, in addition to potential for higher respiration, leaching or fermentation/spoilage losses from increased exposure to soil-borne fungi and bacteria. Solar radiation (light) is the only environmental factor promoting both yield and quality. Light promotes carbohydrate production with every hour increase in day length increasing digestibility by about 0.2 percentage units (Van Soest, 1996).
Figure 4. 30-hour NDFD data from legumes (13261 samples), mixed grass/legumes (10158 samples), and grasses (2407 samples) analyzed by NIR at Cumberland Valley Analytical Services (Ward and de Ondarza, 2007).
The shortening photoperiod in the fall has a negative effect on alfalfa digestibility but is somewhat offset by cooler temperatures. Cloudy weather reduces photosynthesis, causing low sugar and mobilization of nutrients, which results in higher proteins; both of which can be problematic for silage production (Van Soest, 1996). There are also more pentose (5 carbon) sugars in fall-harvested alfalfa, further contributing to fermentation challenges. Drought conditions reduce yield, but the resulting stunted, yet leafy plants, are generally higher in protein and digestibility due to the higher leaf:stem ratio. The digestibility advantages would be greater if they weren‘t somewhat offset by increased lignification from high temperatures which typically accompany drought conditions. Temperature accelerates plant development and warm weather accelerates NDF development and lignification. Every 1°C increase in temperature will generally decrease the digestibility of forages 0.3-0.7% units (Van Soest, 1996). This is one reason why forages produced in northern latitudes or higher elevations (cooler nights) tend to be higher quality. In the spring, light and temperature are positively correlated until June 21 - when maximum day length and light occur - after which light decreases and temperature increases (bad for quality) until the fall, characterized by declining temperatures and decreasing day length and light (good for quality) (Van Soest, 1996).
Alfalfa Silage in a Day
This harvesting approach involves mowing alfalfa into a wide swath to facilitate faster drying followed by merging and chopping all within 24 hours. The most important factors to accelerate the drying of alfalfa are the amount of sunlight hitting the swath (swath density), wind velocity, relative humidity and ground moisture. Being able to harvest more quickly reduces the soluble protein degradation and conserves sugars for use during fermentation or by rumen bacteria. Research from the Northeast has suggested that use of conditioners at cutting time are of no benefit when wide swathing because it interferes with moisture transmission from the leaf stomata. Research from the University of Wisconsin clarified that most of the moisture loss is through stomata openings from fresh cut down to about 70% moisture. For moisture loss to continue beyond that, conditioning of the stem is essential. In that producers are targeting closer to 60% moisture alfalfa silage, most producers continue to condition alfalfa at harvest.
Many seed companies sell heavy-coated seed with the most common coating being 33% limestone. Heavy coat is usually less expensive per pound but more expensive on a basis of pure live seed. There is mixed research on the value of limestone-coated alfalfa seed varying from providing a more suitable micro-environment for seed germination to claims that limestone-coated seed has no advantage in cloddy or dry soil conditions and may actually slow water uptake under moderate to dry soils. Despite the research contradictions, alfalfa growers need to understand how heavy-coating affects both the cost and seeding rate in terms of the number of live seed sown per acre. For example, it takes 21 pounds of 33% limestone coated seed, to equal the same number of seeds per square foot as typical 9% coated seed sown at 15 pounds per acre. When a grower purchases heavy-coated seed without increasing seeding rate, they take on a higher risk of thin stands, stand establishment failure, more weeds during the seeding year and risk that yield over the life of the stand will be reduced.
Lodging Resistant Varieties
Lodged alfalfa is more difficult to harvest. Every inch of uncut stem equates to 0.13-0.15 tons per acre of lost hay yield. Uncut stems left in the field can turn "woody" and lower the forage quality of subsequent cuttings. One of the more recent innovations in alfalfa genetics is the commercialization of lodging-resistant varieties. These varieties have much improved standability when exposed to wind and rain events due to a more upright stem and crown architecture. Research also shows that more vertical plant architecture has no effect on lowering fiber digestibility.
There is a definite lack of consistent and statistically significant results from small-plot university research on the use of fungicides on alfalfa, yet farmer testimonials seem to suggest a positive response to fungicide application. General recommendations are to apply fungicides prior to first cutting when alfalfa is 6- to 8-inches tall. It only requires about 0.1- 0.2 tons per acre of added yield to justify the price of fungicide and application when the crop is selling for upwards of $200 to $250 per ton. Producer testimonials and company literature suggest early application to prevent fungal growth rather than assuming later maturity applications will eliminate disease problems after they have become established. The required yield improvement necessary to justify fungicide use is also less if growers are adding it to tank mixes of insecticide that they are already applying to control leafhoppers.
Positive grower observations may be the result of greater variability in their production-sized fields compared to smaller, replicated research plot studies in terms of canopy humidity levels, fungal loads, trash content and less than optimum soil environments (low pH, low fertility, poorly drained soils) across their larger acreages. More research is certainly needed on the effectiveness of other chemistries given the potential concern of resistant fungal populations. The good news is that, as growers continue to drive this market, more fungicides will likely add alfalfa to their approval list. As more research and producer experience is accumulated, there will likely be improved diagnostics as to when fungicides make the most sense such as in wet springs or on older stands. From a scientific, published literature perspective, the jury is leaning against the economics of alfalfa fungicides. However, fungicides would be expected to be most beneficial in growing conditions conducive to the development of stem and leaf diseases. Wet growing conditions coupled with a heavy crop should theoretically respond to a greater degree to fungicide application. Application in the fall may improve plant health to help stands weather the winter. Fungicides should also be more beneficial in stands which are harvested at later stages of maturity (e.g. lower lignin varieties) which are more susceptible to increased leaf drop (Mahanna and Thomas, 2014).
RFV versus RFQ
Relative Feed Value (RFV) was developed over a quarter century ago as a standard for comparing alfalfa quality based on voluntary animal intake of digestible dry matter. A RFV of 100 describes full bloom alfalfa hay containing 41% ADF, 54% NDF and digestible dry matter of 1.29% of body weight. Relative Forage Quality (RFQ) is an improvement on RFV in that it includes NDF digestibility in the calculation. If sellers and buyers of alfalfa would agree on the same reputable lab and base value on RFQ, there would likely be fewer situations where two lots of hay have the same fiber levels but considerably different results in lactating rations. Producers should remember that when using a RFQ target to stage harvest, it is not uncommon to lose 20 RFQ points during harvest and ensiling.
Lowering the cutter bar obviously results in higher yields of alfalfa. Research shows that alfalfa can be cut as short as 1.5 in. and that each inch above this will result in a half-ton-per-acre reduction in annual yields (Undersander, 2009). However, increased yields must be balanced against the tendency for disc mowers to vacuum soil (which contributes to ash values) into the crop, resulting in lowered digestibility and the potential for increased soil-borne clostridia.
AM versus PM Cutting
The time of day to harvest alfalfa (morning versus afternoon) has research results that fall on both sides of the debate. The basic idea is that cutting later in the day allows the crop to lay down more sugars to improve palatability or aid in silage fermentation. Much of the positive research has been conducted on alfalfa hay harvested in western states. Although morning versus afternoon forages differ in initial composition, these differences don‘t always exist after drying and/or fermentation because cell respiration reduces sugar levels at night and in sections of the windrow not receiving sunlight.
Research in Wisconsin (Undersander, 2003) showed that 11 of 14 Wisconsin farm samplings had higher sugars with afternoon-cut alfalfa, yet only one of the 14 had higher sugar levels in stored forage. There also appear to be adequate sugars to support fermentation when alfalfa is harvested at typical North American moistures/maturities compared to wetter European forages (Nasser et al., 2006). A Miner Institute study (Thomas, 2001, 2007) showed no statistical difference in plant sugars, starch, NDF or in vitro digestibility between am and pm harvested alfalfa. While afternoon-harvested alfalfa was numerically higher in sugar and starch, the small differences either decreased or disappeared entirely by the time the forage was 40% dry matter. Alfalfa mowed in the morning was ready for silage harvest in about nine hours, while alfalfa mowed in the late afternoon was not harvestable until after noon on the following day. Many researchers in the Midwest or East believe it makes more sense to cut early in the day to maximize the hours of drying from solar radiation rather than expose the crop to delayed drying or increased weather risk.
Reduced Lignin Alfalfa
The October 2014 World Dairy Expo in Madison, Wisconsin was the launch site of two new alfalfa technologies: a genetically-modified reduced lignin alfalfa (HarvXtra™) by Forage Genetics International (FGI) that will be licensed to a number of seed brands, and the other being lower-lignin alfalfa from Alforex Seeds developed through conventional plant breeding. The Alforex Seed products (Hi-Gest 360 and Hi-Gest 660) are reported by the company to have 7-10% less lignin and will be available in 34%-coated, non-Glyphosate-tolerant varieties on a limited basis for spring 2015 planting season (Jaynes, 2014).
HarvXtra was developed through a strategic partnership between FGI, The Samuel Roberts Noble Foundation and the U.S. Dairy Forage Research Center in conjunction with Monsanto. There are several steps in the process of lignin synthesis in alfalfa with the lignin biosynthetic pathway involving twelve different enzymes. Each is required for a specific step in the pathway. Noble Foundation scientists identified and suppressed several "lignin genes" that code for specific pathway enzymes. FGI scientists generated and evaluated biotechnology-derived plants with suppression of a specific lignin gene resulting in 10-15% decrease in lignin content, 10-15% increase in NDFD and RFQ when compared to related lines without the HarvXtra™ trait. HarvXtra™ alfalfa also displays a slower change in quality with advancing maturity (compared with conventional varieties) yet maintains alfalfa‘s important agronomic characteristics, including lodging potential equal to most commercial varieties harvested at the same time. HarvXtra™ alfalfa will be sold in a trait stack with Genuity® Roundup Ready® alfalfa. A petition to deregulate is currently under review by the USDA with anticipated limited commercial introduction in 2016 to allow growers the opportunity to realize the benefits of the technology, with 2017 as the first year of a wide-scale commercial launch (Fanta, 2014).
These technologies should provide alfalfa producers with greater harvest flexibility when either adhering to current harvest schedules and harvesting higher RFQ alfalfa or by delaying harvest to capture more yield yet maintaining desirable forage quality. In geographies that typically take four harvests, there is opportunity to improve yields upwards of 15-20% by harvesting only three times, and obtaining the same or better quality compared to lower-yielding late-bud harvest. The improved fiber digestibility of these varieties will likely provide the most benefits in transition and early-lactation diets where dry matter intake is of most concern. Research will be needed to determine desirable physically-effective fiber levels in rations containing low-lignin alfalfa, especially if it is coupled with BMR corn silage (Mahanna and Thomas, 2013).
Condensed Tannin Alfalfa
Researchers at the U.S. Dairy Forage Research Center are conducting studies with condensed tannins (CT) which are compounds found in forages such as birdsfoot trefoil that have the ability to bind proteins to reduce protein degradation during the ensiling process. Researchers are investigating new methods of assaying CT in forages and characterizing alfalfa bioengineered to produce CT. The practical utility of this technology will depend on the need for reducing protein degradation in alfalfa silage, which may not be desirable in high corn silage diets (Zeller et al., 2014).