The authors are research scientists with USDA’s Agricultural Research Service (ARS) and its Dairy Forage Research Center in Madison, Wis.
Alfalfa is known for its high nutritional value, and that makes it a valued forage source. However, alfalfa is often hindered nutritionally because of the indigestible lignin components in the cell wall. Using technology to improve fiber digestibility in alfalfa can provide opportunities to improve feeding flexibility and enhance animal performance. Importantly, high-quality alfalfa can be used to improve cow efficiency, production, and enhance an operation’s economic value.
Alfalfa has a number of beneficial nutritional characteristics that make it a high-quality feedstuff for dairy cows. Alfalfa is sometimes overlooked or left out of the diet because of its high expense relative to other forages. This includes its high establishment and harvest costs. This situation has led to less overall alfalfa production in the United States in the last 20 years. Incomplete fiber digestion reduces animal performance by limiting intake, and that also cascades to more manure production. When that takes place, a dairy farm’s profit potential also drops.
That means it’s critically important to maximize fiber digestibility to take full advantage of the nutrients in forage sources. A one-unit gain in forage fiber digestibility is associated with over 0.5% improvement in both dry matter intakes (DMI) and milk production per day. Additionally, each percentage unit rise in lignin concentration reduces the digestibility of forage cell walls by two units. This could potentially lead to further reductions in cow intake and milk production.
Lignin provides strength and rigidity for the plant. However, it also leads to reduced digestibility as the concentration of lignin climbs with maturity. Lignin content can also be directly related to cell wall digestibility by forming cross-linkages with other cell wall components such as cellulose and hemicellulose, making the forage less digestible. Alfalfa leaves maintain high fiber digestibility throughout the growth cycle, while the stem material accumulates more lignin as the plant approaches full bloom.
Lignin is the focus
Alfalfa varieties with reduced-lignin content have achieved significantly greater fiber digestibility by, as the name suggests, reducing lignin in the plant cell wall. There are a few alfalfa varieties on the market that utilize different approaches to reduce lignin. One such specific variety, HarvXtra, uses a genetic modifier to down-regulate the production of lignin.
Additionally, breeding experts continue to focus on conventionally bred, reduced-lignin alfalfa. Another alfalfa variety, marketed under the name Hi-Gest, attempts to manipulate the leaf-to-stem ratio through conventional breeding methods. This variety improves digestibility by enhancing the percentage of leaves, which are more digestible, in proportion with less stems, reducing the overall content of lignin in the harvested forage.
Added harvest flexibility
Alfalfa has environmental and sustainability advantages when compared to corn silage, another popular forage source. However, because corn silage is harvested one time in the fall, it has a perceived economical advantage over alfalfa . . . a crop often cut three to five times in a season, thus requiring more labor and machinery costs. Under other scenarios, alfalfa is often cut more frequently, sacrificing yield, to maximize quality and improve fiber digestibility. Reduced-lignin alfalfa may offer an advantage to harvest management flexibility without having to sacrifice yield.
Using reduced-lignin alfalfa also may lengthen the time window when alfalfa has suitable nutritive value, allowing for wider optimal harvest windows. This would allow alfalfa growers to accumulate larger amounts of forage by delaying harvest while still maintaining acceptable high forage digestibility for lactating dairy cow diets.
A field experiment conducted at six locations in Kansas, Michigan, Ohio, Pennsylvania, California, and Wisconsin over a two-year period reported that reduced-lignin alfalfa (HarvXtra) consistently reduced neutral detergent fiber and acid detergent lignin and enhanced fiber digestibility compared to two other conventional varieties of alfalfa. This ultimately led to a seven-to-10-day advantage in nutritive value using a 38-day cutting schedule.
Another study reported no differences in yield or nutrient quality when the alfalfa was harvested at 28-day intervals. However, in the same study, extending harvest to a 35-day cutting interval led to greater yields while maintaining nutritional quality compared to a control alfalfa that sacrificed quality for greater yields. Ultimately, the HarvXtra alfalfa had a similar improvement in yield but a 12% to 15% advantage in digestibility.
The science of feeding
Improving the nutritive value of alfalfa, by enhancing fiber digestibility, often leads to improved milk production. This response is primarily because of greater intakes, regardless of whether it is a reduced-lignin variety or a conventional variety.
Improved fiber digestibility or increased milk production may not be expected if the reduced-lignin alfalfa was from a delayed harvest, such as seven to 10 days later. If a normal cutting schedule is maintained, then the higher quality reduced-lignin alfalfa may lead to improved milk production. However, research utilizing reduced-lignin alfalfa in lactating dairy cow diets is very limited.
Two studies recently conducted at the U.S. Dairy Forage Research Center evaluated the inclusion of reduced-lignin alfalfa silage in lactating dairy cow diets. In the first study, reduced-lignin alfalfa silage was included as a replacement for soyhulls and supplemental protein in dairy cow diets. Substitution of supplemental protein and nonforage fiber feedstuffs with reduced-lignin alfalfa silage, up to 18% of the diet (dry matter basis) with forage inclusion ranging from 50% to 68%, maintained milk production and enhanced milkfat percentage and yield. It also improved the conversion of feed to milk.
A second study conducted at the U.S. Dairy Forage Research Center evaluated two different harvest intervals (bud and mid-flower stage) in both a conventional and reduced-lignin alfalfa when fed to dairy cows. It was assumed that reduced-lignin alfalfa may improve milk production if harvested at a similar interval as the conventional alfalfa because of greater fiber digestibility and greater dry matter intake. Conversely, if harvest is delayed seven to 10 days, the reduced-lignin alfalfa may maintain milk production compared to possible losses in production when feeding late-harvested conventional alfalfa.
In this experiment, cows fed the late-harvested conventional alfalfa resulted in the poorest feed conversion efficiency. Because this alfalfa was likely of poorer quality, cows consumed more feed to meet energy requirements to produce milk. Greater digestibility for the early harvested reduced-lignin alfalfa allowed cows to eat more and produce more milk. However, cows were not as efficient as early harvested conventional alfalfa on a fat-corrected milk basis.
Fit for your operation
Compared to conventional alfalfa, reduced-lignin alfalfa can be a useful tool to improve harvest flexibility. Delayed harvest using reduced-lignin alfalfa may or may not reduce total milk production compared to harvesting at shorter intervals. However, delaying harvest using reduced-lignin varieties allows for greater tonnage to be harvested with minimal sacrifices in forage quality while maintaining feed conversion efficiency.