Should dairies use grasses to increase NDF digestibility?
EAST LANSING, MI. — The use of ivNDFD for comparisons within a forage class is worthwhile. Research has shown that increased ivNDFD of forages is associated with higher dry matter intake (DMI) and higher milk production. The increase in DMI and milk yield is highest in high producing cows. The connection of forage ivNDFD and performance has led to speculation that using cool season grasses to increase the ivNDFD of forage mixes can increase DMI and milk production, thereby increasing forage value. .
There are agronomic and feed inventory reasons for growing grasses and legumes together, but mixing grasses with alfalfa as a shortcut to increasing DMI and milk production is a flawed strategy. One of the main reasons is that the digestive processes are different for grasses and legumes. First, legumes pass through the rumen faster than grasses due to the way plant particles break down. Legumes, being more brittle, tend to break down into cube-shaped fragments that can exit the rumen relatively faster, while grasses are broken down into long, thin strands that take longer to exit the rumen, while trapping potentially other small particles in the rumen. . Therefore, although the ivNDFD of the forage mixture may increase with the addition of grass, the potential for DMI and milk production does not necessarily increase. Increasing the proportion of herbs slows the rate of passage, which will generally negate the benefit of increased ivNDFD on DMI. It is important to remember that when considering ivNDFD and its effects on milk production and DMI, we should only compare ivNDFD within forage type.
Agronomic implications of grass/legume mixtures
From a crop production perspective, there are advantages to growing alfalfa and grasses in a mixture. Planting alfalfa with compatible grasses such as late maturing orchardgrass, tall fescue, meadow fescue, or early maturing timothy often results in a higher total forage yield than either component grown alone. This is because alfalfa and grasses have different canopy and root structures, nutrient requirements and microclimate preferences. This allows them to minimize direct competition for nutrient, water, and light resources while growing in the same space. In an ongoing research trial at Michigan State University, seeding alfalfa with orchardgrass, tall fescue, or meadow fescue in a 50:50 mix increased dry forage yield by up to 83 % compared to alfalfa alone. As farmland values rise, increasing forage yield per acre on land already farm-controlled can provide economic benefits.
Adding grass to alfalfa stands also increases stand life, thereby reducing costs associated with reseeding. Grasses can be overseeded in the spring into aging alfalfa stands, filling gaps and maintaining total stand yield as alfalfa declines. The fibrous root system of companion grasses can reduce frost heave of alfalfa and help support the weight of harvesting equipment to reduce traffic damage to alfalfa crowns. Root disease complexes are a major factor in the decline of alfalfa stands and traffic damage is a key factor allowing pathogenic organisms to enter alfalfa crowns. The mixtures also reduce insect pests. MSU research observed that mixtures of alfalfa and grasses reduced alfalfa weevil damage compared to pure alfalfa. Finally, growing forage mixtures increases biodiversity, which improves habitat for beneficial species like pollinating insects and below-ground microorganisms that contribute to soil health.
Adding alfalfa to hay meadows can result in huge savings. In Michigan, a cool season grass hay field should receive 100-150 pounds per acre of nitrogen for optimum yield which costs $100 for 150 pounds per acre when the price of Urea N is $1 per pound . Research in Ontario indicates that a mixed stand that contains at least 30% alfalfa can fix enough biological nitrogen to completely meet the needs of companion grasses and eliminate the need for purchased nitrogen.
Despite all the benefits, growing alfalfa and grass mixtures is not without its unique challenges. The first is that it is impossible to control the exact proportions of alfalfa and grass. Seed ratio at planting does not accurately predict what will establish in a new seeding, as the microsite or weather conditions at planting may favor the establishment of one component of the mix over the other. The proportion of grass generally increases with stand age as alfalfa disappears. Also, the established ratio of grass to alfalfa will not be perfectly uniform across a field at all times, nor consistent from cut to cut. The proportion of grass is usually higher in the first cut because cool season grasses flower in the spring. Summer regrowth cuttings generally favor the alfalfa component. In our aforementioned trial at MSU, alfalfa/grass mixtures averaged 51% grass in the first cut, but only 21% grass in the summer cuts. Compared to pure alfalfa, grass mixtures increased ivNDFD by up to 19% in first cut and up to 23% in summer cuttings. However, as mentioned earlier, this should not result in an increase in DMI or milk production. This example illustrates why dairy managers need to keep a careful inventory of which cuttings went into which silos and the importance of feed evaluation in a forage program.
Another challenge for growing alfalfa-grass mixtures is weed control. While mixes inherently provide some weed control by maximizing resource use by desirable species, problem weeds still sometimes gain a foothold. When this happens, there are no easy herbicide options that will maintain both alfalfa and grass while killing weeds. The best strategy for weed control in alfalfa-grass mixtures is to ensure weed problems are brought under control before the field is seeded with a mixture. Prevention is the key.
At the end of the line
Nutritionally, the mixture of grass and alfalfa may increase the average ivNDFD of forage, but is unlikely to result in an increase in DMI or milk production. This is due to the longer retention of grasses in the rumen, which leads to greater intestinal filling and reduced appetite in the cow. The nutritional value of a legume/grass hayfield can vary greatly from crop to crop and requires careful attention to forage testing and ration balancing. Agronomically, mixing grass and alfalfa in the field increases total forage production per acre, increases stand life, reduces the cost of nitrogen fertilizer for grasses, reduces insect pests, and smothers many weeds. However, weeds that do establish themselves can be very difficult to control. Grass/alfalfa mixtures also improve sustainability by increasing biodiversity and soil health, which improves farms’ resilience to extreme weather events. As with any decision, consider the motivations for establishing a grass/legume mix and think carefully about all the pros and cons.