Should dairies utilize grasses to increase forage NDF digestibility?
Blending grasses in alfalfa fields can increase forage NDF digestibility, but is it a profitable decision?
In our previous article, we discussed how to make use of in-vitro NDF digestibility (ivNDFD) feed analyses. Using ivNDFD for comparisons within a forage class is worthwhile, and research has demonstrated that increasing ivNDFD of forages is associated with greater dry matter intake (DMI) and milk production. The increase in DMI and milk yield is greatest in high-producing cows. Connecting forage ivNDFD and performance has led to speculation that utilizing cool season grasses to increase ivNDFD of forage blends may increase DMI and milk yield, therefore increasing the value of the forage.
There are agronomic and feed inventory reasons for growing grasses and legumes together but mixing grasses with alfalfa as a shortcut to increase DMI and milk production is a flawed strategy. A key reason is that digestion processes are different for grasses vs. legumes. First, legumes pass through the rumen more rapidly than grasses because of how the plant particles break down. Legumes, being more brittle, tend to break down into cube-shaped fragments that can exit the rumen relatively quicker, whereas grasses are degraded to long thin strands that take longer to pass out of the rumen, while potentially also entrapping other small particles within the rumen. Therefore, even though the ivNDFD of the forage mix may increase with the addition of grass, the DMI and milk production potential does not necessarily increase. Increasing the proportion of grasses slows passage rate, which will generally cancel out the benefit of increased ivNDFD on DMI. It is important to remember that when considering ivNDFD and its effects on milk yield and DMI, we must only compare ivNDFD within forage type.
Agronomic implications of grass/legume mixes
From a crop production perspective, there are advantages to growing alfalfa and grasses in mixtures. Planting alfalfa with compatible grasses like late-maturing orchard grass, tall fescue, meadow fescue, or early maturing timothy often results in greater 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 orchard grass, tall fescue, or meadow fescue in a 50:50 mix has increased dry forage yield up to 83% compared to alfalfa alone. As farmland values increase, increasing forage yield per acre on land already under farm control may provide economic benefits.
Adding grass to alfalfa stands also increases stand life, thus reducing costs associated with reseeding. Grasses can be overseeded in spring into aging alfalfa stands, filling in the gaps and maintaining total stand yield as alfalfa declines. The fibrous root system of companion grasses may reduce frost heaving of alfalfa and help support the weight of harvest equipment to reduce traffic damage to alfalfa crowns. Root disease complexes are a major factor in alfalfa stand decline and traffic damage is a key factor allowing disease organisms a pathway into alfalfa crowns. Mixtures also reduce insect pests. MSU research observed that alfalfa/grass mixtures had reduced alfalfa weevil damage compared to pure alfalfa. Lastly, growing forage mixtures increases biodiversity which improves habitat for beneficial species like pollinator insects and underground microorganisms that contribute to soil health.
Adding alfalfa to grass hayfields can result in huge cost savings. In Michigan, a cool-season grass hayfield should receive 100-150 lb./acre of nitrogen for optimum yield which costs $100-150/acre when the price of urea N is $1/lb. 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 advantages, growing alfalfa/grass mixtures is not without unique challenges. One is that it is impossible to control the exact proportions of alfalfa and grass. Seed proportion at planting does not accurately predict what will establish in a new seeding because microsite or weather conditions at planting may favor establishment of one component of the mix over the other. The proportion of grass typically increases with age of stand as the alfalfa fades out. Moreover, the established proportion of grass and alfalfa will not be perfectly uniform across an entire field at any moment in time, nor will it be consistent from one cutting to the next. The proportion of grass is typically greatest in the first cutting because cool-season grasses flower in the spring. Summer regrowth cuttings typically 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 summer cuttings. When compared to pure alfalfa, the grass mixtures increased ivNDFD up to 19% in first cut and up to 23% in summer cuttings – but as we mentioned previously, this is not expected to translate into increased DMI or milk yield. This example does illustrate why dairy managers must keep careful inventory of which cuttings went into which silos and the importance of feed evaluation in a forage program.
Another challenge to growing alfalfa/grass mixtures is weed control. While mixtures inherently provide some weed control by maximizing resource use by desirable species, problematic weeds still sometimes get a foothold. When this happens, there are no easy herbicide options that will maintain both alfalfa and grass while killing the weed. The best strategy for weed control in alfalfa/grass mixtures is to make sure weed problems are brought under control before the field is planted to a mixture; prevention is key.
Nutritionally, mixing grass and alfalfa may increase the average ivNDFD of the forage but is not likely to result in greater DMI or milk production. This is due the longer retention of grasses in the rumen, leading to greater gut fill and decreased appetite in the cow. Nutritional value of a legume/grass hayfield can change greatly among harvests 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 nitrogen fertilizer cost for grasses, reduces insect pests, and chokes out many weeds. However, weeds that do get established may be very difficult to control. Grass/alfalfa mixtures also improve sustainability by increasing biodiversity and soil health which improve resilience of farms to extreme weather events. As with any decision, consider the motivations for establishing a grass/legume mix and deliberate carefully about all the pros and cons.
- Yield and nutritive value of grazed complex legume–grass mixtures under increasing nitrogen application rates.
- Forage grasses decrease alfalfa weevil (Coleoptera: Curculionidae) damage and larval numbers in alfalfa-grass intercrops.
- Rates of particle size reduction and passage are faster for legume compared with cool-season grass, resulting in lower rumen fill and less effective fiber.