I love grass — forages, that is!
There, I said it! I prefer grass forages to alfalfa!
But why? Simply put, good-quality grass forage can perform as well as, or better than, legume forages in certain cases. They tend to be more hardy, especially in Vermont, and they work well with a farm’s nutrient management plan.
• Balanced for NDF, grass forage rations at least equal legumes.
• Grasses are hardier, less expensive and less wasteful than legumes.
• A grass-legume forage mix is the best of both worlds.
Forage nutrient composition varies depending on species, maturity, fertilization and soil fertility, growing environment, harvest and storage conditions. Generally, grass forages tend to have greater neutral detergent fiber, or NDF, content than legumes — making them more digestible.
Best feed for rumen microbes
More digestible NDF means more cellulose, hemicellulose and neutral detergent soluble carbohydrates (sugars, pectins, etc.) available for rumen microbes: a good thing!
More available nutrients for the microbes means greater numbers of rumen microorganisms. More rumen microbes means more microbial protein reaching the small intestine, and more amino acids available for milk, growth and reproduction.
Conversely, alfalfa’s lignin content is greater than grass.’ Lignin, being indigestible in the rumen, speeds passage through it, reducing overall diet digestibility.
Most studies that reported greater milk production on diets containing legume versus grass silage, compared diets that had the same percent forage — but not the same percent NDF. When a Minnesota research study balanced diets for NDF content, for example, milk yields were the same for both alfalfa and grass silage diets.
Crude protein, or CP, concentration in legumes is higher than in grasses. The bulk of CP in fresh legumes or grasses is true protein (amino acids, peptides, etc.). About 10% to 15% is non-protein nitrogen, or NPN.
As CP rises, NPN tends to increase. CP and NPN also increase when grasses are fertilized with nitrogen. NPN takes another jump when either legumes or grasses are fermented. Between 30% and 65% of CP is NPN in fermented forages.
Nitrogen solubility is greater in legumes than in grasses. That means excess soluble dietary protein is more likely with legumes. And that’s not a good thing.
It’s a wasteful waste!
Excess soluble protein converts to ammonia in the rumen. Once this occurs, it diffuses across the rumen wall, where the liver metabolizes it to urea, a harmless nitrogen form, for excretion in urine and milk.
The point: This waste is wasteful and takes energy — energy that could be used for milk, growth and/or reproduction.
Given the variability in forages and forage quality, blanket statements about the superiority of one type of forage over the other for milk production are impossible. For every study or farm that makes more milk on legume forages, there is another study or farm that has had great success with grass forages.
My standard advice is to feed high-quality (low NDF, high digestibility) forages, and feed lots of them. Balance those forages with minimal supplement additions, including starches, degradable and nondegradable proteins, vitamins and minerals.
The best way to ensure optimal use of forages and supplemental concentrate feeds is to model those diets with computer diet evaluation software such as the Cornell-Penn-Miner diet evaluator. And test, test and test!
Test forages for nutrient content, especially NDF and NDF digestibility. Don’t be afraid to feed grass forages. Better yet, a diet with both legume and grass forages is the best of both worlds!
Carson and husband Steve partner in Harkdale Farms of Newbury, Vt. She’s also a professor at Vermont Technical College.
This article published in the January, 2011 edition of AMERICAN AGRICULTURIST.