Recent findings are leading to new insights into how best to improve the nutritional value of forage grasses and which approaches aren”t likely to pay off for producers.
Improved grass varieties are becoming more important in Wisconsin, where producers have increased their reliance on grazing as the low-cost way to produce milk and beef. Almost one-quarter of state dairy farmers now use management-intensive rotational grazing — more than triple the use eight years ago.
“Efforts to select grasses with less fiber or less lignin don”t appear promising,” says Michael Casler, a University of Wisconsin-Madison plant breeder. “To improve animal performance, researchers are likely to have greater success if they select for plants where the lignin is less tightly bound to cell walls or select for plants with more nutrients inside cells.”
Casler”s latest results with smooth bromegrass show that breeding for greater digestibility – less lignin – or greater feed intake – less fiber – can produce plants that are susceptible to disease or produce low yields.
However, those results don”t discourage Casler, who is with the College of Agricultural and Life Sciences. After all, he says, grasses have evolved over millions of years and have developed traits that help them survive animal grazing and withstand diseases.
In trying to improve agronomic traits, researchers must alter the basic nature of grasses, according to Casler. In doing so they often run into problems but they also have experienced successes. “Plant breeders began efforts to develop improved grass varieties 40 years ago,” he says. “They”ve released many new varieties that animals find easier to digest.”
A new variety with just a small improvement in nutritional quality can increase animal production on pastures by more than $20 per acre, Casler says.
Wisconsin graziers plant much more orchard grass, reed canary grass, timothy, tall fescue or meadow fescue in their pastures than smooth bromegrass. Casler has studied many of these forage grasses while working on smooth bromegrass for 20 years. Over those years, Casler has produced bromegrass lines that differ in key traits related to their nutritional value.
“We”re now at the point where we know enough about bromegrass that we can ask important questions and determine the answers,” he says. Those answers are likely to influence how breeders develop new varieties of the other grasses.
Casler believes there are four main ways to improve the nutritional value of grasses for cows — decrease lignin, decrease cross-linking between lignin and carbohydrates in plant cell walls, decrease total fiber, and increase soluble carbohydrates in cells. He is particularly interested in how lignin and cell walls affect the nutritional value of forages.
Lignin is the indigestible glue that holds plant cells together. As the lignin content of plants increases, the ability of animals to digest them decreases.
To increase plant digestibility, researchers have been selecting lines of alfalfa and grasses with less lignin. Although this approach worked with alfalfa, Casler has found that it makes smooth bromegrass more susceptible to crown rust. A common disease of oats in the Midwest, crown rust reduces yields and makes the plants less digestible. Casler says others have found the same relationship between lignin and rust with perennial rye grass and meadow fescue.
“The results are a red flag,” Casler says. “If we use this approach to improving the digestibility of forage grasses, we”re going to have to watch out for rust.”
Either the same genes or closely linked genes make the plants more resistant to rust and less digestible, Casler says. If they are separate genes, then researchers can breed for rust resistance and improved digestibility. Such a program, however, would take a long time, he says. If the same genes affect both traits, breeding for less lignin will inevitably produce plants that are vulnerable to rust.
For years, animal scientists have wanted forages with less total fiber because they dramatically improve animal production. As fiber concentration decreases in feed, animals eat more. But decreasing total fiber is not the way to increase the nutritional value of forages, according to Casler.
Over 10 years, he developed lines of smooth bromegrass with 3 percent less fiber than standard lines. “It may not sound like a big change, but animal scientists sit up and take notice at a reduction like that,” he says. “The field tests of the low-fiber lines were a flat-out disaster. The yields of low-fiber lines were way down. The whole point is to produce more milk and meat from the same acreage.”
Casler found that the same genes that control fiber in bromegrass were responsible for 70 percent of the yield depression. “Fiber is the largest single component of plants,” Casler says. “It makes sense that reducing the fiber content of bromegrass also reduces yields.”
So how should plant breeders proceed? Casler says the best approaches for improving the nutritional value of forage grasses are likely to be selecting for less cross-linking between lignin and the carbohydrates in cell walls, and selecting for more soluble carbohydrates in cells.
The research was supported by state funding to the College of Agricultural and Life Sciences as well as a Hatch grant from the College.