seal  Purdue News

June 12, 2003

Stressed forages require a little TLC

WEST LAFAYETTE, Ind. – Forages are perennials, but farmers can't expect them to come back healthy each year without a little care, says a Purdue University agronomist.

"When farmers completely mow off a forage, it's like putting plants on a starvation diet," said Jeff Volenec, professor of agronomy. "It has to rely on what it's got stored in order to survive, and even thrive."

Forages rely on reserves that can be depleted by stresses imposed by farmers as well as nature. Volenec said farmers need to manage forages to allow the plants to have adequate stored reserves to survive.

Some of the stresses these plants encounter are overgrazing, defoliation, extreme temperatures, improper fertilization, untimely mowing and incompatible forage mixtures.

"When animals overgraze, they are eating the plant's reserves," Volenec said.

Forage roots and lower stems store starch, sugars and protein. These stored reserves help them survive defoliation and overgrazing; however, not all grasses are as tolerant. Orchardgrass, timothy and tall fescue have stolons, or stem bases, to aid in storage. When cut too short, the stem bases can be damaged or removed, reducing the availability of reserves. Other plants, such as birdsfoot trefoil, are highly sensitive to overgrazing because they do not store reserves in summer.

The temperature at mowing affects carbohydrate reserve levels. If it's too cold, the plant may not have had time to build up reserves, and high temperatures reduce sugar synthesis in leaves via photosynthesis and increase the rate of sugar loss.

Sugar synthesis in leaves also is influenced by fertility. If fertility is low, reserve levels may be low, according to a three-year study of potassium fertilizer impact on alfalfa yield conducted by Volenec.

"Plants that are provided at least 200 pounds of potassium per acre had good stands and high forage yield in spring even when cut four times during the growing season," he said. "Unfertilized stands had lost half to three-fourths of the plants by the fourth year."

Not all plants are the same, and that is why compatible forage mixtures are important. The selected species should have synchronized reserve accumulation and use. Alfalfa and orchardgrass are a perfect example, both have a near-identical pattern that permits producers to mow the field and have both plants regrow vigorously.

Volenec said another reason forages don't survive as long is because on many farms corn and soybeans are planted in the best ground and forages are relegated to the poorest ground.

"Farmers then expect these forages to grow and prosper, but the plants struggle because of low fertility and limited water supply that increase stress," he said.

"Forages are incredibly stress tolerant and are perennials, but forage legumes like alfalfa usually only last about five years at best. True perennials should last at least 20 years or more. Farmers should want their forages to last longer because it is expensive and difficult to re-establish a good forage stand."

Volenec estimated it costs about $400 per acre to re-establish alfalfa, and since forages are seeded on many hillsides, the ground is highly erodible.

Producers can learn more about forage management and other related issues on June 26 from 8:30 a.m. to 4 p.m. during Purdue Forage Day. The event will take place at the Dennis Smeltzer Family Farm in Middlebury, Ind., in the field located at the intersection of county roads 8 and 10. For more information, call 1-888-EXT-INFO or visit

Writer: Michelle Betz, (765) 494-8402,

Source: Jeff Volenec, (765) 494-8071,

Ag Communications: (765) 494-2722; Beth Forbes,;

Related Web site:
Purdue Department of Agronomy
Purdue forage information

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