New clover could spell good luck for livestock producers

03/02/05



This clover leaflet has been abraded in order to infect it with bean yellow mosaic virus. Infected, susceptible plants usually begin showing symptoms in 10 to 14 days. (Texas Cooperative Extension Photo by Robert Burns)

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OVERTON – It's common knowledge that the high price of crude oil has driven up fertilizer prices. But studies here have shown cattle can gain 3 pounds per day grazing spring pastures of a new disease-tolerant clover.

"That's with no added nitrogen, as you'd have to do with ryegrass pastures," said Dr. Ray Smith, legume breeder with the Texas Agricultural Experiment Station.

Commercial nitrogen fertilizer costs have increased to 40 cents per pound from about 35 cents a pound last year. Natural gas is used in nitrogen fertilizer production, and natural gas price is linked to other fuel costs, including crude oil, Smith said. Luckily for farmers and ranchers, Smith has developed Apache, an arrowleaf clover that is tolerant to bean yellow mosaic virus, has great seedling vigor, gives early forage production, a long grazing season, and offers high nutritive value, he said.

"This new clover is tolerant to the bean yellow mosaic virus that has plagued arrowleaf for the last 20 years," said Smith, who is based at the Texas A&M University System Agricultural Research and Extension Center at Overton.

Smith would say that science had more to do with the development of the clover than luck, however.

Older arrowleaf clovers varieties such as Yuchi, Meeche and Amclo, are affected by bean yellow mosaic virus in several ways. Some plants are killed outright by lethal wilt. Others go on to survive but suffer a variety of symptoms, including misshapen leaves, yellowed leaves or leaves spotted in a yellow mosaic pattern. Of the surviving plants, yields are reduced by as much as 50 percent. But worse, the productive lifespan of the survivors is shortened and that leaves months during the spring without forage production, Smith said.

Ryegrass is one way to fill the production gap, but unlike clovers, it needs supplemental nitrogen to be highly productive. Smith began work on arrowleaf clover disease resistance in the late 1980s.

He started with the conventional method of growing arrowleaf cultivars in the field and selecting those plants that appeared healthy while rejecting those that were prone to disease infection.

"This conventional breeding method just wasn't working," Smith said.

Why? Because, he said, the bean yellow mosaic is spread by aphids, and the insect's haphazard habits meant that some plants would become infected while those nearby might not.

"So we were selecting those plants that both had tolerance to bean yellow mosaic virus and those which just happened to dodge the bullet, so to speak," Smith said.

With this realization, Smith moved to greenhouse trials where he could control the spread of virus. He inoculated plants by hand with a slurry of tissue from infected plants. At the same time, he continued field trials, but mechanically inoculated those plants with the virus as well.

After years of crossing plants from both the greenhouse and field selections, Smith released the result, Apache clover, in 2001. Seed production made the new clover available to farmers in 2002.

Dr. Monte Rouquette, another forage scientist also based at the Overton center, did a grazing study with Apache that lasted from early March 2003 to the first of June 2003. The test showed calves' average daily gains ranged from 1.7 to 3.5 pounds, depending upon the stocking rate.

For consistency between studies, forage scientists typically use poundage to define stocking rates instead of head counts. In the study, one animal unit was defined as 1,000 pounds of animal per acre. Low-, medium- and high-stocking rates were defined as 1.2, 2.0 and 2.8 animal units, respectively.

As might be expected, the low stocking rates showed the highest rate of daily gain, 3.5 pounds per day. And the highest stocking rate of 2,800 pounds of animal per acre showed the lowest daily gain. But the medium stocking rate of 2,000 pounds of animal per acre resulted in nearly 3 pounds gain per acre per day. To put stocking rate in perspective, figure the average beef cow will weigh in about 1,200 pounds, her calf 300 pounds and young stocker animal 600.

As early as the 1960s, common practice was to mix arrowleaf seed with crimson clover seed and grow it throughout many southern states, from East Texas to Georgia. By mixing the early-maturing crimson clover and late-maturing arrowleaf, ranchers and farmers could have forage from February through early June.

Multiple disease problems, including plant viruses and fungal disease, effectively put a stop to this practice more than a decade ago. Of the diseases, bean yellow mosaic virus was the most prevalent and damaging.

Bean yellow mosaic virus didn't affect crimson clover, but it killed arrowleaf clover, stunted it, or caused it to mature early. "It left a production gap from when the crimson matured to when warm season grasses came on in June that was not filled until the release of Apache," Smith said.

Source: Eurekalert & others

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