A new low-coumarin sweet clover could be in the hands of Texas beef producers in three or four years.
“These new sweet clovers will not cause bleeding disorders in livestock and will produce high-quality grazing and hay due to fine stems,” said Ray Smith, Texas Agricultural Experiment Station legume breeder.
Smith, working with forage researcher Gerald Evers, began the breeding program in 1999. Both researchers are based at the Texas A&M University System Agricultural Research and Extension Center at Overton.
Sweet clovers are well adapted to the alkaline soils and climate of Central Texas. Several varieties were regularly grown throughout the region until the 1950s.
Sweet clover had some drawbacks, such as a thick main stem that limited digestibility and slowed drying when cut for hay, and a high coumarin content.
Coumarin is a fragrant crystalline compound found in several plant species, including tonka beans and sweet clovers that is widely used in perfumes.
Coumarin itself is not toxic to ruminants. But when sweet clover hay is not dried properly and becomes moldy, coumarin converts to dicoumarol, a compound similar to modern blood thinners. Cattle eating dicoumarol-contaminated hay can experience internal bleeding, which can be severe enough in some cases to result in death.
Smith began the program with hand and bee-cage crosses between Denta and Emerald sweet clovers in March through May 2001. Denta is a low-coumarin cultivar of biennial white sweet clover, but because it is a biennial, it is poorly adapted to Texas. Emerald is a fine- and multi-stemmed white sweet clover but has a high-coumarin content, Smith said.
Bee-cage crosses are just what they sound like. Bumblebees are released inside a closed container with both species of plants. The bees naturally cross-pollinate the plants.
“They do a much better job than we can do by hand,” said Indre Pemberton, research associate who worked on the project.
The seedlings were grown for 60 days and tested for coumarin content.
Hybrids between Denta and Emerald were identified by the presence of coumarin.
From 338 hand crosses, 36 hybrids were identified; 47 hybrids were identified from bee-cage crosses. These resulting hybrids were self-pollinated in a greenhouse and about 240,000 seed were produced.
From the resulting seedlings, Smith evaluated 10,500 plants.
“Our objective was to initiate a simultaneous screen for low-coumarin, fine-stem or multiple-stem trait, and annual growth habit,” Smith said.
With so many plants to evaluate, Smith had to develop techniques to rapidly screen for multiple-stem trait and low-coumarin content.
From a preliminary study, he learned to identify the multiple-stem trait in young sweet clover seedlings. This was done by looking for tiny stems emerging from the first leaf juncture of the immature plant. Smith then used sodium hydroxide solution — a single drop per leaf — to chemically change the coumarin in leaf sample into a compound that glowed under ultraviolet light. The test made it relatively easy to distinguish between plants that had high- or low-coumarin content.
About 500 plants passed both the low-coumarin and multiple-stem tests. In order to speed up the program, these crosses were planted in a greenhouse under artificial lighting in November 2002. More plants were discarded because of severe powdery mildew infection and/or general low vigor or failure to flower, reducing the selections to 143 plants. These 143 clovers were planted near Thrall on blackland soils, and the evaluation process continues.
Why Central Texas trials instead of East Texas where Smith is based?
“Sweet clover is more adapted to Central Texas than to East Texas,” explained Charles Long, resident director of research at the Overton center.
“The Experiment Station is a statewide organization. This project is an excellent example of how research based in one part of the state may benefit producers in another part of the state.”