Overton, Texas
May 18, 2004
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 Dr. Ray
Smith, Texas Agricultural Experiment Station legume breeder.
Smith, working with forage researcher Dr. 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 fatal 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 made 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 Dr. 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 other regions." |