Is sainfoin really resistant to Roundup?
Ryan Summerlin September 11, 2013
Sainfoin has increased in popularity for Wyoming forage producers because of its many desirable attributes:
■ High nutritional content,
■ Seems well adapted to Wyoming (particularly northern Wyoming), and
■ Does not cause bloat so it can be grazed.
This deep-rooted perennial legume, which is widely adapted for the Rocky Mountain region, has great potential for Wyoming.
Two varieties of sainfoin have been developed at the University of Wyoming by researchers in the Department of Plant Sciences.
Those varieties are ‘Shoshone’ and ‘Delaney’ and are very well-suited for the area.
Many studies have looked at grazing management and livestock response to sainfoin.
Basic agronomic information is also available such as how much seed to use and what time of year to plant.
However, at least one major management issue facing sainfoin producers remains: How do we manage weeds in sainfoin?
How to Manage Broadleaf Weeds?
Herbicide options for use in actively growing sainfoin are few.
Select (clethodim) or Poast (sethoxydim) herbicides can be applied postemergence to sainfoin, but these products only have activity on grass weeds.
A few promising herbicides have been identified as safe for use on sainfoin, but only the two listed above have been registered for post-emergence use.
According to Wyoming Agricultural Statistics, 620,000 acres of alfalfa were planted for hay in 2011.
Sainfoin acreage is quite small by comparison.
While herbicide producers will devote money and resources to develop products for large-acreage crops like alfalfa, there is less interest in developing herbicides for minor crops like sainfoin — there simply isn’t enough return on investment.
Scientists conduct Weed control studies
There have been suggestions sainfoin has a natural tolerance to glyphosate (the active ingredient in the herbicide Roundup), even in actively growing plants.
Some previous studies have shown that sainfoin does possess some natural tolerance to glyphosate, but these studies did not quantify the effect of glyphosate on sainfoin forage yield.
To test the effects of post-emergence herbicides on sainfoin forage yield, researchers in the Department of Plant Sciences are conducting studies at the Powell Research and Extension Center by applying varying rates of glyphosate to sainfoin.
Yield reduction significant
A range of glyphosate (Roundup PowerMAX) rates were applied to an established stand of sainfoin in the fall of 2011 and again in the spring of 2012.
Sainfoin yield was then collected for three cuttings during the 2012 growing season.
Sainfoin did exhibit tolerance to glyphosate (it survived), but yield reduction was significant even when applied at relatively low rates.
The rate of Roundup used in most Roundup Ready crops to manage annual weeds is 22 to 32 fluid ounces per acre.
Only eight fluid ounces per acre of Roundup applied in the spring reduced sainfoin yield by more than one ton of dry matter per acre — an estimated $198 per acre of lost revenue.
Eventually, the sainfoin outgrew the injury symptoms from the lower rates of glyphosate, and yield loss was higher in the first cutting but less pronounced in the second and third cuttings.
Even fall applications, which resulted in far less sainfoin injury, resulted in significant yield reductions and associated lost revenue.
What it Means
Sainfoin yield loss due to glyphosate application has significant economic implications, especially in a year when hay prices are high due to drought conditions.
Recently, the price for prime quality hay exceeded $200 per ton.
If we conservatively estimate that sainfoin hay could be sold for $160 per ton, the potential revenue lost due to glyphosate application far exceeds the likely benefit we would have received by spraying Roundup for weed control.
Roundup may be an inexpensive herbicide to apply, but the costs associated with sainfoin yield reduction make this an extremely expensive weed management decision.
Herbicides other than glyphosate are being evaluated, and some promising options have been identified.
This research will continue with the hope that at least one of these promising herbicides will be registered.
Until then, growers are encouraged to use one of several registered preemergence herbicides during stand establishment to aid weed control.
If a vigorous stand of sainfoin can be established, the need for postemergence herbicides will be greatly reduced. ❖
This article appeared in most recent edition of the University of Wyoming College of Agriculture and Natural Science’s magazine, Reflections, published annually by the Agricultural Experiment Station. To see the entire issue, go to www.uwyo.edu/uwexpstn/publications/reflections/.