Proceedings of the Sixty-first Annual Meeting of the Northeastern ...

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ALFALFA/GRASS FORAGE MIXTURES USING GLYPHOSATE-RESISTANT
ALFALFA. B.L. Dillehay, W.S. Curran, M.H. Hall, and D.A. Mortensen, The
Pennsylvania State Univ., University Park.
ABSTRACT
Alfalfa/grass mixtures are popular forages that are well adapted to the cool
climates of the northern United States. The addition of a grass to an alfalfa (Medicago
sativa L.) crop in this region aids in weed management, winter survival, and forage
yield, among other variables. During establishment, few herbicides are labeled for
control of weeds in alfalfa/grass mixtures. Although some selective herbicides safely
control broadleaf weeds in alfalfa/grass mixtures, choices are limited due to the lack of
labeled products. In addition, nothing is available to control grassy weeds in seedling
alfalfa/grass forage mixtures. Glyphosate-resistant Roundup Ready alfalfa became
commercially available in late summer 2005. This technology offers unprecedented
weed control and crop safety in pure alfalfa stands, but may benefit alfalfa/grass forage
stands as well. The objective of this research was to evaluate the potential use of
glyphosate for weed control in seedling Roundup Ready alfalfa/grass mixtures. By
varying the planting date of the grass in relation to alfalfa planting and a glyphosate
application, the potential for good weed control exists. In treatments that included
herbicide application, glyphosate was applied 4 weeks after alfalfa planting.
Orchardgrass (Dactylis glomerata L.) was seeded into the alfalfa at 3 different times;
seeded with the alfalfa, seeded 4 weeks after alfalfa, and seeded 5 weeks after alfalfa.
All of the treatments included an untreated check. In alfalfa/grass mixtures not treated
with glyphosate, alfalfa and orchardgrass dry matter was lower, and weed dry matter
was higher than the respective treatments that included a postemergence glyphosate
application. Preliminary results suggest that the trends for increased alfalfa and
orchardgrass, and decreased weed dry matter in the later seeded glyphosate
applications appear to persist into the second year. Initial observations show that
Roundup Ready alfalfa could be a successful addition to weed management for
alfalfa/grass mixtures.
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