Glyphosate-tolerant (Roundup Ready) alfalfa – seeded at a reduced rate and treated with glyphosate – can improve the crop’s yield and forage quality, reports Jim Morrison, Extension educator, crop systems, Rockford Extension Center, University of Illinois.

A multi-state research study, coordinated by Marvin Hall, Penn State University forage agronomist, was conducted at seven locations in Ohio, Oklahoma, Pennsylvania, South Dakota, Utah and Wisconsin. Glyphosate-tolerant alfalfa was seeded in the spring of 2006 into conventionally tilled seedbeds at 6-, 10-, 14-, and 18-lb/acre rates of pure live seed.

Stand density, botanical composition, yield, and forage quality were determined for each seeding rate under three herbicide treatments: glyphosate, a non-glyphosate herbicide and no herbicide.

Here are Hall’s observations:
•The amount of weed infestation differed among locations, but there were no weed infestation x seeding rate or herbicide x seeding rate interactions.
• Fewer alfalfa plants died in lower-rate seedings than in higher-rate seedbeds.
• Seeding rate had no affect on forage quality or weed content at any harvest.
• At only one harvest in the seeding year did the 6-lb/acre seeding rate produce less alfalfa than did other seeding rates.
• In the seeding year and the year after seeding, there were fewer weeds and greater alfalfa yield in areas treated with herbicide than where no herbicides were used.
• Alfalfa plant density didn’t differ by the fall of the year after seeding no matter what the seeding rate.

Hall and his colleagues noted that, as alfalfa plant density continues to decline over time, the lower seeding rate may drop below the minimum density threshold sooner and result in yield decline earlier than the other seeding rates.

In conclusion, herbicides used during alfalfa establishment increased alfalfa yield and forage quality. That indicates that weed control is an important management factor in obtaining a high-yielding and vigorous alfalfa stand, Morrison notes.

The study was reported in the Agronomy Journal’s, May-June 2010 issue. Or see the abstract.