By Kevin Bradley and Robert Kallenbach
Integrated Pest & Crop Management Newsletter  
University of Missouri-Columbia
Vol. 16, No. 3
Article 1 of 7
March 17, 2006

As the time for spring establishment of alfalfa nears, growers in Missouri and throughout the U.S. have a new option to consider - Roundup Ready Alfalfa. Since its introduction in 2005, we have received a lot of questions about the utility of Roundup Ready alfalfa in Missouri. In this article, we give our opinions on the advantages and disadvantages of this new technology.

First, we’ll start with the bad news. The technology fee alone will cost Missouri growers an additional $2.50 per pound of seed planted. Another way of looking at this is an additional $125.00 per bag of alfalfa seed purchased, or total seed costs per acre somewhere on the order of $80.00 to $150.00 per acre (depending on alfalfa variety and seeding rate).

In our opinion, another potential disadvantage is that this is the first genetically-modified perennial crop that is designed to be sprayed with the same herbicide in the same location throughout the life of the stand. This places tremendous selection pressure on the weeds in these environments, which could result in some of these weeds developing resistance to glyphosate. We have already seen glyphosate-resistant weeds appear in annual Roundup Ready crops that are planted without rotation. Granted, most alfalfa producers will probably not need to spray on an annual basis, but how many times will a producer spray glyphosate during the alfalfa’s lifetime? How long will a Roundup Ready alfalfa crop persist? We don’t have an answer for these questions now, but Roundup Ready alfalfa systems will likely last longer and could be sprayed much more than our conventional systems, for many of the reasons that will be discussed below.

One of the clear advantages of this technology, however, is in the control of troublesome broadleaf weeds like curly dock, musk, bull, and Canada thistle, horsenettle, and dandelion. Of these, curly dock is especially troublesome in established alfalfa stands. In many areas, curly dock infestations are the reason that alfalfa stands are eliminated after only 3 or 4 years of establishment. Although we have not been able to conduct research to address this issue yet, it seems clear that we will be able to prolong the life of a stand through the use of the Roundup Ready technology.

Another advantage that we see with the Roundup Ready technology is with spring establishment. Often, spring-established alfalfa is more difficult from a weed management standpoint. This is because many summer annual weeds emerge throughout April and May into newly seeded stands that have little to no canopy. To complicate this issue further, only a few conventional herbicide options are available for application on these newly seeded stands.

Table 1. Influence of herbicides and seeding rate on crabgrass control in RR alfalfa.

This was the rationale behind two research trials that we conducted with Roundup Ready alfalfa in 2005. In these trials, we compared the weed control achieved with a standard glyphosate program (Roundup WeatherMax at 22 fl ozs/A) to that received with conventional programs that have been utilized by growers for a number of years. In these experiments we also evaluated the effect of alfalfa seeding rate on the level of weed control achieved. Our thinking was that with highpriced seed, growers might be tempted to reduce seeding rates. For years, we have used higher-than-necessary seeding rates so that new alfalfa stands can compete with weeds. With Roundup Ready alfalfa, weed control would be less difficult and the need for high-seeding rates to out-compete weeds in the early establishment phase would not be as critical.

As illustrated in Table 1, we observed increasing levels of large crabgrass control with corresponding increases in alfalfa seeding rate with the conventional herbicide programs, but not with glyphosate. Regardless of seeding rate, the glyphosate program provided essentially complete large crabgrass control. We observed a similar response with both giant foxtail and common lambsquarters (data not shown). After one year of research, these results confirm what many have suspected from the beginning; that the flexibility and broad-spectrum weed activity that we have observed with glyphosate in other Roundup Ready systems seems to apply to Roundup Ready alfalfa as well.

However, one of the big questions that each grower will have to answer for themselves is, "Is it worth it?" As shown in Table 1, our results indicate that we can achieve good crabgrass control with many of our conventional herbicide programs. We observed a similar response with both common lambsquarters and giant foxtail (depending on the conventional treatment). Weed control may not be as high for conventional programs as with glyphosate, but producers will have to determine whether or not this is good enough. In other words, is the difference in weed control that you will achieve with a glyphosate program be enough to justify the cost of the technology compared to a conventional program? This question becomes more difficult to answer as the number of different weed species increases, and as you incorporate other factors into the decision-making process.

Kevin Bradley
573-882-4039

Robert Kallenbach
573-882-2801