Although scientists at Monsanto and Forage Genetics International have developed the technology, RR alfalfa varieties will be marketed broadly by a wide range of seed companies. Important characteristics, such as genetic resistance to insects and diseases and yield potential, remain important criteria for selecting a variety. The RR trait enables a unique weed control program to be used in alfalfa.Successful weed control has been a continual challenge for alfalfa producers. The high forage quality requirements of the dairy industry in many areas necessitate nearly perfect weed control. In some regions, growers simply tolerate weed problems due to their inability to effectively control them. Similarly, it is important for the horse industry to have alfalfa hay that is free of noxious, low-quality, and poisonous weeds. This can be difficult with conventional herbicides. Typically no single herbicide controls all the weeds present in many alfalfa fields. Weed management in alfalfa involves two distinct phases: weed control in seedling alfalfa during crop establishment and weed control in established stands. Weed infestation during crop establishment increases weed seed reserves and reduces seedling vigor and alfalfa stand, grow racks which can affect crop production for many years in this perennial crop. However, if protected from initial weed competition, a vigorous alfalfa stand competes well with later-invading weeds.
After the crop is established, alfalfa stands naturally thin over years, making the crop increasingly susceptible to weed invasion. Weeds may reduce alfalfa yield, but more importantly, weeds reduce the forage quality of alfalfa. Most weeds are lower in forage quality or palatability than alfalfa. Therefore weeds can significantly reduce the feeding value of hay for milk production and affect animal growth and health. Weeds such as lamb squarters and pigweed can accumulate toxic nitrate concentrations. Others such as common groundsel and fiddleneck are poisonous to livestock and have been known to harm or even kill animals that feed on them.In 2002 to 2004, field trials were established in the intermountain area , Sacramento Valley, and two locations in the San Joaquin Valley of California to compare the RR weed management system with standard weed control strategies under the diverse environmental conditions and weed spectrums encountered in California. Different Roundup rates, 1 and 2 pounds active ingredient per acre, and application timings were evaluated and compared with conventional standards. Three different herbicide application timings were evaluated based on the alfalfa leaf stage: unifoliolate to first trifoliolate, 3 to 4 trifoliolate, and 6 to 9 trifoliolate. The conventional standards used varied slightly depending on the location and the weed spectrum but included Raptor , a tank mix of Pursuit and Buctril or Prism , and a tank mix of Roundup and Pursuit.
Sequential treatments were evaluated to ascertain the need for multiple treatments to control weeds that emerged after the initial application.The Roundup treatments usually caused no injury. In cases where slight injury was observed, symptoms were short-lived and not evident at the time of first cutting. Raptor and Pursuit plus Buctril tank-mix treatments resulted in higher injury ratings, usually less than 20 percent at most locations. We observed a small percentage of seedling mortality in our RR alfalfa plots after the initial Roundup application. This is normal. Due to the polyploid genetics of the crop , a small percentage of alfalfa seedlings do not contain the gene for Roundup resistance. Plant breeders from Forage Genetics International have indicated that seedling mortality of RR alfalfa varieties from commercial seed will likely be between 3 and 7 percent. This is not a problem with alfalfa, since usually more than 40 percent of seed sown fail to establish before the first or second cutting in a normal process of self thinning during stand establishment. However,if RR lines are not sprayed with Roundup during the seedling phase, these nonresistant plants may have a greater impact on the stand if sprayed at a time when the overall population is much lower . Therefore, it will be important to spray RR varieties at the 3 to 4 trifoliolate stage. Since one of the major incentives to plant RR alfalfa is to control weeds during establishment, this requirement should not be problematic for growers.The importance of application timing during alfalfa seedling development varied depending on weed species, location, and time of year.
Generally, our trials showed that when Roundup was applied at the 3 to 4 trifoliolate stage, weeds were effectively controlled and usually no second application was needed. Early applications allowed late germination of weeds, requiring a second application, while later applications allowed greater weed competition at early growth stages. Roundup does not have soil residual activity, and weeds may emerge after application, especially if the alfalfa is small. At the intermountainsite, a Roundup application made at the unifoliolate to first trifoliolate timing resulted in subsequent invasion of prickly lettuce , and henbit, but shepherd’s purse was completely controlled, as there was no subsequent emergence of this weed after the initial application. A second application in late winter was needed to control all the weeds. Similarly, a second application of Roundup was needed at one of the San Joaquin Valley sites when the first Roundup application occurred at the cotyledon to unifoliolate stage in alfalfa. Early Roundup timing resulted in excellent weed control in the other trials. In the San Joaquin County trial, where all Roundup timings performed well, a second flush of annual bluegrass and canarygrass germinated in all treatments by early December. None of the conventional herbicide treatments controlled this second flush of weeds at this location, whereas a second application of Roundup was effective. The effectiveness of an early treatment appears to be a function of the weed species and its germination period and how soon after application the alfalfa canopy covers the soil surface. Unlike standard herbicides, Roundup controlled weeds in the latest application period, 6 to 9 trifoliolate leaf stage. This indicates a greater flexibility with Roundup compared with other options. However, weed competition was typically greater when weeds are allowed to compete with alfalfa as late as the 6 to 9 trifoliolate stage. The RR system of weed management resulted in the best overall weed control of the treatments evaluated, and there was considerable flexibility in treatment timing. These results indicated that application of Roundup during the 3 to 4 trifoliolate stage is likely to result in the best control of emerged weeds. Extremely early applications, at the unifoliolate to first trifoliolate growth stage, are generally not advised because under some conditions subsequent weed invasion can occur since open areas in the young stand facilitate weed encroachment. The necessity of a second application depended on location, weed spectrum, and timing of the first application. In established stands, Roundup was compared with a commercial standard herbicide treatment for established alfalfa, Velpar and paraquat.
These are commonly used during the dormant period to control the winter complex of weeds that germinate during the rainy period in California. Roundup and the commercial standards were equally effective at most sites, providing excellent weed control. The commercial standards caused visible injury at some locations on both conventional and RR alfalfa. No injury was observed on RR plants from Roundup treatments. Roundup controlled the weeds at all sites except the San Joaquin County site, where burning nettle , was not controlled . This weed is not on the Roundup label. Tank mixes of Velpar with Roundup or paraquat controlled all weeds, planting racks including burning nettle. Our research showed that for difficult-to-control weeds, such as dodder or small cheese weed , applications should be initiated earlier than with other weeds, followed by a second application 2 weeks later. Several herbicides offer an alternative weed control strategy for herbicide rotations or tank mixes with Roundup if there are difficult-to-control weeds in the population.Eventually, an alfalfa stand must be removed when production falls to a level where the field is not as profitable or returns would be greater with a different crop. Roundup has been the primary herbicide used to remove depleted alfalfa stands; however, it obviously could not be used for this purpose in a field of RR alfalfa. The unwanted persistence of RR alfalfa in subsequent crops could potentially be a limitation of the technology if RR residual stands cannot be removed. However, alternative methods appear to be available to remove old RR alfalfa stands, but these methods must be adapted to specific crop-rotation situations. Alfalfa fields are primarily removed by mechanical means such as cultivation with a plow or heavy disk or by undercutting the plants with a blade or rototiller. However, herbicides are sometimes used instead, alone or in combination with cultivation. Trials were conducted at several locations throughout California to evaluate the effectiveness of alternative herbicides for alfalfa stand removal. Numerous herbicides and herbicide combinations were tested including 2,4-D, Banvel , Stinger , Garlon , Liberty , Aim , and Harmony . Treatments containing 2,4-D and dicamba tended to be the most effective, generally controlling greater than 90 percent of the alfalfa with the least crop rotation restrictions . Alfalfa control improved to nearly 100 percent with all herbicides evaluated when followed by cultivation. These results demonstrated that chemical and non-chemical alternatives to Roundup exist for alfalfa stand removal in RR fields .Alfalfa seed is grown primarily in the western United States on less than 100,000 acres . Alfalfa is a cross-pollinated crop that requires bees to “trip” flowers to release pollen for seed production. In the United States, alfalfa seed production fields are pollinated with leaf cutter bees in the Pacific Northwest and honey bees in California. Some growers in Washington use alkali bees for pollination. Although viable seed production requires the transfer of pollen from one plant to another, control of pollen movement from seed field to seed field is important for maintaining genetic purity in alfalfa seed production, whether it is RR or not. Isolation standards are in place to maintain the purity of seed from varieties in adjacent seed fields. However, seed companies and alfalfa seed produc-ers will need to establish new or modified protocols for production of RR seed, since the seed may require higher standard of purity to address specific markets.Understanding alfalfa outcrossing rate, gene flow, and pollinator behavior is very important to proper management of commercial alfalfa seed production. Current isolation standards for alfalfa in the United States are 165 feet isolation from other alfalfa for certified seed and 900 feet for foundation seed production. These are based on data collected in the 1980s, which used pest resistance genes as pollen flow markers between seed production fields . Recent studies used naturally occurring alfalfa marker genes to measure the potential for gene flow from source blocks to alfalfa plants at various distances from the source . This experiment was useful in studying the potential for transgene escape to feral alfalfa. A larger 3-year study was carried out by Forage Genetics International using leaf cutter bees. Both studies indicated that although gene flow can be detected over 1,500 feet from the pollen source, it is reduced to less than 0.5 percent at 900 feet and less than 0.2 percent at distances greater than 1,500 feet . Preliminary studies conducted by University of California scientists in 2003 using honey bees as pollinators evaluated movement of the RR gene into herbicide-susceptible trap crops west and east of a 6-acre RR source plot at distances up to 2.5 miles . A significant decrease in gene flow with increasing distance from the source plot was observed . At 900 feet , pollen-mediated gene flow was less than 1.5 percent, and it decreased to less than 0.2 percent near 5,000 feet . Gene flow continued to decline to a distance of 2.5 miles , where it was detected at a very low frequency . Gene flow to the west and the east was not significantly different. It is clear that pollen-mediated gene flow decreases exponentially with distance, and the safe distance to minimize pollen flow is affected by type of pollinator.The preliminary data from honey-bee-pollinated field studies will be combined with data from future studies to develop strategies to preserve seed trait quality during seed production. Crop isolation practices are being developed by seed companies and producers to ensure seed purity and a minimum adventitious presence in conventional alfalfa seed and hay.