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Eileen Balz, Steve Washburn, and Sue Ellen Johnson
North Carolina State University, Raleigh, NC and the Center for Environmental Farming Systems, Goldsboro, NC
Added August 3, 2012.
Note: Observations herein were adapted from an MS thesis completed in 2011 by Eileen Balz under the direction of Drs. Washburn and Johnson. Financial support was provided through a Southern region SARE grant: Research and educational support for organic dairy farming in the South.
Over two years, two varieties of alfalfa (Haygrazer and Arriba) were planted in replicated grazing plots of approximately 2 acres each (Field 1, Oct 2009) or about 1.25 acres each (Field 2, Oct 2010) on sandy soils under either organic or conventional management. Both fields were planted into a prepared seedbed after appropriate fertilizer and lime were applied according to soil test recommendations and following organic guidelines for the organically managed areas except that organic seed was not used. Before planting, the herbicide Eptam® was incorporated into conventional plots whereas organically managed alfalfa was planted without any herbicide. For Field 1, Matua or Lakota prairie grasses were drilled into organic and conventional plots in late winter (Feb, 2010) whereas for Field 2, Matua was planted with the alfalfa in half of each of the organically managed plots in Oct 2010. Stands of the prairie grasses were better with fall planting. Field 1 had several years of annual ryegrass in winter and sorghum-Sudangrass hybrid in summer whereas Field 2 had been in hybrid Bermudagrass sod for several years and a smother crop of sorghum-Sudangrass was planted in May, 2010 to suppress the Bermudagrass before the field was disked and planted to alfalfa in the fall.
Because Eptam was in the conventional plots, there was more winter weed pressure in organic plots for several months after planting as well as volunteer forage grasses. This was particularly true for henbit during the first winter in Field 1. Henbit has been documented as a potential ovipositioning site for alfalfa weevil and may have contributed to higher weevil counts in organic plots earlier than in conventional plots as observed in March, 2010 (see below). Primary weed species observed in organic plots were henbit in winter and early spring, chickweed in spring, and spiny amaranth (pigweed) in late spring and summer. Forage grasses including the planted prairie grasses and volunteer ryegrass in winter and spring, and volunteer crabgrass in summer. In Field 2, there was not much residual Bermudagrass after using the summer smother crop procedure. Presence of weeds was very low for the first winter and early spring for conventional plots but by mid summer, the amount of spiny amaranth was similar in both systems. Winter weeds during the second year were also similar across management systems in Field 1. Interestingly, the different history of Field 2 resulted in very little henbit in either conventional or organically managed plots.
Each year scouting for Alfalfa weevil started in February and continued through April as needed. For organic management, alfalfa was grazed by lactating cows at an economic threshold of 3 weevil larvae per stem regardless of the maturity of the alfalfa. Otherwise, plots were grazed when maturity reached late bud to early bloom. Alfalfa weevil counts did reach 3 larvae per stem in some of the conventional grazing plots in Field 1 on April 6th, 2010 and on March 29, 2011 but because of the 14-day withdrawal time for the insecticide, we chose not to spray. In 2012, plots in both fields were scouted a few times and as weevil larval counts began to rise, it was decided to graze both conventional and organically managed alfalfa plots relatively early when alfalfa plants averaged between 15 and 20 inches in height (Figure 1, March 28, 2012). Re-growth about 3 weeks after an early grazing is shown in Figure 2 (April 19, 2012). Cows used for the grazing system included Jerseys, Holsteins, and Jersey-Holstein crosses and were separated into organically or conventionally managed groups.
We did observe in late April, 2011 that many pupating alfalfa weevil larvae had been parasitized with Bathyplectes anurus, (Figure 3) a parasitic wasp first imported to Utah from Italy in 1911 for biological control of the alfalfa weevil. Because we managed some of the alfalfa using organic principles and managed the conventional alfalfa using scouting data rather than spraying at the first sign of feeding damage, we likely facilitated greater survival of parasitic wasps as well as other beneficial insects.
Yields of alfalfa and total forage yields were examined for both conventional and organically managed areas. In the spring of 2010 and again during the summer, we experienced periods of mild to moderate drought and the alfalfa varieties we used were nearly dormant for a while. Therefore, overall yields in Field 1 were lower than expected in the first year. Yields of alfalfa by itself were similar for both organic (2.53 vs. 2.48 +/- 0.2 tons DM/acre) and conventional (across 4 grazing periods in 2010. However, there was a tendency for yields of all forage species (alfalfa + forages grasses including ryegrass, prairie grass, and crabgrass) tended to be greater for organically managed areas vs. the conventional system (3.12 vs. 2.56 +/- 0.2 tons DM/acre, P = 0.06). Weed yields were only 8.9% and 8.7% of the total biomass for organically vs. conventionally managed areas, respectively and the respective proportions of those weeds were 80.9% vs. 87.9% spiny amaranth (pigweed) observed in the summer. Common chickweed was the second most common weed and in grazing systems, cows readily consume it.
For Field 1 in the second spring (2011), a grazing was done in early February in both organically and conventionally managed systems to potentially reduce egg masses of alfalfa weevil in alfalfa stems and in henbit. Two additional grazings for each plot were done in March, April, or May with timing based on alfalfa weevil pressure and/or alfalfa maturity. In this situation, collective spring alfalfa yields tended to be greater for conventionally managed alfalfa (1.43 vs. 1.21 +/- 0.06 tons DM/acre, P = 0.09) but yields of ryegrass and prairie grass more than offset the difference (1.43 C vs. 1.58 O +/- 0.07 tons DM/acre, N.S). Weeds made up 15.8% of the total biomass in the conventional plots but only 8.4% in the organic plots. Primary weeds were henbit in February and chickweed in March to May with over 80% of the weed biomass being chickweed.
For Field 2, winter weeds were not as much an issue nor were the alfalfa weevil the first spring after planting. Yields for one grazing period were collected in both organically and conventionally managed plots in early May. Yield of alfalfa alone was actually greater for the organic area (0.64 vs. 0.49 +/- 0.02 tons DM/acre, P= 0.004) suggesting that use of Eptam may have an inhibitory effect on new alfalfa stands. When yields of total forage species (alfalfa + ryegrass + prairie grass) were included, differences were even greater in favor of organically management: 0.69 vs. 0.49 +/- 0.03 tons DM/acre, P = 0.001). Primary weed was chickweed which made up 3.1% of the total biomass in organic areas and 7.5% in conventional areas.
Because organically managed alfalfa was planted with or without prairie grass, those yields were compared but differences were not significant. Alfalfa yields were numerically greater for plots without prairie grass (0.65 vs. 0.61 tons DM/acre) whereas total forage yields were numerically greater (0.73 vs. 0.66 tons DM/acre) for plots with prairie grass. Weeds (mostly chickweed) were only about 3.1% of total biomass for that single spring grazing.
Summary: Conventional management generally produced more actual yields of alfalfa than organic management. However, alfalfa grass mixtures yielded as much or more useable forage without the need chemical control of weeds and insects. Interseeding prairie grass may be useful in suppressing weed production but may or may not boost total forage production. Less weed pressure (particularly henbit in fall/winter) could reduce ovipositioning sites for alfalfa weevil. Longevity of stands and productivity over time remain to be determined. However, it is clear from our initial efforts that managing organic stands of alfalfa and alfalfa-grass mixtures is a feasible option in eastern North Carolina.
Posted: to Organic Production on Fri, Aug 3, 2012
Updated: Fri, Aug 3, 2012