How do you know what the best cover crop for your system is? Unfortunately, there is no easyanswer. The primary goal of using cover crops is to improve the ecological function of your productionsystem, but specific objectives for a location and grower at any given time may vary widely. A firststep is to decide what the most important function(s) you want your cover crop to serve are.
Break the pest cycle?
Improve nitrogen nutrition for the succeeding crop?
Increase soil organic matter?
Weed suppression?
Erosion control?
Habitat for beneficial insects?
Nematode suppression?
Figure 1. Young papaya trees with living mulch of sweet white clover and hairy vetch 'Lana' and ‘Naomi' (Top). Trees several months later with vetch grown up around the trees and mowed between rows (Bottom).
"All of the above" is an obvious answer, but no single crop will serve to optimally address all thoseneeds all the time, so priorities need to be set.Also, several practical questions should be asked:
How long can I keep the area out of a cashcrop?
What cash crop will be growing with/after thecover?
Can I afford to irrigate the cover crop?
What equipment do I have to mow and/orincorporate the cover?
Is seed available? How much does it cost?
What information is available on the species Iam interested in?
What are my neighbors and others doing?
It is important to be aware that cover crops must bemanaged properly to achieve their full potential.Management strategies to maximize the potential forlegumes to contribute nitrogen and other benefits tothe system include liming acid soils, improvingphosphorous availability if soil is deficient, avoidingexcess N, inoculating with the appropriate rhizobia,and timing killing and residue incorporation properly(Cassman, 1980; Habte, 2000; Hooks, 1997;NIFTAL, 2000; Yost et al, 1981; Yost and Evans,1988). When used as a living mulch, all plantsincluding legumes have the potential for competingwith the concurrent cash crop. Strategies to minimizecompetition include planting the living mulch outside of crop rows, controlling growth of living mulch through mowing/ herbicides, avoiding genotypes withvining/climbing habit and ensuring nutrients are supplied directly to the cash crop (Evans et al., 1988;Glover, 1998; Leary et al., 2006; Radovich et al., 2009; Figure 1). Alternating strips of cover cropswith cash crops in a simple rotation has been suggested as a strategy to reduce the economicproduction area lost to cover cropping(http://www.ctahr.hawaii.edu/sustainag/Downloads/Strip-till_row-switching.pdf). Grasses are often underrated as cover crops and are particularly well suited toretain excess nitrogen in the system, produce large amounts of biomass and suppress weeds (Evanset al.,1988; Leary et al., 2006; Smith, 2002;). However, grasses can vary in their culturalrequirements, and a large biomass producers like sorghum x sudangrass may present challengeswith regards to killing and incorporation in annual vegetable systems (Fig. 2).
Figure 2. Japanese millet (Echinochloa sp.) growing at ~2000 ft in Kula, Maui (A), growing at ~60 ft in Waimānalo, O'ahu with taller volunteers of sorghum x sudangrass (B), and Japanese millet killed by mowing while root mat of sorghum x sudangrass regrows (C).
An important factor that impacts decision-making with regard to cover crop selectionis the extent to which the crop is tolerant toimportant pests. Sunn hemp is the superstarof Hawai‘i cover cropping, in part becauseof its ability to suppress root-knotnematodes. Many other legumes with highpotential for use in Hawai‘i are susceptibleto this important pest (Sipes and Arakaki,1997). Marigold is another good nematodesuppressing crop, but is a host for thripsand mites (Wang et al., 2007). Hairy vetchhas potential as a vigorous weedsuppressor and nitrogen fixer in rotationwith nematode suppressing covers, but is ahost toSclerotinia minorwhich causeslettuce drop, and may be unsuitable forsystems where lettuce is a primary cashcrop.
So, there is a lot to consider with no easy answers when it comes to cover crop selection.Nevertheless many producers in Hawai‘i have successfully included cover cropping into theirsystems, and CTAHR researchers and others continue to address questions to assist with selection(see marigold and Pioneer articles in this issue). The list of references and resources below isintended to provide a representative sampling of materials available to help with decision-making incover crop selection and management.
References and Resources
Boyd, C., P. Rotar, R. Guyton, J. Thompson, R. Kimoto. 1992. The influence of seeding rates and cutting heights on dry matter yields of alfalfa in Hawaii. CTAHR research series 068.
Cassman, K., A. S. Whitney, and R. L. Fox. 1980. Symbiotic nitrogen fixation requires extra phosphorus. http://www.ctahr.hawaii.edu/oc/freepubs/pdf/IC-17.pdf Hānai‘Ai/
Crop Care Hawai‘i. 2007. Accelerating the Adoption and Implementation of Proven Cover Crop Technologies in Hawaii. http://www.ctahr.hawaii.edu/sustainag/NewFarmer/downloads/Accelerating%20_Adoption_of%20_Cover%20Crops.pdf
Evans, D.O., R. J. Joy, and C. L. Chia. 1988. Cover crops for orchards in Hawaii. http://www.ctahr.hawaii.edu/oc/freepubs/pdf/RES-094.pdf
Glover, N. 1994a. Cover crops for weed control in dry land taro fields. On-farm Research Leaflet #3. In: N. Glover and L. Ferretinos (eds.). Pacific Islands Farm Manual. Agricultural Development in the American Pacific (ADAP). http://www.ctahr.hawaii.edu/adap/Publications/ADAP_pubs.htm
Glover, N. 1994b. Glycine. Cover Crop Leaflet #6. In: N. Glover and L. Ferrentinos (eds.). Pacific Islands Farm Manual. Agricultural Development in the American Pacific (ADAP). http://www.ctahr.hawaii.edu/adap/Publications/ADAP_pubs.htm
Guyton, R., J. Thompson, C. Boyd, P. Rotar, D. Matsuyama, and R. Kimoto. 1992. Performance of 15 alfalfa varieties in 31 consecutive harvests in Hawai‘i. CTAHR research series 067.
Habte, M. 2000. Mycorrhizal fungi and plant nutrition. http://www.ctahr.hawaii.edu/oc/freepubs/pdf/pnm14.pdf
Hooks, C.R.R., K. Chandarab, D. Fallon, K-H. Wang, and R. Manandhar. 1997. The impact of sunn hemp cover cropping on belowground organisms and nutrient status associated with a cucumber planting. http://www.ctahr.hawaii.edu/oc/freepubs/pdf/SCM-21.pdf
Hooks, C.R.R., K.-H. Wang, and D. Fallon. 2006. An ally in the war against nematode pests: Using sunnhemp as a cover crop to suppress root-knot nematodes. http://www.ctahr.hawaii.edu/oc/freepubs/pdf/PD-32.pdf
Hooks, C.R.R., A. Fereres, and K.-H. Wang. 2007. Using protector plants to guard crops from aphid-borne non-persistent viruses. http://www.ctahr.hawaii.edu/oc/freepubs/pdf/SCM-18.pdf
Leary, J. K.; DeFrank, J.; Sipes, B. 2006. Tropical eggplant (Solanum melongena L.) production with a buffelgrass (Pennisetum ciliare L.) living mulch system in Hawai‘i. Biol. Agric. & Horticulture 24: 105-116,
NIFTAL. 2000. Biological nitrogen fixation. University of Hawai‘i at Mānoa, College of Tropical Agriculture and Human Resources (CTAHR). http://www.ctahr.hawaii.edu/oc/freepubs/pdf/pnm13.pdf
Radovich, T., L.J. Cox, J Sugano, and T. Idol. 2009. Benefits and costs of using perennial peanut as living mulch for fruit trees in Hawai’i. http://www.ctahr.hawaii.edu/oc/freepubs/pdf/SCM-27.pdf
Sipes, B. and Arakaki, A. 1997. Root-knot nematode management in Dryland Taro with Tropical Cover crops. Journal of Nematology 29:721-724.
Smith, J. (ed.). 2002. Cover crops and green manures for Hawai‘i. http://www.ctahr.hawaii.edu/sustainag/cc-gm/index.html
Valenzuela, H., R. Hamasaki, and T. Radovich. 1999. Cover Crop Studies. Pp 32-42 in: Organic Farming Compost Experiments in Waimanalo, Hawai‘i. http://www2.hawaii.edu/~hector/sustain%20site/MOA%205%20yr%20report%20pm6.pdf
Wang, K-H., C. Hooks, and Antoon Ploeg. Protecting Crops from Nematode Pests: Using Marigold as an Alternative to Chemical Nematicides. http://www.ctahr.hawaii.edu/oc/freepubs/pdf/PD-35.pdf
Yost, R.S., D.O. Evans, N.A. Saidy, and R.L. Fox. 1981. Green manuring- renewed interest in an old concept. http://www.ctahr.hawaii.edu/oc/freepubs/pdf/IC-25.pdf
Yost, R. and D. Evans. 1988. Green Manure and legume covers in the tropics. CTAHR research series 055
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