1.6 Selecting and Using Cover Crops Introduction 3 Instructor’s Lecture Outline 5 Detailed Lecture Outline for Students 7 Demonstration: How to Estimate the Nitrogen Contribution of a Cover Crop 11 Assessment Questions and Key 13 Resources 15 2 | Unit 1.6 Cover Crops Introduction: Selecting and Using Cover Crops Overview MOdes Of instructiOn Cover crops can be a key soil > LECtURE (1.5 HOURS) fertility and pest management tool A lecture covers the advantages and disadvantages of using cover crops and the three-step process of choosing a cover for growers. This unit introduces crop based on the agricultural system’s needs. students to the role and selection > ASSESSmENt QUEStIONS (0.5 HOUR) of cover crops for organic farming Assessment questions reinforce key unit concepts and skills. and gardening systems. The lecture addresses the influence of Learning Objectives cover crops on soil’s physical and CONCEptS chemical properties, the role cover • The influence of cover crops on the physical and chemical crops play in pest management, properties of agricultural soils and the factors to consider when • The role cover crops play in pest and disease management selecting a cover crop for a farm • The factors to consider when selecting a cover crop for or garden. The resources section farm or garden use includes an annotated list of print SKILLS and electronic references that • How to access cover crop information students may use in selecting cover • How to estimate nitrogen contributions of cover crops crop species or mixes for specific agricultural applications. Unit 1.6 | 3 Introduction Cover Crops 4 | Unit 1.6 Cover Crops Lecture Outline: Selecting and Using Cover Crops for the instructor A. Pre-Assessment Questions 1. What benefits can be derived from the regular use of cover crops in a farming system? 2. What role do cover crops play in the management of soil fertility in organic farms and gardens? 3. How can cover crops be used to ensure adequate soil nutrient levels/nutrient budgeting? 4. How can cover crops improve the physical properties of agricultural soils? 5. What role can cover crops play in pest management? 6. What are the factors to consider when selecting a cover crop for your farm or garden? B. Why Use Cover Crops? 1. The role of cover crops in organic farming systems: Benefits and disadvantages 2. Cover crop, “green manure,” and “catch crop” defined a) Cover crop: Mainly used to prevent soil erosion by covering soil with living plants b) Green manure: Crop grown mainly to be turned under for soil improvement c) Catch crop: Used to “catch” nutrients left after harvest of a cash crop and prevent leaching d) Most cover crops serve multiple functions C. How to Choose a Cover Crop 1. Three steps in selecting a cover crop a) Step 1: Identify what you want the cover crop to do i. Defining primary function of cover crop based on needs of system • Provide nitrogen • Increase soil organic matter (SOM) and improve nutrient availability • Scavenge nutrients • Prevent soil erosion • Improve soil structure • Improve drainage • Protect water quality • Provide mulch to conserve soil moisture and/or suppress weeds • Provide habitat for beneficial insects and spiders • Suppress weeds • Suppress soil borne pests and diseases b) Step 2: Identify planting niche i. Defining when and where the cover crop fits in your crop rotation ii. Examples of cool weather and warm weather cover crops c) Step 3: Select cover crop species or mix to meet the goals and requirements from steps 1 and 2. Once you have set the priorities for what cover crop effects you are looking for, and you have identified when the cover crop can be grown, the final step is to match the potential candidates identified in step 2 with the required characteristics identified in step 1. Unit 1.6 | 5 Instructor’s Lecture Outline Cover Crops i. Additional considerations • Consider the characteristics you don’t want as well as those you are looking for • Consider the cost and availability of seed • Consider the number and types of field operations required for the different options in order to make a sound economic assessment of the alternatives D. How to Roughly Estimate N Contribution from a Cover Crop 1. Calculate above-ground weight for a given area as follows: a) Take a number of samples from the field (clip at ground level, and use a yardstick or frame to measure area to be sampled) b) Dry for a few days in sun, greenhouse or oven (140ºF) until “crunchy” or brittle c) Calculate the dry weight produced in pounds/acre (lbs/ac) as follows: Dry wt (lbs/ac) = weight of samples x 43,560 sq.ft # of sq. ft sampled 2. Multiply dry wt by the % N content of the biomass (see below) to give total N in cover crop in lbs/ac: Total N (lb/ac) = dry weight x %N 100 3. To estimate how much of the N will be available to the crop that season: If conventionally tilled divide total N by 2 If left on surface divide by 2 in warmer climates, by 4 in cooler climates 4. Typical % N contents for cover crop types Before flowering: Annual legumes 3.5–4% Perennial legumes: 2.5–3% Grasses, brassicas: 2–3% When flowering: Annual legumes: 3–3.5% Perennial legumes: 2–2.5% Grasses, brassicas: 1.5–2.5% From: Managing Cover Crops Profitably, 2nd Edition, published by the Sustainable Agriculture Network (see Resources section). Used by permission. 6 | Unit 1.6 Cover Crops Instructor’s Lecture Outline Detailed Lecture Outline: Selecting and Using Cover Crops for students a. Pre-assessment Questions 1. What benefits can be derived from the regular use of cover crops in a farming system? 2. What role do cover crops play in the management of soil fertility in organic farms and gardens? 3. How can cover crops be used to ensure adequate soil nutrient levels/nutrient budgeting? 4. How can cover crops improve the physical properties of agricultural soils? 5. What role can cover crops play in pest management? 6. What are the factors to consider when selecting a cover crop for your farm or garden? b. why use cover crops? 1. the role of cover crops in organic farming systems Cover crops are one of the primary fertility and soil management tools available to the organic farmer, and are an important strategy for preventing nutrient and soil loss from a field. they are crops grown primarily for soil or ecosystem improvement rather than cash, and can provide a variety of services, from increased nitrogen (N) input, to soil protection, to weed and disease suppression. However, they can also have negative consequences if they are managed incorrectly or the wrong species are chosen. these issues are covered in Unit 1.1, Soil Fertility management. 2. Cover crop, “green manure,” and “catch crop” defined though the terms are used interchangeably, cover crop, green manure, and catch crop refer to different primary functions of the crops planted a) Cover crop: mainly used to prevent soil erosion by covering soil with living plants b) Green manure: Crop grown mainly to be turned under for soil improvement c) Catch crop: Used to “catch” nutrients left after harvest of a cash crop and prevent leaching these are not mutually exclusive functions. For example, a fall-planted cereal + legume crop that is incorporated the following spring can serve as a cover crop, green manure, and catch crop. However, different species and mixes may perform one function better than another. c. How to choose a cover crop 1. Deciding whether it is desirable or feasible to include a cover crop in a given production system, and which cover crop to use, involves the following three steps a) Step 1: Identify what you want the cover crop to do the first step is to identify your goals by prioritizing the main effects you want the cover crop to have in your system. this will depend on what you think are the most important factors limiting the productivity and sustainability of your production system. For example, is it low fertility, poor soil structure, weed or pest populations? Deciding this is critical before choosing which cover crop to use. i. Defining primary function of cover crop based on needs of system Unit 1.6 | 7 Students’ Lecture Outline Cover Crops • Provide nitrogen. Here the best choice would be a legume that is well adapted to your area. It is important to ensure that sufficient rhizobia bacteria are present in the soil to give good root nodule formation, and hence N fixation. If it is the first time a 2 legume cover crop has been grown in a field it is recommended that you purchase the appropriate rhizobium inoculant for the species being planted. A rough guide to estimating the N contribution from a cover crop is shown on page 10 (excerpted from Managing Cover Crops Profitably, 2nd Edition, published by the Sustainable Agriculture Network). • Increase soil organic matter (SOM) and improve nutrient availability by increasing soil biological activity. to build SOm you would look for a high biomass-producing cover crop. possible options include non-legumes such as annual rye grass, cereal rye, triticale, or sorghum/sudan grass if a summer niche is identified. High-biomass legumes such as sub-clover or woollypod vetch would also work and provide N, as would a mix of cereals and legumes. • Scavenge nutrients left in the soil after the cash crop and prevent loss by leaching. to maximize nutrient scavenging the cover crop should have an extensive root system that develops quickly after planting. Non-legumes such as small grains, cereal rye, triticale, rapeseed, annual rye grass, oilseed radish, and mustards work well, but some legumes are also suitable. • Prevent soil erosion. Here the key is to choose a species that rapidly covers the soil surface. many of the species that are good nutrient scavengers also provide excellent ground cover. However, while annual rye grass is a good nutrient scavenger, it has fine leaves and is slow to cover the soil surface. • Improve soil structure. Increasing SOm is the key to improving soil structure, and similar species can be used. It may be beneficial to include a non-legume to provide organic material that breaks down more slowly than a legume alone. • Improve drainage. Some deep-rooted species can help to break through compacted layers in the soil and improve drainage. the roots of soil-penetrating cover crops also create channels through which water can move as the root systems decompose after death or incorporation. Examples include bell beans (Vicia faba) and clovers (Trifolium spp.). In addition, many grass species with extensive, fibrous root systems add large quantities of organic matter to the soil by sloughing off roots. Such organic matter additions stimulate biological activity and the formation of soil aggregates, which improve soil’s drainage. Examples include annual rye , perennial rye, and oats. • Protect water quality. this is achieved by selecting species that both prevent soil erosion and scavenge nutrients during periods of high rainfall. It is also important to avoid turning in high N cover crops when the soils will be vulnerable to leaching, e.g., when there is little or no crop cover and high rainfall. • Provide mulch to conserve soil moisture and/or suppress weeds. A combination of high above-ground biomass and moderate or high C:N ratio residues is desirable. most legume residues with their high nitrogen content will decompose too rapidly to be effective. • Provide habitat for beneficial insects and spiders. this aspect of cover crop ecology has not been well studied for annual systems, and there may be more potential than has been recognized. From research in orchards and vineyards it is clear that cover crops provide good habitat for beneficials, especially when species with food sources such as extrafloral nectaries (e.g., vetch) or flowers are used. the key in annual systems is to make sure that when the cover crop is turned in there are alternative habitats for the beneficials to move to. 8 | Unit 1.6 Cover Crops Students’ Lecture Outline • Suppress weeds. Cover crops can reduce weed populations either by outcompeting weeds by more vigorous growth and dense canopy cover, or by releasing allelopathic compounds that inhibit weed seed germination and growth. Some good options include cereal rye, triticales, sorghum/sudan and other cereals, as well as brassicas such as rapeseed and oilseed radish, and high biomass or allelopathic legumes. A well-balanced mix can also work, providing the canopy closes quickly. It is also important to remember that certain cover crops can themselves become weeds if they have persistent or hard seed. Finally, just as crop rotation is important, it is advisable to avoid using the same cover crop every year, particularly a single species. Otherwise populations of weeds that are most competitive with that species will build up, as will pests and disease organisms that also attack that species. • Suppress soil borne pests and diseases. Certain species are known to suppress particular disease or pest organisms. Similarly others are known to be excellent hosts to certain pests and diseases. If your field is known to have a history of bad pest or disease problems this will affect your choice of cover crop. For example, cereal rye, sorghum/sudan, selected rapeseed varieties (e.g., c.v. humus), oilseed radish, and white mustard are known to suppress root knot nematodes, and soil-borne diseases such as rhizoctonia and verticilllium wilt. Conversely many legumes are excellent hosts for nematodes and populations will increase rapidly in the soil with their use unless the crop is turned in before the nematodes complete their life cycle. If the timing is correct the legume may actually reduce nematode populations by stimulating the nematodes’ emergence but killing them when the crop is turned in and the infected roots die before their life cycle is complete. b) Step 2: Identify the cover crop planting niche When and where can the cover crop fit in your crop rotation? What are the climatic and soil conditions at that time? Once these two questions are answered, then you can select the best species or mix to plant. i. Defining when and where the cover crop fits in your crop rotation to choose when to plant cover crops, timing of field operations to avoid interfering with cash crop production is critical. Once the opportunity for cover crop growth and incorporation is identified then the species selection will depend on the climatic and soil conditions during the window. the publications and web sites in the Resources section of this unit provide information on climatic and soil requirements for many cover crops and will enable you to identify potential species to consider. • Define timing of critical field operations for cash crop production. Field operations and labor needs for cash crop production will take precedence over cover crop management, so it is important to minimize these conflicts. ii. Examples of cool weather and warm weather cover crops • Winter cover crops. Most cover crops are planted in the fall to provide cover over the winter months. Examples include: Vetches, fava/bell beans, peas, annual and perennial clovers, rape seed and oilseed radish, and grasses such as oats, annual and perennial rye grass, or barley. • Summer cover crops. When temperatures are high during the summer and providing water is available, fast-growing species such as sorghum/sudan grass, cowpeas, annual buckwheat, lab lab, Crotolaria (sun hemp), Sesbania, and others can provide a good biomass return in a short growth period. Unit 1.6 | 9 Students’ Lecture Outline Cover Crops c) Step 3: Select cover crop species or mix to meet the goals and requirements from steps 1 and 2 Once you’ve set the priorities for what cover crop effects you are looking for, and you’ve identified when the cover crop can be grown, the final step is to match the potential candidates identified in step 2 with the required characteristics identified in step 1 i. Additional considerations: • Consider the characteristics you don’t want as well as those you are looking for. It is rare that the “perfect” cover crop will exist and trade-offs will need to be made amongst different goals. • Consider the cost and availability of seed • Consider the number and types of field operations required for the different cover cropping options to make a sound economic assessment of the alternatives d. How to roughly estimate n contribution from a cover crop 1. Calculate above-ground weight for a given area as follows*: a) take a number of samples from the field (clip at ground level, and use a yardstick or frame to measure area to be sampled) b) Dry for a few days in sun, greenhouse, or oven (140ºF) until “crunchy” or brittle c) Calculate the dry weight produced in pounds/acre (lbs/ac) as follows: Dry wt (lb/ac) = weight of samples x 43,560 square feet # of sq. ft. sampled 2. multiply dry wt by the % N content of the biomass (see 4, below) to give total N in cover crop in lbs/ac: total N (lb/ac) = dry wt x %N 100 3. to estimate how much of the N will be available to the crop that season If conventionally tilled, divide total N by 2 If left on surface, divide by 2 in warmer climates, by 4 in cooler climates 4. typical % N contents for cover crop types Before flowering: Annual legumes: 3.5–4% perennial legumes: 2.5–3% Grasses, brassicas: 2–3% When flowering: Annual legumes: 3–3.5% perennial legumes: 2–2.5% Grasses, brassicas: 1.5–2.5% *From: Managing Cover Crops Profitably, 2nd Edition, published by the Sustainable Agriculture Network (see Resources section). Used by permission. 10 | Unit 1.6 Cover Crops Students’ Lecture Outline