1. COVER CROPS FOR NEW AMERICAN CROPPING SYSTEMS
Seth Dabney
USDA-ARS
National Sedimentation Laboratory

2. Organization Introduction: why cover crops (and no-till)?
Concepts and Terms
Cover Crop Management (killing cover crops)
Cover Crop Selection
Cover Crop Resources

3. Why Grow Cover Crops? Soil Management Water Management Pest Management
Nutrient Management
I was invited to critique a CEAP (conservation effects assessment project) literature review that was being conducted by the SWCS. The first order of business was to divide the conservation practices among four groups of authors dealing with: 1) water management, 2) soil management, 3) nutrient management, or 4) pest management. The authors of all four areas wanted the “cover crop” practice because it was important in all these management areas.

4. Winter rye cover crop reduced runoff throughout year average of 10 years and in three vegetable crop systems Freehold loamy sand, 3% slope, New Jersey
No cover
Brill and Neal
Agron. J. 42:
Rye cover

5. Cover crops, high-residue crops, and no-till management can reduce runoff (and erosion even more!) average of 6 years, Grenada silt loam, 5% slope, Mississippi
Reduced winter runoff with cover crops is mainly due to increased transpiration (and biomass production).
Meyer et al Trans. ASAE 42:

6. Insufficient Residue = Soil Crusts (no-till won’t work with bare soil)

7. Cover crop residue mulch increases infiltration
No-till
No Cover Crop
Cover Crop
Conventional-till
Sprinkler
Irrigation
trigger
Sprinkler-irrigated rice on Crowley silt loam
Dabney (1998) J. Soil and Water Cons. 53(3):

8. Why Grow Cover Crops? Soil Management Water Management Pest Management
Nutrient Management
I was invited to critique a CEAP (conservation effects assessment project) literature review that was being conducted by the SWCS. The first order of business was to divide the conservation practices among four groups of authors dealing with: 1) water management, 2) soil management, 3) nutrient management, or 4) pest management. The authors of all four areas wanted the “cover crop” practice because it was important in all these management areas.

9. Weed suppression in no-till corn by sub clover (background is conventional tillage, no cover)
Problems: cost of cover crop, hard to kill cover crop, stand problems

10. Stripped cover crops of rye and a clover mix in Georgia for beneficial insect habitat.
Cotton was planted into the killed strips of rye while the clover was left growing until an application of Roundup at the 4 to 6 leaf stage
Photo: Harry Schomberg

11. Why Grow Cover Crops? Soil Management Water Management Pest Management
Nutrient Management
I was invited to critique a CEAP (conservation effects assessment project) literature review that was being conducted by the SWCS. The first order of business was to divide the conservation practices among four groups of authors dealing with: 1) water management, 2) soil management, 3) nutrient management, or 4) pest management. The authors of all four areas wanted the “cover crop” practice because it was important in all these management areas.

12. N management concepts and terms
green manure: cover crops grown mainly to improve the nutrition of subsequent main crops; may contain legumes that can add N to the cropping system
catch crop: cover crops grown to catch available N in the soil and thereby prevent leaching losses of N already in a cropping system
pre-emptive competition: uptake of soil nitrate by cover crops that would not have been lost to subsequent crops by leaching, thereby reducing availability of N to the subsequent crop

13. Concepts and terms
To be most effective, green manure crops should winter kill, be grazed, or be killed early in the spring to prevent pre-emptive competition and so that green manure N can be rapidly mineralized.
To be most effective, catch crops should be planted early in the fall to maximize root growth and N uptake.
High residue cover crops can increase yield potential and build soil C, but may also increase the economically optimal fertilizer N rate.

14. Catch crop effectiveness is highly correlated with rooting depth, but not with root density With warm weather, broad leaf plants may root more deeply more quickly than grasses
Cover crop rooting depth varies with interactions of species, soil properties, climate, and planting date. When tested during warm weather (planting 1 Aug in Denmark following horticultural crops), broadleaves cover crops grew deeper roots faster than cereals or annual ryegrass (Lolium multiflorum).
Phacelia (Scorpion weed) Hydrophyllaceae (water-leaf family). Native to America (chiefly to western U.S. and Mexico)
Thorup-Kristensen, K Plant and Soil 230: 185–195.

15. Catch crop effectiveness is highly correlated with rooting depth, but not with root density With warm weather, broad leaf plants may root more deeply more quickly than grasses
Cover crop rooting depth varies with interactions of species, soil properties, climate, and planting date. When tested during warm weather (planting 1 Aug in Denmark following horticultural crops), broadleaves cover crops grew deeper roots faster than cereals or annual ryegrass (Lolium multiflorum).
Phacelia (Scorpion weed) Hydrophyllaceae (water-leaf family). Native to America (chiefly to western U.S. and Mexico)
Thorup-Kristensen, K Plant and Soil 230: 185–195.

16. Synchrony – temporal match between N supply and demand
Asynchrony between N availability and crop demand can lead to mineral N excess, susceptible to loss, or deficit that limits crop growth. Asynchrony can occur with either inorganic fertilizer N additions or through green manure or soil N mineralization.
(Crews and Peoples Nutrient Cycling in Agroecosystems 72: )

17. Rice and legume cover crops: no-till improves synchrony
Source: Dabney et al Agron J. 81:
My first cover crop research was done with Steve Eastman, an MS student from a rice farm in Jennings, LA.
He wanted to work on rice, I wanted to work on cover crops and no-till.
We chose crimson clover and subterranean clover.
We used a Moore no-till grain drill that had a 4 ¾” drill spacing and heavy cast iron press wheels. Seeding rice with this drill killed the clover even when it wasn’t sprayed. Any escaped weeds were controlled with Stam prior to the flood.
No-till increased rice yields with or without cover crops, cover crops increased rice yields with or without tillage.
Clover N acted as a slow release N fertilizer with better N synchrony and increased rice yield (higher than could be done with inorganic N fertilizer).
Tillage lead to mineralization and subsequent denitrification resulting in less efficiency (poor synchrony).
Flooding also enforced seed dormancy resulting in thick volunteer stands after draining the flood.
In a related study with sprinkler-irrigated rice: without a cover crop mulch, infiltration is greater under CT than under NT (where it was not even possible to maintain the soil water potential target), but with a mulch, infiltration is greater under NT than under CT without a cover crop.

18. Reseeding crimson clover biomass and N accumulation in GA
Sampling (of above ground biomass, litter, or roots) indicates net growth, not gross biomass production or N uptake
In reseeding legume systems, seed may contain ~ 50 kg N/ha
Anthesis is on ~ April 4, Spray is ~ May 12
Point in time sampling measures net shoot and root biomass production and N uptake, not total production or uptake. To maintain constant litter levels, cover crop litter additions must occur to balance decomposition, even while canopy growth continues. In reseeding systems, seed N becomes a significant component of the N balance, reducing the green manure benefit. Harper et al (1995) found total crimson clover N exceeded 300 kg ha-1 even though N available to subsequent crops was less than one third of this.
Harper et al Crop Sci. 35:

19. Reseeding crimson clover biomass and N accumulation in GA
Sampling (of above ground biomass, litter, or roots) indicates net growth, not gross biomass production or N uptake
In reseeding legume systems, seed may contain ~ 50 kg N/ha
Anthesis is on ~ April 4, Spray is ~ May 12
Point in time sampling measures net shoot and root biomass production and N uptake, not total production or uptake. To maintain constant litter levels, cover crop litter additions must occur to balance decomposition, even while canopy growth continues. In reseeding systems, seed N becomes a significant component of the N balance, reducing the green manure benefit. Harper et al (1995) found total crimson clover N exceeded 300 kg ha-1 even though N available to subsequent crops was less than one third of this.
Harper et al Crop Sci. 35:

20. Plowing cover crops into soil does not increase soil organic matter
Tillage controls weeds, loosens soil, and smoothes fields
burns up soil organic matter
speeds organic N mineralization and nitrification
Cover crops make no-till more successful
(make no-till vegetables possible)
No-till with cover crops increases soil C

21. Worried about ammonia loss from no-till residues?
Look how clearly the far shore of Cayuga lake could be seen on clear days when the wind blew from the NW.

22. Even when hay is rained on, total NH3 losses to the atmosphere are minor
I borrowed some (ARS) anemometers from him and set out to study the annual cycle of ammonia exchange over an alfalfa field.
But in the course of developing the sampling methodology, it became clear to me that I needed to separate gaseous from particulate forms of ammonia. My major professor, Dave Bouldin, resisted the change because he wanted me to remain focused on the primary goal, but I insisted that I needed to broaden the study.

23. No-till with cover crops
wheat cover
No-till with cover crops
Among the finding of this project was improvements in cotton productivity with time in no-til for light colored loessal alfasols.
I believe that the improvement in productivity relative to conventional tillage was a combination of several factors: degradation of tilled soils with time from sod, development of stable organic mulch in no-till, but also improved know how on the part of the farmers (researchers).

24. Cover crop residue mulch increases soil temperature
NT often is thought to result in cooler soil. This is true only while soils are warming, and only during the day. Night temperatures and temperatures day and night during cold snaps are often warmer under NT than CT.
Average of 4 observations of soil temperature at 2.5 cm depth, daily rainfall and daily solar radiation.
The 20 degree refrence line is the morning temperature minimum recommended for planting. Significant diffrences between tillage temperature minima and maxima are shown *(P<0.05) and **(P<0.01).
Dabney, Delgado, and Reeves Comm. in Soil Sci. and Plant Anal. 32:

25. Improved early season cotton growth and survival with no-till
mycorrhiza hyphal counts per mm2
Mycorrhizae are another soil quality factor that we found to be contributing to enhanced survival and early season growth of cotton. As part of his PhD studies, Ernie Flint found this benefit was not associated with the plant, but with the soil surrounding the plant.

26. Why Not Grow Cover Crops?
Time
Cost
Lack of water
Stand problems
Seed placement
Pest and disease risks

27. 10 ton biomass yield in California ~50 mm (2 inches) of water per ton of cover crop biomass
Photo: Jeff Mitchell; Rolling rye cover crop,
T & D Willey Farms, Madera, CA, April 21, 2006

28. Rhizoctonia infection of sorghum seedlings grown in soil with and without cover crop residues
Dabney, Schrieber, Rothrock and Johnson Agron. J. 88(6):

29. No-till planter: coulter to cut residues; double-disk opener set slightly deeper to plant into firm soil
On most corn planters, coulters are typically placed as close to the planter unit as possible to ensure uniform cutting depth and seed placement. They are often bolted directly to the front of each planter unit and set to run at a depth of 1 to 2 inches.
In most soil conditions, the double disk openers and coulters should be set to operate at about the same depth. When soils are dry, coulters should be set slightly deeper than the seed openers to ensure that loose soil is present at the bottom of the seed slot to provide good seed-soil contact. When soils are wet, coulters should be set slightly shallower than seed openers to ensure that seed is placed in firm soil in case the seed zone later dries out.

30. Enhanced no-till planter: Tined-wheel row cleaner, scalloped double-disk openers reduce “hair-pinning” and tined press wheels close no-till seed slot without compaction.
Extra features are needed to plant no-till into heavy cover crop residues
Row cleaners
Scalloped coulters
Non-crust creating press wheels

31. Row cleaners can clear heavy residues, improving stands of direct seeded crops

32. Cover Crop Management (Killing Cover Crops without Tillage)
Spray (works best early)
Mechanical (works best late)
Mow (flail gives most uniform residue distribution)
Undercut
Roll (roll chop, knife roll)

33. Mowing – flail mowers leave residues evenly distributed, but operation and maintenance costs are high.

34. Undercutter (Nancy Creamer, NC State)
Suitable for high value crops on permanent beds

35. South American Knife-rollers

36. American Rollers, Roll-choppers, and Roll crimpers

37. GA farmer, Lamar Black, rolls a 2 meter tall rye cover crop prior to planting corn or cotton
The resulting mulch suppresses weeds, conserves water, and lowers peak soil temperature.

38. Cover Crop Management (killing cover crops)
Cover crops make no-till vegetables possible.
Direct seed no-till pumpkin into rye cover crop in Southwest VA (Ron Morse)

39. No-till transplanting cabbage into rolled rye, Hillsville, VA (Ron Morse)

40. Percent kill of summer cover crops in North Carolina (Creamer and Dabney, 2002, Am J. Alt. Ag. 17:32-40)
Cover crop
Growth stage
Mow
Undercut
Roll
(smooth)
Cowpea
Vegetative 98 85 5
Soybean
Early bloom
100 99 12
Buckwheat
Mature
Pearl Millet
Heading 73 18
German Millet
Green Seed
Sorghum-sudangrass 89 25

41. German or Foxtail Millet (Setaria italica)
Mowed or Rolled
doesn’t re-grow

42. No-till no-herbicide broccoli transplanted into rolled foxtail millet (Ron Morse)
Weed control lasts longer after rolling than after mowing.

43. Sunn Hemp – Crotalaria juncea rapid legume growth following corn in Alabama killed by frost ($2.5/lb from

44. Sunflowers grow fast, has wildlife benefits, and can be killed by rolling; mulch degrades quickly

45. Cover Crop Selections for the mid-South (humid zones 6+)
Winter
Hairy Vetch
(Vicia villosa)
Balansa clover
(Trifolium michelianum)
‘Paradana’ or ‘Frontier’
Rye
(Secale cereal)
‘Abruzzi’ or ‘Merced’
Oat
(Avena sativa)
Summer
Cowpea
(Vigna unguiculata)
Sunn hemp
(Crotalaria juncea)
Foxtail millet
(Setaria italica)
Sunflower
(Helianthus annus)
Forage turnip
(Brassica rapa)
Forage radish
(Raphanus sativus)

46. First cultivar was released by SCS in 1952, but like other cover crops, disappeared from seed trade in US.
Commercially available form australia.

47. Mississippi County, AR 7A
Balansa clover Trifolium michelianum Savi – a superior reseeding winter cover crop
Mississippi County, AR 7A
Moorhead, MS 8A
Tiptonville, TN 6B
First cultivar was released by SCS in 1952, but like other cover crops, disappeared from seed trade in US.
Commercially available form Australia.

48. On-farm reseeding demonstration/adoption
Thighman Lake
Reseed for 2 to 4 years from 1 seed crop. 70 to 150 kg N/ha.
The demonstrations illustrated were located in the Delta, near Thighman lake.
While crimson clover (Trifolium incarnatum) is the most common reseeding winter legume cover crop, balansa clover (Trifolium michelianum), shown above in Mississippi, produces more hard seed. Balansa is winter hardy through zone 7, is adapted to wet soils, flowers by the end of March, produces hard seed 30 days after blooming, and will regenerate stands under no-tillage for several years from a single hard seed crop.

49. Seed size: 1000 to 1400 per gram
Plant 5 lb/a (8 if coated); ~$2.00/lb
Kamprath Seed ( ), Manteca, CA

50. Rye, Balansa, or No cover (Average of NT and RT) silty clay soil, Stoneville, MS
N Fertilizer Applied NC
Rye
Clover
Kg N/ha
2001
140
2002
112
134 45
2003
2004
157
179 90
Although challenges associated with clover seemed to occur more in the first year than in subsequent years, delay in emergence and early growth were observed every year. At the beginning of the study, it was hypothesized that once established, the clover would contribute sufficient nitrogen so that supplemental amendments could be reduced. Therefore, 140 kg N ha-1 was added to all treatments the first year, but in 2002 and 2003, N was added at rates if 45, 112, and 134 kg N ha-1, respectively, for clover, no cover, and rye treatments. Because cotton yields in clover plots were less than that of rye and no cover, the supplemental N levels for clover plots were increased from 45 to 90 kg N ha-1 in The N also was increased for rye and no cover plots to 179 and 157 kg N ha-1, respectively. The highest rates were made to the rye crop because the slower decomposition of rye residue was assumed to be immobilizing nutrients.
Locke et al. (unpublished)
NT with rye was most profitable over four years

51. Sweet Potato

52. Rodale Roller at DCDC, Metcalf, MS, in 2006 part of a Conservation Innovation Grant
Things that went wrong:
Rye Stand Marginal (Rita)
Rye too old (roller delivered late)
Soil too dry
Planter depth set too shallow (poor cotton stand)
Needed herbicide (thin mulch, poor stand), but herbicide was caught on mulch (poor weed control)
Fertilizer N not increased (pre-emptive competition)
Result: poor stand, poor growth, poor weed control, poor yield

53. Let cover crops grow longer (only if water is available)
increased residue can be managed
Increased residues control weeds
increased residue conserves water later
maintains mycorrhiza hyphae network for early growth boost
Do not till needlessly
it is like taking money out of the bank!

54. Conclusions: Cover Crop Opportunities
Catch crop or green manure
Synchrony (i.e. N recovery in no-till rice)
Balansa clover as a reseeding legume
Cover crops for bio-fuel production fields in humid zones 6+ (available water and sunlight)
Research to identify insect/disease complex when planting crops other than rice into recently killed legume cover crops

55. Selected resources on the web
ATTRA
NAL/SAN
ARS
California
Michigan
Cedar Meadow (Steve Groff)
Rolf Derpsch

56. Questions?
Rolf Derpsch uses rolled black oat cover crop residues to make no-till successful in Paraguay

58. Why Grow Cover Crops? Increase yield (nutrients, mycorrhiza, water)
Build up soil carbon
Decrease runoff and erosion (wind and water)
Reduce N leaching
Decrease weed growth
(Cover crops must be easy to control or they could become weeds)
Increase beneficial insects
I was invited to critique a CEAP (conservation effects assessment project) literature review that was being conducted by the SWCS. The first order of business was to divide the conservation practices among four groups of authors dealing with: 1) water management, 2) soil management, 3) nutrient management, or 4) pest management. The authors of all four areas wanted the “cover crop” practice because it was important in all these management areas.