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SWEEP Report #57b

Effect of Winter Rye Mulches and Fertilizer Amendments
on Nutrient and Weed Dynamics in No-Till Soybeans

R. Samson and J. Omielan, Resource Efficient Agricultural Production (REAP) Canada, Ste. Anne de Bellevue, Quebec and C. Drury, Agriculture Canada, Harrow, Ontario

Executive Summary

Evaluation Summary (Tech. Transfer Report Summaries)

View / Download Final Report [408 KB pdf]

Associated SWEEP/LSP Research



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Completed: June, 1992

Key Words:

no-till, soybeans, rye, cover crop, mulch, weed control, allelopathy, nutrient effects, physical effects

Executive Summary

In the early 1980's, weed scientists identified winter rye to be an effective weed-suppressing mulch for no-till production systems. The term allelopathy is frequently used to describe the weed-suppressing effect of the winter rye. However, comprehensive field data supporting allelopathy as a major mechanism of weed suppression for high C:N ratio cover crops such as winter rye is lacking. No studies in the literature could be identified which examined the effect of a winter rye cover crop on soil nutrients and weeds. However, a number of studies by soil scientists have recently indicated that rye cover crops reduce soil nitrogen. As well, weed scientists have identified that some annual weeds are more responsive to high soil nutrient levels than other weeds or crops.

This study examined if rye cover crops contribute to weed suppression in no-till systems by reducing soil nutrient levels and hence weed pressure from N-responsive annual weeds. Experiments were conducted to examine: (1) the mulch (or physical) effects and (2) the fertility effects of a no-till rye cover crop on nutrient and weed dynamics in no-till soybeans. The objectives were to: (1) identify the effects of a rye cover crop on soil nutrients and (2) to separate the nutritional effects from physical and chemical (allelopathic) ones that are responsible for weed suppression in no-till soybeans.

Soil nutrient data indicated that rye plots had low soil nitrate levels at the time of soybean planting but nitrogen levels in control plots were also relatively low when measured shortly after planting. Soybeans sampled 4 weeks after planting showed delayed growth where rye had been planted as a fall cover crop in both experiments. However, nutritional deficiencies in the soybeans were not evident 8 weeks after soybean planting in both experiments. In the mulch experiment, rye cover crops grown in place and left in place (providing physical, chemical and soil nutrient effects of a mulch) resulted in approximately 1/5 the weed biomass of bare plots (no mulch) and 1/3 the weed biomass of: a rye mulch moved in place (providing physical and chemical effects of a rye mulch); a poplar excelsior mulch moved in place (providing a physical effect of a mulch ) and a plot in which the rye was grown in place but removed after soybean planting (providing a soil nutrient effect of a mulch). In the fertility experiment, the addition of 75 kg N/ha to the rye plots increased weed growth by 400% and caused lamb's-quarters to become a major weed species.

In summary, one of the weed-suppressing characteristics of a rye cover crop appeared to be related to its effect on nutrient availability. The physical mulch effect also appeared to be significant in reducing weed biomass. This study indicated that these two mechanisms together play an important role in providing weed control for farmers developing low herbicide, no-till soybean systems.


Evaluation Summary

(From Technology Transfer Report Summaries - A. Hayes, L. Cruickshank, Co-Chairs)
The purpose of the project was to study the effect of no-till rye mulches on nutrient and weed dynamics. It was thought that the growth of rye depleted the soil nitrates therefore reducing the amount available for weed growth. The high C:N ratio is thought to create enough nutrient immobilization to have affected weed growth.

The experiment consisted of 5 treatments:

  1. rye grown and left (physical, chemical and soil nutrient effects of the mulch)
  2. rye removed (soil nutrient effects of the mulch)
  3. rye moved in place (physical and chemical effects of the mulch)
  4. poplar wood mulch moved in place (physical effect of the mulch)
  5. control (bare plot, no mulch)
The nutritional and physical effect of rye mulch on weeds appeared to be additive. The physical effect of a mulch, either rye or poplar added, reduced weed biomass by about 36%. Allelopathy was reported as not being a significant factor in weed suppression as there was no apparent additional weed suppression by rye moved in place over poplar mulch moved in place. The soil nutrient effect with rye grown and removed reduced weed pressure by 36%. The combined effect of rye mulch grown and left reduced weed biomass by about 80%.

The nutrient effect of the rye mulch was also apparent in the lower soybean biomass and N content. The addition of 75 kg of N/ha in the fertility experiment quadrupled the weed biomass in the mulch plots but this was only half of what was observed in the bare plot fertilized with 75 kg N/ha.


This is one of the very few studies attempting to sort out the causes of weed suppression by cover crops. It is unfortunate that there is only one year's data. The main areas of concern are the use of 2,4-D in the burndown herbicide treatment, which could have affected the beans detrimentally, and the fact that the rye grew to the heading stage. Current recommendations are to kill the rye at 30-60 cm height, so the nutrient effects may not be as evident.

Associated SWEEP/LSP Research:

  • SWEEP Report #19 - Studies on the Control of Problem Weed Species in Conservation Tillage Systems
  • SWEEP Report #19A - Weeds of Corn, Soybean and Winter Wheat Fields Under Conventional, Conservation and No-Till Management Systems in 1988 and 1989
  • SWEEP Report #27 - Cereal Cover Crop Study
  • SWEEP Report #29 - The Effect of Organic Mulches on Soil Moisture and Crop Growth
  • SWEEP Report #43 - The Use of Cover Crops for Nutrient Conservation
  • SWEEP Report #52 - Field Scale Tests of Cover Crops I and II

Future Research: ( ) indicates reviewers suggestion for priority, A - high, C - low.

(B) As the report indicated, further study is needed to better understand soil nutrient dynamics under rye mulch. There is a need to determine if early season deprivation of nitrogen by the rye cover crop affects final soybean yield.




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Created: 05-28-1996
Last Revised: Thursday, May 19, 2011 04:15:00 PM