WEED COMPOSITION IN MAIZE (ZEA MAYS L.) FIELDS UNDER SMALLHOLDER CONSERVATION FARMING

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Focus group discussion

Prior to the field study, focus group discussions were conducted in November 2008 in each ward. To select farmers who participated in FGD, a list of all farmers in the ward compiled by the AGRITEX officer was used to group farmers according to adoption or non-adoption of CF since its inception in 2004. This gave two groups, the CF and non-CF farmers. For the non-CF group, 20 farmers were randomly selected and invited to attend an FGD at the local ward meeting place on an appointed date. The CF group was further stratified into early and late adopters of CF and ten farmers were randomly selected from each group and invited to attend a separate FGD. The two FGD meetings were conducted on the same day but at different times per ward. The aim of the focus group discussion was to get a view of farmers’ perceptions of constraints to crop 95 production in CONV tillage and CF. Each focus group discussion lasted about 2 hours. In Ward 14, two separate focus group discussions were conducted and were attended by 16 non-CF and 15 CF farmers. However, in Ward 12 over 100 farmers were found assembled at the meeting point.

Field study

Farmers whose fields were monitored during the 2008/09 season were drawn from the sample of farmers that attended the focus group discussion in each ward. During the FGD, interested farmers were invited to participate in the study. From the group of interested non-CF farmers, farmers were selected based on the presence of field that had been under the same method of conventional tillage for a minimum of 5 years, farmer had not adopted CF on any of their fields in the previous seasons and field accessibility.
In both wards, the majority of non-CF farmers used mouldboard ploughing tillage with some farmers without draught animal power using traditional hand hoe tillage. Six non-CF farmers (three per ward) were selected and the fields that were monitored ranged in size from 0.1 to 0.6 ha. Most of the fields monitored were outer fields that in the past four years (2005-2008) had had a two-year maize-groundnut rotation The majority of farmers used the ox-drawn Zimplow® VS200 mouldboard plough to prepare fields in early summer with planting done using third furrow planting. The decision to use farmers who had not previously adopted CF before was based on the observation that farmers that adopted a technology usually changed some of their traditional practices to those used in the new technology (Romney et al., 2005; Pedzisa et al., 2010). This group is hereafter referred to as CONV tillage.

Above-ground weed flora

Weed composition and density were estimated from the 24 experimental fields from Ward 12 and 14 during the period from November 2008 to April 2009. All weeds that were present in each of the 8 quadrats that had been used for soil sampling in November 2008 were identified to species level and counted. Thereafter, one quadrat from each diagonal transect was marked out with tall pegs and maintained as a permanent quadrat for weed assessments throughout the 2008/09 cropping season. Originally, weeds were to be sampled before each hoe weeding operation but since timing of weeding varied with farmer this approach was found to be difficult to implement. Furthermore, during the second weed survey some CF farmers reported that they had delayed weeding their fields until after the weed counts had been done and thus, this approach was interfering with farmer weed management. To avoid this, farmers were advised to weed their fields except for the area under the permanent quadrats.

Tillage effect

There was no evidence of a decline in the total seedling density of the soil weed seed bank with years under PB as seed bank size did not differ (P > 0.05) between CONV and PB tillage systems. The upper 15 cm soil weed seed bank was estimated 422 seedlings m-2 for CONV tillage, 760 seedlings m-2 for PB3- and 655 seedling m-2 in PB3+. There was also no difference in weed seed distribution through the 15 cm soil layer between CONV and PB systems. In all tillage systems most weed seed was found in the upper 10 cm soil layer. Tillage, however, had a significant (P < 0.05) effect on weed seed bank diversity with a two-fold increase in the Shannon’s evenness and diversity indices recorded in the PB seed bank compared to that under CONV tillage (Table 5.3). This indicated an increase in weed diversity in CF systems relative to CONV tillage although the length of time a field had been under PB had no significant effect on seed bank diversity.

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Weed composition

Nineteen weed species were identified in maize fields during the 2008/09 season (Table 5.4) of which 15 species were also found in the soil seed bank (Table 5.2). However, B. pilosa, D. stramonium, Digitaria spp. and S. asiatiaca were not identified in seed bank while S. alba was not sampled in any of the fields during the maize growing period. The weed species S. asiatica was not present in the weed seed bank because there was no cereal host in the plastic pots used for seed bank enumeration. Most of the species that were not common to both seed bank and above-ground weed flora were usually identified in only one tillage system. Furthermore, the weed species had low RIVs showing that they were not generally important weed species in the study area. The ranking of the top five weed species based on RIV was influenced by tillage system. The top three weed species varied with tillage system but were found in all the tillage systems.

CONTENTS :

Title page
Declaration
Acknowledgements
Contents
List of Tables
List of Figures
List of abbreviations
Abstract
CHAPTER 1 GENERAL INTRODUCTION
- 1.1 Background
- 1.2 Rationale of study
- 1.3 Research questions
- 1.4 Outline of thesis
CHAPTER 2 LITERATURE REVIEW
- 2.1 Introduction
- 2.2 Smallholder agriculture in sub-Saharan Africa
- 2.2.1 Constraints to crop production
  - 2.2.2 Crop production in the semi-arid tropics
- 2.3 Conservation agriculture
  - 2.3.1 Principles of CA
  - 2.3.2 Benefits associated with CA
  - 2.3.3 Challenges to CA adoption
- 2.4 Weed dynamics under CA
  - 2.4.1 Tillage effect on weeds
  - 2.4.2 Crop residue mulching effects on weeds
  - 2.4.3 Weed response to diversified crop rotations
- 2.5. Weed management in CA
- 2.6 Weed management in smallholder agriculture in Zimbabwe
- 2.7 Conclusion
CHAPTER 3 CROP YIELD AND WEED GROWTH UNDER CONSERVATION
- AGRICULTURE IN SEMI-ARID ZIMBABWE
- 3.1 INTRODUCTION
- 3.2 MATERIALS AND METHODS
  - 3.2.1 Location
  - 3.2.2 Treatments and experimental layout
  - 3.2.3 Crop management
  - 3.2.4 Data collection
  - 3.2.5 Statistical analysis
- 3.3 RESULTS AND DISCUSSION
  - 3.3.1 Seasonal rainfall
  - 3.3.2 Weed density and biomass
  - 3.3.3 Crop performance
- 3.4 CONCLUSION
CHAPTER 4 RESPONSE OF WEED FLORA TO CONSERVATION AGRICULTURE SYSTEMS AND WEEDING INTENSITY IN SEMI-ARID ZIMBABWE
- 4.1 INTRODUCTION
- 4.2 MATERIALS AND METHODS
  - 4.2.1 Experimental design and crop management
  - 4.2.2 Data collection
  - 4.2.3 Statistical analysis
- 4.3 RESULTS AND DISCUSSION
  - 4.3.1 Seasonal rainfall
  - 4.3.2 General effects on weed species and density
  - 4.3.3 Specific weed densities
  - 4.3.4 Weed community diversity
- 4.4 CONCLUSION
CHAPTER 5 WEED COMPOSITION IN MAIZE (ZEA MAYS L.) FIELDS UNDER SMALLHOLDER CONSERVATION FARMING
- 5.1 INTRODUCTION
- 5.2 MATERIALS AND METHODS
- 5.2.1 Site description
  - 5.2.2 Focus group discussion
  - 5.2.3 Field study
  - 5.2.4 End of season farmer-feedback workshop
- 5.3 RESULTS AND DISCUSSION
  - 5.3.1 Seasonal rainfall
  - 5.3.2 Adoption of CF practices by farmers
  - 5.3.3 Weed dynamics
  - 5.3.4 Maize grain yield
  - 5.3.5 Farmer perceptions
- 5.4. Conclusion
CHAPTER 6 WEEDS IN COMPOST APPLIED IN SMALLHOLDER
- CONSERVATION FARMING
CHAPTER 7 GENERAL DISCUSSION
- REFERENCES
- APPENDICES