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Maize emergence
Despite previous reports that plant residues on the soil surface reduce crop emergence through mechanical resistance, reduced light reaching the soil surface, and interference with heat and water transfer between the soil and atmosphere (Teasdale & Mohler 2000; Teasdale et al., 2007), contrasting results were obtained from the present study. As maize planting furrows were effectively devoid of residues because of the furrows being drawn with a v-shaped hoe, growth inhibition of maize could not have been due to a physical constraint contributed by the residues.
If, hypothetically, residues were present in the planting furrows, light should not have been a limitation to maize seedling emergence. Crops with big seeds seem to be less affected by the presence of residues than small seeds, because of the relatively large amount of resources available in the former (Putnam et al., 1983; Teasdale 1993). Due to the relative large size of maize seeds enough resources should exist within the seed in order for coleoptiles to have emerged unimpeded through the cover crop residues in the present study. The reduction in emergence percentages in the residue treatments could not have been the result of nutrient imbalances, as the emerging seedling is totally dependent on seed reserves and thus not yet influenced by the nutrient status of the soil (Purvis 1990).
The optimum mean daily temperature for maize to germinate is between 18 and 20°C with growth being inhibited at temperatures below 10°C or above 30°C (Smith 1991). Although soil temperature was not measured in the experiment, it is possible that fluctuations in soil temperature over the four seasons could have contributed to some of the reductions recorded for crop emergence as soil temperature could have been lower under the residue, especially with the lower air temperatures in 2005 and 2007 accompanied by higher rainfall. Kravchenko and Thelen (2007) found that the lower soil temperatures under wheat shoot and root residues decreased maize emergence more compared to plots with no wheat residues. Teasdale and Mohler (1993) suggested that a delay in germination could be expected with lower soil temperatures under cover crop residues.
Differences in soil moisture were not responsible for the differential emergence, as lower soil moisture values were measured in the non-residue treatments, yet emergence was not suppressed in these treatments.
In this study it was the type of residue, rather than the amount thereof, that impaired maize seedling emergence. Burgos and Talbert (1996b) reported similar results when the number of southern pea (Vigna unguiculata) plants were reduced in annual ryegrass residues, despite the latter crop’s residues having had a similar amount of biomass to oats and a lower amount of biomass compared to sorghum-sudangrass (Sorghum bicolor x Sorghum vulgare var. sudanense).
Woodland species emergence was significantly reduced under grass residues compared to woodland residues (Donath & Eckstein 2008), while pasture species proved to be more restrictive to crop establishment than cereal grains (Weston 1990).
Investigating the effect of cover crop residues on crop and weed emergence revealed the involvement of putative allelochemicals with benzoxazinones and various phenolic compounds previously identified in stooling rye (Wójcik-Wojtkowiak et al., 1990; Sicker et al., 2004; Belz 2004). Allelochemicals are released from plants through leaching, decomposition, volatilization and root exudation (Belz 2004) and the effect is concentration dependant. The decomposition rate, leaching of water-soluble allelochemicals and the available concentration under field conditions and the prevailing temperatures, which can vary from year to year, as well as on the soil microbial activity (Purvis 1990; Facelli & Pickett 1991). Significantly more cover crop biomass was produced in 2003 compared to 2006, yet there were no significant differences in emergence between the two seasons. After spraying the cover crops in 2003, warm and dry conditions prevailed, rendering decomposition of residues possible, but limiting the leaching of putative allelochemicals. Leaching of allelochemicals into the root zone conceivably was further reduced by the low rainfall received (29.00 mm) during the maize germination and emergence period. Similar temperatures but more rainfall occurred in 2006 during the period between killing the cover crops and maize planting making the leaching of potential allelochemicals possible.
However, only 13.20 mm of rainfall fell during the emergence period, limiting the absorption of allelochemicals which could explain the similarity in emergence percentages.
Relatively similar amounts of cover crop residues were left on the soil in 2005 and 2006, but significantly more maize seedlings emerged in 2006, compared to 2005. Warm, moist climatic conditions prevailed during the decomposition period in 2005 and 2006, probably increasing the decomposition of residues and the availability of allelochemicals. Warm and dry conditions occurred during germination and emergence in 2006, which might have reduced the availability of allelochemicals and thereby reducing the possibility of a reduction in maize emergence. In contrast, conditions in 2005 were cool and moist, which could have exposed the emerging seedlings to stressful conditions and putative allelochemicals, resulting in a significant reduction in the number of seedlings that emerged.
Maize growth
In the present study, the possibility that differences in soil water content were responsible for growth differences is small, as seedlings growing in the nonresidue treatments had higher dry weights compared to the residue treatments in spite of the former having generally lower soil moisture levels. The soil water moisture levels between the weeds and annual ryegrass treatments were similar, but maize seedlings were less suppressed by the weed residues than by residues of annual ryegrass. Maize seedling growth could have been reduced by possible lower soil temperatures due to the presence of residues on the soil surface. The soil temperature under the weed residues could have possibly been higher than the cover crop residues due to the lower amount of biomass present. Both cover crop species had higher amounts of biomass present and, due to their slow decomposition residues would have been present for a longer period (Reddy 2001; Fourie et al., 2001) reducing the maize growth for longer.
The residues in the current study were not incorporated and additional N was applied at planting, thereby reducing the probability that N immobilization could have suppressed growth. According to Kuo and Jellum (2002), the growth of the main crop is mainly dependant on the available N and subsequent uptake and less on the cover crop species. N mineralization is dependent on soil moisture, temperature, soil pH, the amount of available N in the soil and the C:N ratio of the residues (Kuo & Jellum 2002). The C:N ratio of cereals is mostly dependant on the time of desiccation. If killing the cover crops occurs at a late growth stage, the material would contain more carbon and the ratio could exceed 30:1, which is higher than 25:1, at which stage N immobilization would occur (Reeves 1994). In addition, N immobilization is generally greater if the residues are incorporated (Smith & Sharpley 1990). Applying N at the beginning of the growth of the main crop can reduce the initial N deficiency (Hairston et al., 1987; Reeves et al., 1990).
INTRODUCTION
1. References
CHAPTER 1 Literature Review
1. Weed management
2. Conservation tillage
3. Cyperus esculentus
4. Cover crops
5. References
CHAPTER 2 Influence of cover crops Secale cereale and Lolium multiflorum on the growth of Zea mays and Cyperus esculentus under field conditions
List of tables and figures
1. Introduction
2. Materials and methods
3. Results
4. Discussion
5. Conclusion
6. References
Appendix A Statistical analyses
CHAPTER 3 Influence of cover crops Avena sativa, Secale cereale and three cultivars of Lolium multiflorum on the growth of Zea mays and Cyperus esculentus under controlled conditions
List of tables and figures
1. Introduction
2. Materials and methods
3. Results
4. Discussion
5. Conclusion
6. References
Appendix B Statistical analyses
SUMMARY
1. References
