Effect of winter cover crops on nutrition and weeds of maize
Sakala, A. (2019). Effect of winter cover crops on nutrition and weeds of maize (Thesis, Master of Science (Research) (MSc(Research))). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/12741
Permanent Research Commons link: https://hdl.handle.net/10289/12741
Cover crops are being used in New Zealand to manage soil nutrients and weeds and can have positive or negative effects on the growth of the following main crop. This study investigated the causes of lower yield of maize following a ryegrass cover crop compared to fallow or other cover crops at the Foundation for Arable Research Northern Crop Research site. Cover crops were sown in mixtures faba (Vicia faba L.), gland clover (Trifolium glanduliferum Boiss.) and rye (Lolium perenne L.) with fallow as a control and followed by maize. The effects on soil carbon to nitrogen ratios, and maize growth, yield and nutrition were monitored. Additional experiments in the laboratory and glasshouse investigated the effect of cover crops on weed seed germination. Winter cover crops affected nutrient dynamics during growth of the maize crop. The total soil carbon to nitrogen (CN) ratio decreased as the proportion of clover in the cover crop mix increased. In the fallow and clover treatments nutrients were available earlier for maize to grow rapidly, resulting in higher yields compared to the rye and rye faba treatments. In the rye treatment, the CN ratio had fallen and nutrients had become available to the crop by harvest time, resulting in lower maize tissue CN ratios and higher tissue nutrient contents. Rye had a clear effect on the CN ratio in the cover crop mix whether sown at full rate or half rate. Rye also appeared to suppress clover and faba in the mixed treatments, possibly reducing the potential for supplementation of nutrients through the nitrogen fixation process. A clover-based cover crop was better than a rye-based cover crop, probably because it resulted in a lower soil CN ratio and higher extractable soil nitrogen at the time of establishment of the main crop, and may therefore release nutrients faster for the benefit of the main crop. A laboratory experiment tested the inhibition effect of cover crop extracts on the germination of weed and crop species. Five gram aliquots of cover crop powder were soaked for 24 hours in 100 ml of water and the filtered extracts diluted before application to seeds of a range of test species. Cover crop extracts almost always inhibited seed germination, by up to approximately 40%, but the inhibitory effects on seed germination varied with the species of cover crop and the test species. Serial dilution of extracts indicated that there was a dose response effect in the level of germination suppression by the cover crop species. Clover had the largest potential for reducing the weed and crop species germination and therefore was the most promising cover crop for exhibiting a true allelopathic effect on weed seed germination. To further test the inhibition effect of clover and rye cover crops on germination of weed and crop species in the soil, root and shoot residues were tested on seeds of weed and crop species planted in pots. Root residues inhibited germination of volunteer and planted test species more than shoot residues, possibly because of a higher availability of inhibitory compounds, or because of differences in the time of setting up the root and shoot pots. The inhibitory effect on germination exerted by cover crop varied with the test species. This was shown by inhibition affecting one species but not others, which might suggest that some species were more sensitive to a particular cover crop treatment. Cover crop roots were more likely to affect germination, but cover crop shoots were more likely to reduce the growth of planted test species. The lack of a root effect on growth might be explained by loss or dilution of any allelopathic compounds that affected germination, or because their mode of action was specific to germination. Overall, clover root and shoot treatments have the potential to inhibit germination and growth of weed and crop species. The results support the hypothesis that clover either as a crop residue or as an extract has a higher potential to suppress germination of other species compared to rye and is the most promising cover crop for exhibiting a true allelopathic effect on germination of weed seeds. It was also shown that a clover-based winter cover crop might be better than a rye-based cover crop in terms of its effects on nutrient availability to the main crop. The results have implications for the decision to keep crop residues in the field to reduce the germination and growth of weeds. There may be an economic benefit from using cover crop shoots for other activities (e.g. silage or cut-and-carry forage) while still benefiting from the weed controlling effects of roots residues that are left in the field. Further experiments on the effects of winter cover crops for maize production in New Zealand are recommended.
The University of Waikato
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