Abstract
2.4 Discussion
In non-fungicide treated maize, there were significant increases in grain yield with increased N applications, despite increased leaf blighting. However, in the non- fungicide treated K maize, potential grain yields srer-e not achieved as can be seen from the increased losses in grain yield between fungicide and non-fungicide treatments with increasing K rates. Grain yield losses increased from 1.4 -3.15 tonnes ha-1 in 1996/97 and from 1.57-3.56 tonnes ha^iin 1997/98. This loss in grain yield between fungicide treated and non-fungicide treated maize was not reflected with increasing N levels, possibly because of the reteirfion and build up of N on the clay soils of the trial site.
In the non-fungicide treated maize in the Ahrens tiriaB, there was a significant increase in grain yield with increasing N, P and K applications T his was probably due to the fact that at zero and low application rates of N and K, tfi e soil was more severely depleted of these nutrients from the long-term trial at Ahrenstthan at Cedara. In addition, levels of final percentage leaf blighting at 146 DAP wer«e< 10%, which probably had little effect on grain yield with increasing N, P and K application rates.
In fungicide treated maize, highest grain yields weir« -obtained using 120 kg N ha A and 150 kg K ha"1, and in non-fungicide treated rnaice, 60 kg N ha'1.and 50 kg K na'1. Highest added gross margins (relative to minimum gross margins) were also obtained using these fertilizer applications (see Chapter 3).
Analysis of dry matter in pig, chicken, cattle (kraal)*, horse and sheep manure show that they contain 1-3% N and 0-3 % K (CADI -unpublished data). Therefore, a small-scale farmer would have to transport and apply 1 -3 tonnes of manure to substitute for 60 kg inorganic N and 50 kg inorganic K ha"1. This would toe an impractical and uneconomical exercise.
In the absence of high N and K applications, the relative impact of GLS is minimized, e.g., in small-scale farming systems, maize may produce grain yields higher than expected as"the plants reach physiological maturity "without significant foliar blighting, and resultant grain yield losses.
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Grain yield alone should not be used to justify the use of fertilizers for increased grain yields. Grain yield losses from increased GLS result in reduced grain yield potential.
More important are the economic implications of the extra costs of added fertilizers and the grain yield related to disease.
The significant positive N:K interaction of SAUDPC in the fungicide treated and non- fungicide treated plots in 1997/98 illustrates the compounding influence of high levels of fertilization on the incidence of disease and the associated increase in grain yield loss between fungicide treated and non-fungicide treated maize.
These results have important implications for farmers. For commercial farmers who are financially able to spray for control of GLS, increased applications of N result in increased yields and gross margins. However, for small-scale farmers, who are financially unable to embark on costly fungicide control programmes, increased N fertilization results in increased GLS severity and consequent grain yield losses. Best yields and gross margins are obtained using 60 kg N ha'1 and 50 kg K ha"1.
Maize is produced on millions of hectares in Southern Africa, much of which is subject to GLS incidence. Hybrids less susceptible to GLS, rotational cropping, cultural control methods and the use of fungicides to control GLS are economically viable for commercial farmers in RSA (Ward et a/.,1993; Ward andNowell, 1997). In the present economic climate for commercial farmers, the price-cost squeeze tends to reduce the profitability of maize production, and to increase the break-even yield level. A small yield loss for commercial farmers may result in reduced gross margins but could also determine whether maize production is economically viable. However, for the small - scale farmer, fungicide control may not be an option because they do not have access or finances to buy and maintain fungicide equipment, and often do not have the knowledge of the use of chemical sprays and maintenance of equipment. They will have to rely on using less susceptible hybrids, cultural control and rotational cropping as a small loss in yield could result in a family food shortage.
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Cercospora zeae-maydis is considered to be a high sugar disease. This is possibly the reason why GLS is not considered to be a major problem where maize is grown in nutrient deficient soils in RSA. In poorly fertilized crops in Cameroon, Kenya, RSA and Zimbabwe, the severity of GLS was generally low and disease development was slow (Nowell, 1997).
Reducing nutrient levels has been suggested as a control measure for many diseases, but control at the cost of grain yield and gross margins is unacceptable. Increased N and K fertilization does increase the susceptibility of maize to GLS. This must be taken into account when encouraging the use of fertilizers for small-scale farmers to increase production if increased disease offsets the responses expected from increased fertilizer application. It also follows that the more progressive small-scale farmers who are able to fertilize their lands will have a greater problem with GLS, and will need to take precautions against disease to realise maximum grain yields and gross margins.
As it was important to ensure that the cultivars most susceptible to GLS, e.g., ZS 206, were affected by soil inorganic fertilizers, the results presented from this trial represent the worst case scenario. Future research needs to investigate the effects of N, P and K on maize hybrids grouped into the three categories of susceptibility to GLS, i.e., highly susceptible, moderately susceptible and resistant (Ward et al., 1999). From this trial in the southern hemisphere and Smith's trial (1989) in the northern hemisphere, we propose that N and K will increase GLS blighting and that P will have little effect in other highly susceptible and moderately susceptible hybrids but that the magnitude of increased GLS will decrease with hybrids_showing more resistance to GLS.
It is noted that in this trial, the primary effect of fungicides was to control GLS. There may have been secondary effects from fungicide applications, e.g., control of other maize pathogens and other physiological effects, e.g., hormonal effects. In addition,
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root morphology of different hybrids, the effect of fungicides on root and stalk health relative to nutrient uptake, translocation and utilization may prevent general organic soil fertilizer recommendations to be made. However, this research does provide a framework for further investigation into the effects of soil organic and inorganic nutrients on GLS and other foliar fungal pathogens of maize.
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