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included in the trial, such as might be used in an exploratory trial in a research domain, 6 environments is an adequate number. For 4 treatments, 10 environments would be required, and in a verification trial with only two treatments (the recommended treatment and the farmer check, for example) 23 environments is adequate. These suggestions, of course, are approximate. The appropriate number of environments is a function of the variance and the required sensitivity all case-by-case situations.
Numbers of years
Experience has indicated that if three conditions are met, the estimates of environment by treatment response stabilize in one year. These conditions are:
1. The range of environmental indices (El) should be at least as great as the mean of the
indices.
2. The range of environmental indices should approximate what would normally be
expected over a period of years.
3. The distribution of environments should be reasonably uniform from good to poor.
However, it should be remembered that at least two years of data will be available for estimates if both an exploratory trial (in a research domain) and a validation trial (in a recommendation domain) are carried out prior to making firm recommendations. Also, preliminary data often are available from on-station trials, conducted over previous years, as the technology is being developedo._The treatmentsLthat.arecommon -from. among these current and previous trials can be combined in a single MSA. The data from previous years can also help to verify whether the range of environments included in a current trial is adequate.
RECENT EXAMPLE
Singh (1990) reports on recent research conducted near Manaus, Brazil, that illustrates
many of these concepts. The on-farm portion of his research was conducted in two small farming communities in the municipality of Rio Preto da Eva, Amazonas, Brazil, where the government was initiating a small watershed management program. The Brazilian national agricultural research institution (EMBRAPA) has a mandate to develop appropriate technology for different farming conditions in this relatively inaccessible area. Also collaborating in the research were EMATER (extension) and SEPA, the state development planning entity, TROPSOILS, and the University of Florida.
Secondary information regarding indigenous farming practices of the area .were collected from published sources. A-rapid appraisal of the area, was conducted with a multidisciplinary team of persons from EMBRAPA, SEPA and EMATER who, visited the area on three different occasions. Farmers' knowledge of indigenous technology, agronomic practices, and land types being used were recorded. An extensive soil sampling program was carried out to understand soil physical and chemical characteristics and relate them to farmers' rationale for assigning a particular cropping pattern to a given land type.
Three treatments, based on previous on-station research, were selected for comparison with farmers' practices (FP) for growing maize (Za nays L) and cowpea (Vina .nguicata Only results from the cowpea are reported here. All three treatments with amendments received K (60 kg ha'- broadcast.. -Processed city waste (PCW), chicken manure (CM)-and triple super