Increasing Plot Length Reduces Experimental Error of On-Farm Tests

Abstract

Farmer conducted on-farm research is an effective tool for development of crop management practices. The randomized complete-block experimental design is being used in on-farm tests, with blocks consisting of two or more long, narrow, side-by-side plots. This study examined the relationship between plot length and experimental error, and assesses the probable statistical outcome of on-farm tests performed in the dryland region of the Pacific Northwest, USA. Fourteen trials were conducted in wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) fields to measure yield variance of combine-width plots ranging in length from 250 to 1500 ft. The relationship between plot length and variance for each site followed a logarithmic decay model (average r2 = 0.88). Variance declined rapidly as plot length increased from 250 to 750 ft at most sites. Averaging the ten least variable sites, the LSD (0.05) with three degrees of freedom declined from 6.5 to 2.6 bu/acre as plot length increased from 250 to 1500 ft. At the same 10 sites, power for mean separation (α = 0.05) of treatments with 4 bu/acre true difference was >0.80 with six replications and 750 ft plot length, or four replications and 1250 ft plot length. With adequate replication and plot length, on-farm tests can be designed for highly variable dryland regions with good control of experimental error. Research Question The use of side-by-side, combine-width experimental plots in farmer-conducted on-farm tests is increasing. Information on how plot length affects the success of these tests is lacking. It is also desirable to know what level of precision to expect from on-farm tests performed in highly variable dryland cereal production areas. This study investigated the relationship between the length of combine-wide, side-by-side plots and experimental error under the dryland grain production conditions of the Pacific Northwest. Literature Summary Researchers studying the performance of on-farm tests have found that the randomized complete block design used in the Midwest produces results comparable in precision to research station small plot experiments. These designs use 1200 to 1300 ft long plots 20 to 40 ft wide, and primarily involve corn or soybean. The performance of on-farm tests under dryland small grain production conditions has not been evaluated. There has also been little research to date that can be used to recommend minimum, maximum, or optimum plot lengths. Study Description Fourteen uniformity trials were harvested in commercial wheat and barley fields in Washington, Idaho, and Oregon. A uniformity trial measures natural variability between plots by harvesting plots where no treatments have been placed. Side-by-side strips 1500 ft long were harvested in 250 ft segments to allow recombination of the data into plots of different lengths. The grain yield data (bu/acre) were analyzed to determine variance between pairs of side-by-side plots. Ten of fourteen sites had a variance of <5 at plot lengths of 1500 ft, and were classified as low variance. The remaining four sites ranged from 6 to 22 (high variance). Figure 1 shows least significant differences at α = 0.05 (LSD 0.05) for an individual on-farm test with two treatments and four replications based on average variances for the low and high variance groups. Applied Questions How long should on-farm test plots be? Figure 1Open in figure viewerPowerPoint The effect of increasing plot length on LSD values at α = 0.05 is shown for experiments with different numbers of replications (reps). (A) is based upon data from the 10 fields with low variance and (B) from the remaining four fields with high variance. Note that the scales on the vertical axes differ. In most fields there was a large decrease in variance as plot length increased. Therefore, on-farm tests will produce more reliable results as plot length increases from 250 to 750 ft or more. In some very uniform fields even short plots will have acceptably low variability, but in every field measured, variability decreased as plot length increased to 1500 ft. To ensure the best results, we recommend that plots be as long as is practical. How does replication affect precision? The variability encountered in field experiments makes replication the key to a successful test. Figure 1 shows how LSD (0.05) decreases when replications are added. A low LSD is important because it allows detection of smaller differences between the performance of the treatments, or if there is no difference, it allows a high confidence that the treatments do not perform differently. On-farm tests can provide valuable information to farmers and researchers. Small differences can be detected with a high degree of confidence in most fields with four or more replications of 1000 ft or longer side-by-side plots.