Field research: Replicated comparisons vs. side-by-side comparisons

Producers are interested in knowing what works best, yields the most, and especially what is most profitable during these tight economic times. Some may want to compare products or practices on their own farm or look at information from other farms or industry studies.

How should a basic study be set up or laid out in the field? One very common approach is to divide a field in half and compare the halves, or possibly compare two fields in close proximity, and see which variety or practice yields highest. This approach can produce very misleading results because of the variability that exists across a field or fields due to many factors. Some sources of variability include: variations in soil type, topography, varying management practices, drainage, pesticide residues, disease pressure, compaction, and weather events. Just as you can count on yield monitor results to change across a field, you can essentially count on sources of variability within a field or between fields that would impact study results if you just split the field in half or compared fields across the road from each other.

A better approach, which provides a better estimation of future performance of a treatment you want to test, is to put out replicated studies with random placement of treatments in each replication. This simply means that the same treatment is put out at more than one place across the area of study to be assured that treatment performance is not based on location in the field. Using three to six replications is common in most agricultural studies. The more replications, the more reliable the results will be in a given comparison. Repeating the replicated comparisons for more than one year is also a good idea to test performance over more than one environment. This will allow you to come to stronger conclusions and better estimations of real differences between treatments.

As an example, an on-farm trial completed in 2016 is described below, showing how replication affected the results. This study compared two systems commonly used in planting pinto beans in Nebraska. The treatments were applied and replicated six times with random placement. One treatment was pinto beans planted in 30-inch rows at a population of 90,000 plants per acre; the second treatment was pinto beans planted in 7.5-inch rows at 120,000 plants per acre (Figure 1). This was a large field trial with each treatment being 60 feet wide by 1,400 feet long. The randomization was as follows:

Rep 1 Rep 2 Rep 3 Rep 4 Rep 5 Rep 6 7.5” 30” 30” 7.5” 7.5” 30” 7.5” 30” 30” 7.5” 30” 7.5”

Figure 1. Left, 30-inch rows at 90,000 plants/acre; right, 7.5-inch rows at 120,000 plants/acre.

The average yields from the treatments in the six replications were as follows: 7.5-inch with 120,000 population yielded 52 bu/acre and the 30-inch treatment with 90,000 population yielded 44 bu/acre. The 7.5-inch treatment yielded 8 bu/acre more than the 30-inch treatment. Having analyzed the yield data statistically (at the 0.05 probability level), yields were significantly different, with the least significant difference being 2 bu/acre. This means that due to variability within the study, a yield difference of less than 2 bu/acre would not indicate any treatment differences.

During early August a hail storm damaged the field, with the most significant damage occurring on the half of the field containing replications 4, 5 and 6. If the field had just been split with one treatment on each side, results would have looked different.

If we lump the 7.5-inch treatments from the hailed side of the field together we would find an average yield of 49 bu/acre. In comparison, if we lumped the 30-inch treatments together on the side with minimal hail, average yield for this treatment would have equaled 45 bu/acre. This equals a difference between treatments of 4 bu/ac (half the difference that was detected by the full, replicated trial).

Conversely, if we had the 30-inch treatments on the side of the field that received the most hail, yield for this treatment would have been 43 bu/acre and yield for the 7.5-inch treatment on the side receiving minimal hail would have equaled 54 bu/acre, for a difference of 11 bu/acre (Figure 2).

It is clear that when the six replications were spread out across the field we found a more accurate estimation of the impact of these systems on yield than splitting the field in half. In all three layouts the 7.5-inch treatment yielded the most. The split field design either exaggerated or diminished the yield advantage of the 7.5-inch treatment, depending on which treatment was exposed to the heavier hail damage (Figure 2). Poorly laid out field studies can generate misleading data and can lead to incorrect conclusions. Also keep this in mind when you are looking at data from other studies you encounter. In our modern era with GPS guidance, it is relatively easy to put in replicated, randomized studies, even on large field-scale comparisons.

Figure 2. Change in yield advantage of the 7.5-inch treatment as compared in split field layout vs. a replicated randomized field layout. An early August hail storm had greater damage on one half of the field. Like treatments were lumped together on the hailed half vs. the lightly hailed half to get the above yield averages in the split field comparisons.

John Thomas, Cropping Systems Extension Educator, University of Nebraska-Lincoln

thomas2@unl.edu

Sara Berg, Extension Agronomy Field Specialist, South Dakota State University

sara.berg@sdstate.edu

Josh Coltrain, Wildcat District Crops and Soils Agent, K-State Research and Extension

jcoltrain@ksu.edu

Lizabeth Stahl, Meeker County Extension Educator, Crops, University of Minnesota

stah0012@umn.edu