This is Part 4 of a multi-part series on On-Farm Variety Trials. Part 3: “Designing a Screening Trial” can be found here.
Last month, I outlined the design of a screening trial. In this installment, I will discuss the design of a replicated variety trial.
In contrast to screening trials, replicated variety trials can answer more complex questions like yield differences or disease resistance because their design controls for the effects of field variation. This type of trial accomplishes this by repeating or “replicating” the experiment multiple times in different locations, ensuring the results hold across space. Because of the more robust design, replicated variety trials are able to evaluate traits that are likely to be influenced by environmental factors, detect subtle differences between varieties, and obtain much more reliable information in comparison with a screening trial.
A replicated variety trial has two main parts, “plots” and “blocks”. Each variety is planted in a “plot” and the complete set of varieties you are testing are arranged into “blocks” that are replicated a certain number of times, usually 3-5 times. Typically one plot of each trial variety and a check variety is planted within each block. The check variety should be one that you are familiar with or one that is industry standard and will serve as a point of comparison for the test varieties. Just as with the design of a screening trial, it is very important to randomly assign the order with which the varieties are assigned to each plot. This can be done using an online random number generator. Assign a number to each variety (e.g. 1-7 if there are seven varieties) and the random number generator will randomly pick the order for you. And, as with a screening trial, it is important to mitigate edge effects by planting a border around your trial area.
Your knowledge of the field is very important in choosing the layout of the blocks. The aim is to limit field variation to between blocks, rather than within blocks. The figure above illustrates how the blocks are oriented to keep the slope as uniform as possible within the blocks. Soil type can also have a great effect on plant outcomes and is another field variable that is important to keep in mind when determining block orientation.
Replicated variety trials are a robust way to achieve more complex goals but they come at the cost of more challenging management and investment of more time. When deciding whether or not to conduct this type of trial, make sure you consider the time commitment required to see it through. The same number of notes and measurements will be required of each block and can add up quickly. Keeping to a focused research question and the minimum number of necessary replications can help keep the time investment at a reasonable level.
In the next installment of this blog series on on-farm variety trials, we I will outline planting densities, plot layout, and management strategies. Until then, I hope you enjoy this beautiful fall weather!