Estimation of the optimal plot size and number of replications in a field pepper crop experiment with varying irrigation depths and application frequencies

Abstract

Abstract The precision of an agronomic field trial, or its ability to detect true treatment effects, can be improved by using both plots of suitable size and an appropriate number of replicates. The objective of this work was to determine the optimal size and number of replications of the experimental plots, by examining differences in plant height, stem diameter and number of fruits per plant, of a bell pepper (Capsicum annuum L.) crop that was irrigated with different water heights and frequencies of application. Five treatments were compared: T0, control with no irrigation; T1, water depth 50% of crop evapotranspiration (ETc) and daily frequency; T2, water depth 100% of ETc and daily frequency; T3, water depth 50% of ETc and frequency every two days; and T4, water depth 100% of ETc and frequency every two days. In the experimental plots, 4 m wide × 12 m long (48 m2), a total of 120 bell pepper (cultivar ‘Impacto’) plants were arranged in four rows. Plants in each row were irrigated with a drip irrigation system with drippers placed between them 0.2 m apart and having a flow rate of 0.8 L h−1. The growth rates of the pepper crop and the number of fruits per standing plant were lower in the treatment without irrigation than in the irrigated treatments. Among the irrigated treatments, daily irrigation provided higher growth rates when the lowest depth of water was applied. In the plants irrigated every two days the highest growth rates were observed in those that received the largest water depth. The number of fruits per standing plant was greater in the daily irrigated plants than in those irrigated every two days, regardless of the amount of the water applied. The optimal plot size was determined by the modified maximum curvature method (MMC) and the linear model with plateau effect (LPR). For each of the three variables, the criterion followed to establish the optimal plot size was to choose the largest one, regardless of whether the method used in its determination was MMC or LPR. The size of the experimental plot was always smaller in the case of treatments with irrigation than in the treatment without irrigation, as well as for the crop variables exhibiting lower variability. Therefore, the optimal sizes of the experimental plots for the plant stem height and diameter, and for the number of fruits per plant was estimated at 3.2 m2 (8 plants); 3.6 m2 (9 plants); and 10.8 m2 (27 plants), respectively. Regardless of the experimental design, complete randomized or complete randomized block, the number of replications of the experimental plots was conditioned by the variability of the three variables. Thus, 10 replicates were required to detect between the five irrigation treatments differences of 10% in the mean value of plants height, 5 replicates in the case of the stem diameter of the plants and 15 replicates for the number of fruits per plant.