Sample Size for Morphological Traits of Sunn Hemp

Sunn hemp (Crotalaria juncea L.) is an annual leguminous plant used for crop rotation, biomass formation, biological nitrogen fixation, and nematode control. To this end, sampling is an important tool owing to its advantages, such as low cost and fast data acquisition. This study aimed to determine the sample size required to estimate the mean morphological traits of sunn hemp and to assess sample size variability among traits, crop development stages, and sowing seasons. Two uniformity trials were performed in the field during the 2014/2015 harvest. Crops were sown in October and December on an experimental area of the Department of Crop Science at the Federal University of Santa Maria, Brazil. An area of 1,200 m2 was allocated for each trial. Each trial was divided into 2 m × 2 m grids, which formed 25 rows and 12 columns. One plant was marked per plot, totaling 300 plants in each trial. Leaf number and plant height were assessed weekly. Stem diameter and root length were measured at flowering. Normality and randomness tests were performed on each trait. The sample sizes (plants number) were calculated for the confidence interval half-widths (estimation errors) of the 2, 4, …, 20% mean estimates. There is variability of the sample size (plants number) between morphological traits, crop development stages, and sowing seasons. When choosing a single sample size to evaluate the morphological traits of sunn hemp, at least 70 plants should be sampled for an accuracy of 10% and a 95% confidence level.