Comparision the Effectiveness of Sampling Methods, the Shape and Size of Quadrat for Estimating Weed Population in Corn Fields under Different Irrigation Systems

In weed management planning, the most important step is to know the weed population, which starts with sampling. In order to investigate the effectiveness of random, zigzag and diagonal sampling methods with 28, 29, and 28 sampling units, respectively, the shape (square and rectangle) and the size of the square (0.25 square meters and one square meter) for estimating the weed population of corn fields under drip, Furrow, and sprinkler irrigation systems, a research was conducted in 2020 in six corn (Zea mays L.) fields. In order to estimate the accuracy of the sampling methods, first, the studied area was gridded with the standard grid sampling method of 5 m × 5 m (171 sampling points) and at the intersection of the lines using a square quadrat (100 × 100 cm), an area equal to one square meter. The data related to the species density and diversity of weed seedlings by species were recorded at the four-leaf stage of corn and before herbicide application in one stage. The results showed that regardless of the quadrat shape and sampling methods, one square meter quadrats had a better estimate of species diversity and weed population density in corn fields under three irrigation systems. But when only weed density is considered, the use of a rectangular square of 0.25 m2 (100 × 25 cm) was able to estimate weed density with acceptable accuracy. Due to the different pattern of soil wetting in different irrigation systems and consequently the appearance of weeds and the way of navigation in each sampling method, the sampling methods showed different estimates of the population density of weeds; so that the diagonal sampling method in the drip irrigation system and the sampling methods Zigzag and random in sprinkler and furrow irrigation systems showed less error compared to standard grid sampling method. Meanwhile, in the investigation of single weed species, the diagonal systematic sampling method in all three irrigation systems showed the least error in the accuracy of the density estimation of the studied species compared to the standard grid method.