The additive method comprises the increase in the number of weeds, keeping constant the number of crop plants, and the analysis of the variables is given by regression. The substitutive method, in contrast, provides that the final population is constant being a given plant substituted by other of the opposite species when it disappears.For the substitute method, pre-testings are necessary for all species involved. With the pre-test, the objective is to determine the minimum density of each species from which the mass per unit area increase is negligible even with an increase in planting density . Each study method is different in its essence, where the additive experiments aim to investigate the impact on increasing the density of the weed compared to crop plants when the latter is not sown in density which gives the maximum competitive potential,cannabis grow system which is similar to the observed in commercial crop fields.
Therefore, it is proposed that results from additive experiments tend to be more practical. The replacement series, in contrast, aim to compare theoretical situations not observed in the field, where both crop and weed are planted in densities which confer them maximum competitive ability. We hypothesize that this method may be more suitable for studies of competition for morpho-physiological and environmental resources when a complete understanding of the competitive process is required. Given the above, this study aims to evaluate the inter-specific competition between plants of soybean and corn competing with the weeds Bidens pilosa and Digitaria insularis by using and comparing two different study methods: additive and replacement series experiments. Four experiments were conducted by the substitutive method, under completely randomized design with four replications. The experimental units were kept equidistant, so that the surface area available for the development of the plants corresponded to the area of the experimental unit composed of plastic pots with a volume of 10 liters of soil. Seeds of beggar tick or sourgrass were randomly allocated in the vases, according to the treatments, being enough to establish at least 20 and 60 plants per vase, respectively.
Thereafter, soybean or corn were planted, according to the treatments , aiming to establish at least 40 and 16 plants per pot, respectively , according to the results of the pretests . Ten days after emergence, thinning was accomplished being kept the number of crop plants and weeds per pot according to the treatments .Fifty days after emergence, all plants of each experimental unit were cut close to the ground, separated according to the species ,marijuana grow system packed in paper bags and placed into an oven with forced air circulation at 65˚C, until constant mass for subsequent measurement of total dry mass. Dry mass accumulation for soybean when competing with beggar tick shows that when both co-emerge, there is reduction in dry mass of crop plants of the order of 30%. This reduction in dry mass of plants may, especially under unfavorable conditions, reflect on crop productivity. Drastic reduction was observed when a single soybean plant competed with two or more plants of beggar ticks ; considering the densities of normal soybean plants in the field , it can be proposed that plant densities of beggar tick equal to or greater than 20 plants∙m−2 , concomitantly emerged with the crop, can cause severe damage to yields. Beggartick caused damage to the development and dry mass accumulation of corn, linearly to the increase in density, and for each additional beggar tick plant per square meter emerged concomitantly to the crop, each corn plant reduced accumulation of 2.63 g of dry mass .
This level of infestation is commonly observed in non-GM soybean crops because of the difficulty in controlling beggar tick, especially the resistant biotype to ALS inhibitors that occurs in much of the state of Mato Grosso do Sul. Sourgrass, on the other hand, caused virtually no damage to the development of soybean when emergence is concomitant, since there are no limiting factors for the production system. Crops have been improved and selected to respond to higher levels of factors such as the availability of nutrients and water, and its lack causes more damage to crops compared to weeds Another study did not find any correlation between the dry mass of different soybean genotypes and their productivity, although this relationship is usually significant when considering the same plant genotype, being more accurate when dry mass is assessed closer to harvest .