Evaluation of Growth Indices of Potato (Solanum tuberosum L.) Affected by Density and Time of Emergence Pigweed (Amaranthus retroflexus L.)

Weed is one of limiting factors in potato production.Weed management systems require comprehensive and accurate data on behavior of weeds and their impacts on the farming systems. Pigweed (Amaranthus retroflexus L.) is third dicotyledonous weeds in the world that due to photosynthetic pathway of C4 and indeterminate, in high temperature and high light intensity conditions particularly in the fields of summer crops such as corn (Zea mays L.) and sunflower (Helianthus annus L.) shows more competitive. Weed density is the most important factor affecting on crop by weed competition and defines part of the decline in crop by weed competition. The emergence time of weed and competition period with crop are two important aspects that are often considered in competitive studies. Quantitative analysis of growth is a method for interpretation of plant response to environmental conditions that plant is encountered during its life. Plant growth analysis can be used as a factor to illustrate the competitiveness of the crop and weeds. Growth indices such as dry matter accumulation, crop growth rate (CGR), net assimilation rate (NAR), and leaf area index (LAI) have been used for assessing the effects of weed competition on crop yield. The aim of this study was the evaluation of growth indices of potato affected by density and time of emergence pigweed.
The experiment was conducted as a factorial based on randomized complete block design (RCBD) with two factors and three replications at the experimental station of Borujen city in Charmahal and Bakhtiari province, Iran. Treatments were included a combination of pigweed density (5, 10, 15 pigweed plants m-1) and time of emergence pigweed (four and eight days before potato emergence, coincided with the potato emergence, and four days after potato emergence). Pigweed evaluated traits were included as plant height, and total dry weight as well as potato evaluated traits were included as LAI, total dry matter (TDM), (CGR), relative growth rate (RGR), and tuber yield. Plot size was 15 m2 with four lines of 60 cm. Potato cv. was Bourne which is an early ripening cultivar was planted at 10 April. Irrigation was performed based on requirement of the plants, temperature and environmental conditions in 7 to 9 days. Potato tuber harvested at 20 June. Data were analyzed by SAS software and the mean data were compared according to least significant difference (LSD) Test at 5% probability level.
Results showed that the relative time of emergence and Pigweed plant density were significant on plant height and TDM of pigweed. Pigweed plant height and dry weight decreased, when pigweed emergence was later than potato. Pigweed plant height and TDM increased with increasing density of pigweed. Their interaction was significant only for potato tuber yield. Earlier relative emergence and increasing density of pigweed was reduced potato tuber yield. Five pigweed per m-2 at emergence of four days after potato and 15 pigweed per m-2 at emergence of eight days before potato had the highest (39800 kg ha-1) and the lowest (18400 kg ha-1) tuber yield, respectively. Also low density of pigweed in treatment of emergence at eight days before potato was caused more economic losses. Whatever emergence of weed delayed, more number of weed plants is needed to economic losses causes. Therefore emergence of weeds is important equally that weed density is important and should pay attention to the fact in Weed management. In general, the results showed that the pigweed can damage to potato production because its height is higher than potato and it also is C4, when relative appearance is earlier can be strong competitor to absorb light even in the less dense. Previous researches about crop-weed competition for light, have revealed that higher plant with lower LAI could overcome the plants with lower height and higher LAI. These findings showed that plant height have a main role in competition and height is one of the determinant factors for light competition.Therefore, it seems that pigweed which has this feature by itself, is a strong competitor for potato plant. This study concluded that because of that pigweed is C4 and its plant height is more than potato, so pigweed is superior competitor for light even in lower population.
Generally, according to the results it can be concluded that the relative earlier rise and increasing pigweed density through height gain and dry matter further, reduced tuber yield of potato. Furthermore, pigweed, due to a higher height than the potato as well as its C4 the if a relative early rise occurs, even at low density for absorb light, is still a strong competitor and can cause heavy damage to potato production. The relative earlier rise of this weed in potato fields can reduce the potato yield. After amaranth weed control in potato farming is very important and should be done as soon as possible.