Effect of stand weediness and seeding date on seed yield of amaranth

In two field experiments on luvic chernozem (weak acid soil reaction, good content of potassium, and medium content of phosphorus and magnesium) in altitude of 176 m (48 degree 34 min. N, 17 degree 45 min. E), the effect of weeds on amaranth grain yield was studied. In the first experiment, there were compared the yields of four cultural varieties (A. hypochondriacus - varieties 1008, Koniz and selection of K-343, A. cruentus - variety Olpir) with the yield of local redroot pigweed (A. retroflexus) land race (date of sowing from the 15th to 20th May). In the second experiment, two varieties (A. cruentus - variety No-29 and A. hypochondriacus - variety K-343) were sown at three dates (the 15th April, 15th May and 15th June). Row spacing and seed rate in both experiments were 125 mm and one million of emerged seeds per hectare respectively. Weeding was done by hands in both experiments. The amaranth grain yields were in average more affected by some agrochemical characteristics of soil than by weather conditions of the year. On the other hand, the negative effect of weeds depended more on weather conditions of the year. Natural weediness decreased significantly the yields of all varieties with the exception of redroot pigweed local land race and variety Olpir. In the stands of these two genotypes, the lambs quarters presented more than half (55-79%) of total dry matter fytomass weight of weeds. In the stands of remaining varieties, more than half (52-71%) of total dry matter fytomass weight of weeds was produced by redroot pigweed. Variety 1008 A. hypochondriacus had the highest grain yield 3.85 t per ha in average and the variety Koniz the lowest one 2.31 t per ha. In conditions of the year with the lowest yields, redroot pigweed outyielded Koniz variety and reached the same level of yield as the remaining varieties. Seeding date influenced the yields of investigated varieties, expression of experimental year conditions, and weediness effect. The stands seeded on the 15th May reached the highest yields in two of three experimental years. The stands seeded on the 15th June had on the contrary the lowest yield every year. The stands seeded on the 15th April reached the same level of yield in average as those seeded on 15th May, but in two years they yielded significantly less, and on the other hand, one year they reached the highest yield.