The effect of weed infestation on alfalfa yield without cover crop in first cutting

In field experiments on degraded chernozem, 167 m above sea level, with long-term average of annual temperatures 9.2 degrees C and 15.5 degrees C during vegetation period, with average annual precipitation 593 mm, the effects of weeds number and weight on alfalfa yield have been studied (Palava variety). Besides forage yield also weeds effects on the stand density and height as well as alfalfa root weight have been studied. In weeds of the highest abundance, Echinochloa crus-gali, Chenopodium album, and Polygonum persicaria the effects of their number and weight on the yield, stand height and density have been studied, too. The yield of alfalfa without weeds has been analyzed. The highest number of weeds (152.m(-2)) was found in 1989, the lowest one (20.m(-2)) in 1988. The weight of weeds was highest in 1987 (287 g.m(-2)) and the lowest one in 1988 (119 g.m(-2)) (Tab. I). The highest alfalfa yield was obtained in 1988 (2.69 t.ha(-1)) and the lowest in 1987. Only in 1988, one of three experimental years, weeds did not affect the forage yield (Tab. II). The yield was affected highly significantly negatively by weeds number and weight, and on the contrary, stand density (stems number per 1 m(2)) and alfalfa root weight exerted a positive effect. In accordance with our expectations, weeds number correlated highly significantly with their weight and alfalfa stand density with alfalfa root weight. Negative relation between alfalfa height and alfalfa root weight results probably from preferential root growth and development in the year of alfalfa sowing. Positive correlation between weeds number and alfalfa height represents obviously the reaction of alfalfa to the certain level of weed competition. Weeds number and weight influenced alfalfa yield in a different way (Tab. IV). The effect of weeds number was mainly direct, but the effect of weeds weight was obviously indirect, especially through their number and reduction of alfalfa stand density. From the analyzed yield-forming elements in alfalfa, density influenced the yield directly and root weight indirectly through the stand density and weeds number. Seemingly illogical indirect positive effect of root weight through the number of weeds consists probably in the fact that at the same weeds weight, a higher number of weeds can result in areal reduction of adverse effect. Out of the three most frequent weeds (Tab. V), Polygonum persicaria achieved the highest number of plants, 47 per 1 m(2), as well as the highest weight, 194 g.m(-2). On the basis of path analysis it can be supposed that Echinochloa crus-galli had the highest competition ability (Tab. VI). With as few as 3 plants per 1 m(2) and 22 g.m(-2) of dry matter produced by this weed had a negative effect on alfalfa yield. Also Chenopodium album competed with alfalfa significantly, 9 plants per 1 m(2) affected negatively alfalfa yield during two years, when dry matter weight of 163 g.m(-2) did not reach a significant effect one year, while in another year as few as 39 g.m(-2) reduced the yield highly significantly (Tab. VII). These facts confirm the difficulty in determination of harmfulness threshold and at the same time they indicate that weeds number can play more decisive role in this respect. Between the three mentioned weeds a different level of correlations could be seen. In Chenopodium album and Polygonum persicaria the highest level of mutually supporting effect can be supposed. Mutually supporting effect, on the contrary, was the lowest, or was absent between Echinochloa crus-galli and other weeds.