Abstract:
Yield reductions due to competition of weeds with crop plants
can be extremely important. Early weed competition can be as important
in reducing crop yield as weed competition for the entire
season.
The objectives of this study were (1) to determine the weed
control achieved by narrow row-cropping patterns, (2) to measure
the competitive effect achieved with narrow row-cropping patterns,
with low photosynthetic capacity crop plants or with high photosynthetic
capacity plants, in competition with high photosynthetic capacity
weeds, (3) to determine at what time weed control becomes important
and (4) to determine if the time at which competition begins
could be determined by change in weight, sugar content, and/or
nitrogen content of the commercial crop. Also, two greenhouse
studies were made to evaluate the differential uptake of nitrogen by
two competiting species, corn (Zea mays L.) and pigweed (Amaranthus
rectroflexus L.), and to evaluate the effect of competition for
soil and light between two broadleaf species, snap beans and pigweed.
Yields of bush snap beans (Phaseolus vulgaris L.), corn (Zea
mays L.), and onion (Allium cepa L.) were all decreased significantly
by the presence of weed competition throughout the entire
growing season. Reduction in total area occupied by an individual
crop plant resulted in decreased effects on vegetative and reproductive
parts due to weed competition and increased the effect of crop
plant-to-plant competition. Early weed control was important in all
crops. In 1969, yield reductions in corn and beans allowed, to compete
with weeds for a period of five to six weeks after crop emergence
were equal to those from plots with weed competition for the
entire season. In 1970, corn required two weeks of cultivation after
emergence and snap beans required three weeks of cultivation after
emergence to decrease losses due to weed competition. Weed weight
yields were significantly reduced in snap beans at the 5 x 5 inch
spacing when compared to the 15 x 5 and 35 x 5 inch spacings.
As measured by leaf fresh weight, competition was determined
to begin between snap beans and redroot pigweed at 36 to 44 days
after crop emergence at crop plant spacings of 35 x 5 and 15 x 5
inches. Significant weed competition did not develop after 51 days
at crop plant spacings of 5 x 5 inches. Competition between corn and
redroot developed 41 days after emergence for all plant spacings.
This study indicated the time at which competition between
crop and weeds could be determined by leaf fresh weight measurements
or by measuring the leaf area of the crop plants.
At a fertilizer rate of 77-101-64 pounds of N-P-K per acre,
corn made greater gains in total green weight than did pigweed in the
greenhouse. However, with the further addition of 300 pounds of
ammonium nitrate per acre, pigweed continued to make gains while
corn growth was unaffected. At all fertility rates, pigweed had
higher levels of total leaf nitrogen than corn.
When snap beans had emerged and become established before
the emergence of pigweed, the total green and dry weight of pigweed
was significantly reduced if growing in full competition or in competition
for soil factors only.
