Abstract:
Purple nutsedge [Cyperus rotundas L.) has been recognized as one of the
most troublesome perennial weeds of agricultural lands in tropical and some
temperate regions. This research sought to determine the effects of timing of
herbicides, shading, and soil moisture on plant population growth and tuber
production of purple nutsedge through field and greenhouse experiments. The
results of these experiments were used to validate a purple nutsedge population
matrix model constructed with observed and reported data. Purple nutsedge
control options were evaluated with model simulations.
Glyphosate reduced shoot number, tuber number, and tuber viability of
purple nutsedge, and the herbicide efficacy was higher when applied from 2 to 4
weeks after shoot emergence as compared to the first 2-week growth period. The
more effective period for the herbicide coincided with the tuber initiation phase
of purple nutsedge growth. Metolachlor caused only temporary suppression of
purple nutsedsge.
Sunlight intensity by 30%, 47%, 63%, and 90% caused in successively
greater reductions in shoot number, tuber number, leaf area, and total dry weight
of purple nutsedge. Shading decreased partitioning of plant biomass into tubers
and increased partitioning into leaves. These responses remained essentially the
same irrespective of timing of shading from early emergence through the first 4
weeks of plant growth. Depletion of available soil moisture from 25% to 75%
also reduced the number and dry weights of shoots and tubers produced.
However, proportional biomass allocation to shoots, leaves, and tubers and
relative growth and net assimilation rates remained unaltered with soil moisture
depletion, suggesting that purple nutsedge is fairly well adapted to low soil
moisture levels.
The importance of intraspecific competition on population regulation of
purple nutsedge was evident from model simulations. Model predictions of
maximum population size closely agreed with reported plant and tuber densities
of purple nutsedge. Model simulations of proportional changes in population size
of purple nutsedge, as influenced by soil moisture depletion or shading, also
closely followed the field results. Model simulations indicated that seasonal
application of herbicides resulting in 90% shoot kill will provide a successful level
of control and that herbicide efficiency will be higher when shoots are killed
during the second to fourth week of the growing period than from earlier
applications. However, model simulations showed that a better strategy than
using a highly effective, short duration herbicide is to provide a moderate level of
purple nutsedge control extending through the growing season.
