Agriculture Reference
In-Depth Information
M
ATERIALS
A
ND
M
ETHODS
On 29 September 2005 we sampled a rainfed commercial field of mature soybeans,
cultivar Pioneer 9344 'Roundup Ready
®
', 10 miles south of Hays, Kansas that we had
previously confirmed was infested with
D. texanus
. The crop was fully mature and the plant
population was estimated at around 280,000 plants/ha. Individual plants were removed by
snipping them off at ground level with a strong set of pruning shears. The stalk of each plant
was then snapped open in several places to determine presence or absence of larval tunneling
and the entire plant was then placed into a paper bag and labeled with a letter and number
code. Plants were selected at random until a minimum of 100 plants of each type (infested /
uninfested) were collected. Since uninfested plants were more scarce than infested ones, once
104 infested plants were obtained, all further infested plants were discarded until a similar
number of uninfested plants had been obtained.
After harvest, the bags containing plants were placed in a series of cardboard boxes and
dried at 40 °C for a 7-10 d in a large drying room. Boxes were then transported to the
laboratory where each plant was carefully removed from its bag and placed on a large sorting
tray. Since considerable abscission of leaf petioles had already occurred, the remaining
petioles and leaves were stripped from plants to leave only stems and pods. The stalk
diameter of each plant was estimated at the soil line by taking a maximum and minimum
measurement with a digital caliper and averaging the two values. All pods containing filled
seed were removed from the plant, counted, and then shelled. The stalks of infested plants
were split lengthwise and the length of larval tunnels measured with a flexible tape measure.
Stalks and seeds where then weighed separately on an analytical balance (Mettler model
PL202-5).
On 23 September 2006 we sampled a 0.8 ha. field of Asgrow Roundup Ready
®
soybeans
on the experimental farm at K-State Agricultural Research Center-Hays, in Hays, Kansas.
The field was rain fed and plant population was estimated at around 165,000 plants/ha. Plants
were fully mature and identical procedures were followed as in 2005.
The data were analyzed using PROC ANOVA (SAS Institute 2003) with 'year' and
'infestation' as fixed factors. Linear regression was then used to establish relationships among
components of yield in both infested and uninfested plants.
R
ESULTS
The data for all components of yield are reported for both years in Table 1. The effect of
year was significant for all four dependent variables (
P
< 0.001 in all cases), with all values
significantly larger in the second year. There were no significant interactions between year
and infestation (
P
> 0.05 in all cases).
In 2005, infested plants tended to be slightly larger than uninfested plants with marginally
larger stem diameters and greater seed weights, although differences in stem weight and pod
number were not significant (Table 1). Given these apparent differences in plant size, we
divided seed weight by stem weight to generate an index of plant productivity that would be
independent of plant size and compared infested and uninfested plants again, but found no
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