cotton - Greenmount Press

Furthermore, during the season the monitoring of the crop in
terms of water requirements is critical. The use of C-probes has
helped in indicating when crops will require further water. The
effect of delayed watering on Bollgard II crops has been well
documented and will affect yield. As an example, when Bollgard
II is grown using conventional crop irrigation management, a
yield loss is seen in the Bollgard II compared to the conventional
due to the Bollard II requiring different irrigation management.
A trial was setup using Sicot 71BRF in a Bollard II field and
also in a conventional field to look at what impact conventional
management would have on a Bollgard II variety.
The results can be seen in Figure 3. The impact of a late first
irrigation was stressing the Bollgard II variety in the conventional
trial (Crop B) and overall caused the Sicot 71BRF to be shorter
with less fruiting branches and caused premature cut-out,
compared to the normal Bollgard II field (Crop A).There were
less secondary fruit, as well as less top fruit which meant that
under the conventional irrigation management regime the Sicot
71BRF in the conventional field yielded less than the conventional
varieties in the trial. Therefore irrigation management is critical,
particularly as we get into flowering where there is peak demand
for water by the crop. Timing is critical so that the crop does not
stress as Bollgard II varieties generally have a higher early fruit set
than conventional varieties.
The impact of water stress on the plant also can affect cotton
quality. Water stress occurring during peak flowering will have
an impact on fibre length while late flowering moisture stress
will affect fibre maturity or micronaire. Typically, the top fruit
under moisture stress will be aborted which will allow more
carbohydrates to be redistributed to lower developing bolls. This
can lead to high micronaire in the crop. Hence, the importance of
watering on time for the whole season and developing the top
fruit on the plant.
Nutrition management
Crop nutrition is an important part of managing the crop to
maximise yield potential. Outside of the weather we can control
the amount of fertiliser and soil nutrition through knowing what
status the soil is in, in terms of macro and micro nutrients via
thorough soil, petiole and leaf testing.
There is a definite improving yield trend with the right
FIGuRE 4: Crop nitrogen uptake kg/ha versus
yield (two decades work at ACRI, CRC 2008)
application of nitrogen as seen in Figure 4. What is important
is getting the balance right to maximise yield potential while
reducing the chances of too much nitrogen being left in the soil
profile which can cause issues with regrowth and poor defoliation
at the end of the season.
The most important point to make is not to be short on
nutrition as this can have a major impact on yield. Constable and
Bange (2006) assessed the likely nutrient requirements of a four
bale per acre versus a seven bale per acre crop. This can be seen
in Table 1 where the requirement just for nitrogen alone differs
by 125 kg N/ha when comparing a four bale per acre crop versus
a seven bale per acre crop at an uptake rate of 10 kg N per bale.
As important is the removal of nitrogen from the soil. A seven
bale per acre crop would take 173 kg N per hectare from the soil
at an uptake of 10 kg N per bale. This could impact on following
crops if not managed properly.
Simular responses from high yielding crops can be seen with
the uptake and removal of both potassium and phosphorus.
TABLE 1: Comparing the nutrient requirements
of a 7 bale/acre and 4 bale/acre crop
4 7
bales/acre bales/acre
12 kg N
198 347
per bale
Uptake (kg/ha)
10 kg N
165 290
per bale
Nitrogen
12 kg N
120 208
per bale
Removal (kg/ha)
10 kg N
100 173
per bale
Uptake (kg/ha) 200 350
Potassium
Removal (kg/ha) 80 140
Phosphorus Removal (kg/ha) 26 45
Based on Constable and Bange (2006).
Getting the balance right in terms of knowing what the soil
nutritional status is, what yields came off the previous crop and
how much removal of nutrients occurred in that crop gives a
good indication for future crop requirements.
Vegetative growth management
The balancing act for the grower or manager is keeping a lid
on excessive vegetative growth while maintaining good fruit set.
An out of control vegetative plant puts all its energy into leaf and
stem and produces a poor node to height ratio with excessive
shading of lower bolls. It can be prone to poor fibre development
particularly in relation to micronaire and also the potential for
boll rots at a later stage of the crop’s life if humid and rainy
conditions exist. Internode length in this circumstance is usually
quite long (9–10 cm) which leads to poor overall node and fruit
development.
Monitoring the crop through this period using the Crop
Development Tool (CRC website) will help with monitoring
the vegetative growth rate (VGR) through this period. It works
by recording the crop height and number of nodes on two
occasions, four to seven days apart. The VGR is plotted against
day degrees accumulated for the crop during the flowering
period. The aim is to maintain the crop’s VGR between the upper
and lower limits. Warmer regions and more fertile soils will tend
to have higher VGR. A VGR value greater than 5.5 cm per node
would require the use of a plant growth regulator.
56 — The Australian Cottongrower December 2012–January 2013