06 March, 2018
Many irrigated crops are fast approaching or have already reached cut out. Cut out occurs when the cotton plant reaches 4-5 nodes above white flower (NAWF), and demand for assimilate (products of photosynthesis) finally exceeds supply, preventing further growth and halting the production of new squares.
The date of the last effective flower can be used to plan irrigations after cut out. End of season water requirements can be determined by estimating the number of days until defoliation and the amount of water to be used in this period. The prime objective of the last irrigation is to ensure that boll maturity is complete without water stress. At the time of the last irrigation, all bolls have been set.
The objective of the last irrigation is to ensure that boll maturity is completed without any stress, in order to maximise the crop’s yield potential. Cutting off water one irrigation earlier than would normally be the case, is one method by which some irrigation water may be saved. In doing so, the plant/crop will suffer stress but the timing will be such that it should have minimal impact on both final yield and fibre quality.
If the crop is one irrigation short, boll size in later set bolls will generally be reduced rather than there being any significant reduction in boll number. Fibre quality may also be affected, for example reduced total micronaire, although there is unlikely to be any effect on fibre length. If the crop is two irrigations short, boll number will be reduced. Yield reductions can occur, particularly in vegetative crops that stress prior to boll opening.
Crops that experience stress before 60% of bolls have opened or before 4 Nodes Above Cracked Boll (NACB) can suffer yield and quality reductions. If bolls don’t reach maturity before harvest there will be high levels of immature fibre.
There are several methods available to accurately time final irrigation and defoliation, but measuring NACB is the method most commonly used. On average, bolls will sequentially open at a rate of one node every three to four days. This will depend on several factors, particularly climatic conditions.
By Chris Barry: CSD Extension & Development Agronomist – Queensland.
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