by Blake Onken, PhD, CPSS, Lindsay’s “Dr. Pivot”
While cotton is relatively drought tolerant, optimum yields are only obtained when water stress is minimized or eliminated throughout the growing season. Cotton benefits from particular moisture conditions at each growth stage, and pivots provide growers the control they need to achieve this goal.
Cotton is vulnerable to yield reductions from drought stress at each growth stage. Drought stress causes poor emergence and stand establishment early in the season; slow plant development; square shedding and reduced fruiting branches in mid-season; and excessive bloom and boll loss later in the season. Drought stress during late bloom and boll fill causes the largest negative impact on yield.
Beginning the growing season with a full soil profile, through early season rains or pre-irrigation, will encourage deeper rooting and ensure that mature cotton plants exploit the moisture and nutrients that are available throughout the soil profile. During subsequent growth stages optimum soil moisture increases yield by:
1. Emergence: Adequate early moisture helps insure timely and uniform seedling emergence, stand establishment, and early vigorous growth.
2. Squaring: Water use increases rapidly as leaf area increases and setting early squares is vital to achieving high yields.
3. First White Bloom: Cotton responds well to frequent, low-volume applications of water that minimize water stress, thereby increasing fruit retention. Water stress during this phase will cause excessive square and bloom shed.
4. Peak Bloom: Water use reaches a maximum during peak bloom. At this growth stage, plants are blooming and filling early bolls, adding to total water requirements. Frequent irrigation (every 2-3 days) of small amounts of water (.20 –.25 in/5.08-6.35 mm) has been shown to be very effective.
5. First Open Boll: Control of late-season irrigation is essential. Irrigation should continue until 20% of bolls are open (1-2 weeks after first open boll). After this point, soil moisture depletion should be sufficient to bring the crop to full maturity. Excess water in late season can encourage unproductive vegetative growth and subsequent shedding of mature bolls.
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