Optimizing chemical topping in cotton: Effects of planting density and variety sensitivity to mepiquat chloride

Abstract

Context

Chemical topping using plant growth regulators like mepiquat chloride (DPC) effectively inhibits apical dominance in cotton. However, the effectiveness of chemical topping may vary depending on planting density and the sensitivity of cotton varieties to DPC, which remains unclear.

Methods

A split-plot factorial AB+C design field experiment was conducted to evaluate the effects of variety, planting density, topping method, and their interactions on cotton agronomic traits, canopy structure, and yield. The main A plots included cotton varieties with different DPC sensitivities (insensitive and sensitive), main B plots included low, medium, and high planting density (PD₁₄, 14 plants m⁻², PD₂₈, 28 plants m⁻² and PD₅₅, 55 plants m⁻²), while subplots were assigned no topping, manual topping (remove of the main-stem tips), and chemical topping (A mixed liquor with 180 g ha⁻¹ mepiquat chloride and 150 ml ha⁻¹ special additives).

Results

In DPC-insensitive varieties, plant height and number of main stem internodes decreased with the increasing plant density, while sensitive varieties showed no significant changes. Sensitive varieties exhibited stronger inhibition of plant height, plant width, and internode length (1.8–35.9 %) than insensitive ones. At medium and low densities, upper internodes in sensitive varieties were reduced by 14.7–43.2 %. At high density, an increased leaf area index (LAI) in sensitive varieties before topping improved light interception rate (LIR). This effect was further enhanced LIR at medium and low densities by sustaining a higher LAI. Compared with manual topping, seed cotton yield increased by 8.7–29.5 % with no topping and by 4.4–33.3 % with chemical topping at low density for both sensitive and insensitive varieties. At medium density, seed cotton yield in sensitive varieties with no topping increased significantly by 6.3–14.6 % under no topping. At high density, the highest seed cotton yield in sensitive varieties was observed with no topping compared with manual or chemical topping. Yield improvements in insensitive varieties under chemical topping were linked to a higher harvest index at medium and high densities, whereas in sensitive varieties, yield increases were driven by a higher total number of bolls per unit area at high density.

Conclusions

Chemical topping with DPC can be a substitute for manual topping at moderate or high planting density in DPC-insensitive varieties, provided systematic chemical control establishes a balanced canopy structure prior to topping. For DPC-sensitive varieties, no topping or continued low-dose chemical control with DPC is recommended for optimal topping effects. These findings provide valuable insights into optimizing chemical topping strategies by considering cotton variety sensitivity to DPC.