Optimized Planting Patterns Alleviate the Adverse Effects of Improper Harvest-Aid Spraying Time on Machine-Harvested Cotton

Ill-timed harvest-aid spraying can reduce cotton yield and fiber quality. However, determining optimal spraying times is challenging when air temperature fluctuates strongly. Here, we analyzed whether cultivar type and planting pattern alleviate the adverse effects of improper spraying time. We conducted a three-factor field experiment in Xinjiang, China, during 2024–2025 with two cotton cultivars, three planting patterns (H76, H66, and L76), and three spraying times (T1, T2, and T3), plus an untreated control, then compared the defoliation rate (DR), boll opening rate (BOR), yield, and fiber quality among groups. We also determined the net contributions of harvest aids to DR and BOR by subtracting the natural (control) values (NDR and NBOR, respectively). Spraying time significantly affected DR, BOR, yield, and fiber quality. DR and BOR were highest at T2; lower values at T1 and T3 were attributed to physiological immaturity and lower temperatures, respectively. Yield losses and reductions in fiber length and strength of upper-canopy bolls increased with earlier spraying. Across the T1–T2 and T2–T3 intervals, the mean seed cotton yield loss decreased from 15.18% to 8.15%, and its average daily rate of change decreased from 0.95% to 0.36%. In contrast, the mean fiber length loss decreased from 8.44% to 3.81%, whereas its average daily rate of change increased from 0.36% to 0.53%. The moderating effects of cultivar on DR, BOR, and yield losses were susceptible to external environmental factors. Conversely, planting pattern had no significant effect on fiber length and fiber strength losses but had significant independent main effects on DR, BOR, and yield losses by altering NDR and NBOR: NDR and NBOR generally decreased in the order L76 > H66 > H76, whereas yield losses increased in the order L76 < H66 < H76. Therefore, when optimal spraying time is difficult to determine, we recommend reducing plant density or optimizing row configuration, rather than changing cultivar, to improve the efficacy of harvest aids.