In response to the issue of insufficient endogenous enzyme activity in Xinjiang cotton field soils limiting cotton growth and development, the study investigated the effects of foliar application of exogenous bio-enzymes at different growth stages on cotton growth and the formation of yield and quality, clarifying their regulatory effects and suitable application periods.

Field experiments were conducted in Shawan City, Xinjiang in 2024 and 2025, using Zhongmian 7700 as the experimental material. Set three drip application periods: bud stage (T1), initial flowering stage (T2), and full bell stage (T3), applying 30 kg·hm^-2 of bio-enzyme with irrigation, with clear water as the control (CK). The effects of drip-applied bioenzyme at different growth stages on cotton agronomic traits, canopy parameters, photosynthetic parameters, dry matter accumulation, yield and its components, and fiber quality were analyzed.

Compared with the CK, the application of bioenzymes at the T1 stage significantly increased the cotton plant height, stem thickness, and number of fruit branches, with a boll-setting rate increase of 20.7%-28.4%, and optimized canopy parameters, specifically showing a 12.2%-26.4% increase in leaf area index, a 14.8%-16.0% increase in average leaf inclination angle, and a 29.4%-30.5% decrease in canopy openness, thereby improving the population light distribution. At the same time, bioenzyme treatment at the T1 stage significantly enhanced leaf photosynthetic capacity, with net photosynthetic rate increasing by 30.0%-39.1%, and intercellular CO₂ concentration significantly decreasing by 13.3%-14.9%, which was more conducive to the transport of assimilates to reproductive organs. In terms of yield composition, bioenzyme treatment at the T1 stage simultaneously increased the number of bolls per unit area (11.0%-15.0%) and the single boll weight (1.6%-2.9%), with seed cotton yields reaching 7196.2 and 7754.7 kg·hm^-2 over two years, respectively, representing an increase of 14.1%-16.9% compared with CK; the average fiber length in the upper half, breaking strength, and uniformity index of fibers were also improved. Correlation analysis further indicated that applying bio-enzymes at the bud stage (T1) showed a highly significant positive correlation with cotton plant height, stem thickness, boll set rate, relative chlorophyll content (SPAD value), reproductive organ dry matter, boll number, and seed cotton yield, while canopy openness and intercellular CO₂ concentration showed a highly significant negative correlation; the regulatory effect at the initial flowering stage (T2) was weaker, and the effect at the full boll stage (T3) was not significant, indicating that the regulatory effect of bio-enzymes on cotton has a clear period-dependent characteristic.

The period of foliar application of biological enzymes had a significant regulatory effect on cotton growth and yield formation. Applying 30 kg·hm^-2 of biological enzymes during the bud stage could optimize canopy parameters, enhance photosynthetic capacity, and promote the accumulation of dry matter in reproductive organs, achieving a coordinated increase in bolls per unit area and individual boll weight, ultimately improving seed cotton yield, which was the suitable period for applying biological enzymes in cotton fields in Xinjiang.