Shaping of plant form for high output and concentrated boll formation using slow-release nitrogen application in summer direct seeding cotton

High output and mechanized harvesting of summer direct seeding cotton (SDSC) require both high yield and concentrated boll setting. However, it is unclear how to achieve these goals using slow-release nitrogen fertilizer. We performed a field experiment designed to enhance nitrogen use efficiency using a simplified fertilizer application method. The experiment, designed to examine the relationship between plant form and boll-setting speed, comprised three slow-release nitrogen rates (45, 90, and 135 kg N hm −2 ) and two application times (two-leaf and four-leaf stages). The experiment took place at Yangzhou University during the cotton growth seasons in 2021 and 2022. The results showed that the highest seed cotton yield (about 4000 kg hm −2 ) and boll-setting speed (more than 3.0 × 10 4 bolls hm −2 d −1 ) were achieved with 90 kg N hm −2 of slow-release nitrogen at the four-leaf stage. This treatment also achieved suitable agronomic nitrogen use efficiency (ANUE, 10.2–16.4 kg kg −1 ), nitrogen recovery efficiency (NRE, 52.4–54.8%) and partial factor productivity of nitrogen (PFPN, 39.9–40.7 kg kg −1 ), significantly outperforming the CK. There were significant quadratic regression relationships between plant height (PH), the ratio of fruit nodes to fruiting branches (RNB), and the total boll number and boll-setting speed. Fruit branch curvature (FBC) was correlated with population total boll number and boll-setting speed. Positive correlations were found between the fruit branch orientation value (FBOV) and the total boll number and boll-setting speed. These models indicated that a suitable PH (81.2–90.5 cm), RNB (2.6–2.7), low FBC (5.8–5.1 from the lower to upper sections), and high FBOV (36.7–46.5 from the lower to upper sections) comprised a suitable plant form for the tested conditions. The results suggest that plant form regulation by a one-time application of an appropriate slow-release nitrogen rate at the four-leaf stage could optimize SDSC production.