PGR-IV is a plant growth regulator (PGR) containing gibberellic acids, indolebutyric acid and a proprietary fermentation broth, while Pix is a plant growth retardant with mepiquat chloride as its active ingredient. Both PGR's are widely used in cotton (Gossypium hirsutum L.) by producers in the Cotton Belt, but little is known about their compatibility. The current study was conducted to investigate the effect of the combination and timing of PGR-IV and Pix on plant growth, development and yield of cotton grown under field conditions, and the possibility of tank-mixing these two for convenience and cost reduction.

Plants of cotton cultivar Deltapine 51 were grown on a Captina silt loam (Typic Fragiudults) in Fayetteville, Arkansas with 39 inches between rows and 10 plants per meter in-row spacing. Each plot consisted 3 rows 5 meters in length. The experimental design was a randomized complete block with 7 replications. The treatments included 1) An untreated control; 2) Pix at 8 oz/A at pinhead square (PHS) and first flower (FF); 3) PGR-IV at 4 oz/A at PHS and FF; 4) Pix plus PGR-IV at PHS and FF; 5) Pix plus PGR-IV at 5 days after PHS and 5 days after FF; 6) Pix plus PGR-IV at 10 days after PHS and 10 days after FF; and 7) Pix plus PGR-IV at 15 days after PHS and 15 days after FF. Growth analysis was conducted during boll development. Plant height, leaf area, number of squares and number of bolls were recorded. At harvest, the center row of each plot was hand-picked, and seedcotton and lint yields were obtained.

Plants of the untreated control and PGR-IV alone were the tallest, Pix alone the shortest, and the Pix plus PGR-IV treatments were intermediate. The highest number of squares and bolls were recorded from plants treated with the combination of Pix and PGR-IV at PHS and FF, resulting in higher seedcotton and lint yields. It was concluded that PGR-IV and Pix had counteracting effects on plant height, and synergistic effects on number of squares, number of bolls, seedcotton yield and lint yield. These two PGR's can be safely applied together to increased cotton yield, especially at pinhead square and first flower.