The invention relates to a plant growth regulator system comprising a synergistic combination of prohexadione and ethephon. The invention further relates to a method for treating plants with at least prohexadione and ethephon, wherein a water based spraying liquid is sprayed on plants, wherein the liquid comprises water and suitable amounts of prohexadione and of ethephon, wherein the amount of prohexadione is between 20 and 400 g per hectare, and wherein the amount of ethephon is between 100 and 1000 g/ha. The plants preferably are rape seed, flax or cereal crops. Cereal can be barley, oat, wheat, rye, triticale, grain or corn. The combination shows substantial synergistic effects on stem reduction, and therefore reduce risk of lodging and improve yield.
