Thank you Millet.
Split opening is a serious problem with Robinson and Nova mandarin and it can split up to %30 of total yield.
Academic reports recommend 3 application of %2Ca(NO3)2 (CALCIUM NITRATE); 20 ppm GA3 (Gibberellic acid) and 20 ppm 2,4-D (2,4-Dichlorophenoxyacetic acid) and this helps keeping splitting fruits up to %60.
We make this but on the other hand some farmers noticed if they DON'T apply Gibberellic acid at the beginning of the season for fruit set, the splitting occurs in smaller ratios. Somehow, with an unknown mechanism to us, GA3 increases split ratio as they say.