On wood left in the round, even small stuff, as the outter rings dry, they shrink more than the inner rings, and to make up the difference the wood will usually check...you can relieve some of this shrimkage stress by drillling a hole down the center of the of the branch, but that may ruin it as a horn plug unless you fill it...in my experience, "most" big branchs of hard wood, even those cut when the sap is down in February eventually check during drying. Now and then a branch cures with out cracking, but by using splits from bigger stock to begin with you may fare better?
TCA