On or before June 1 of each year, the normal and accrued contribution rates, as determined on the basis of the last actuarial valuation, shall be certified by the board of trustees to the director or chief administrative officer of each employer having members in its employ. Each such employer shall make provision in its annual budget for funds with which to pay to the board of trustees an amount equal to the normal and accrued liability contributions on the earnable compensation of all contributing members under their administration and for an additional amount as expense for the operation of the retirement system.