AFSCME, Local Union No. 3148, AFL-CIO and the Wisconsin Employment Relations Commission appeal from an order reversing a commission decision. The issue is whether the commission properly concluded that Sauk County refused to implement an arbitration award under the Municipal Employment Relations Act, sec. 111.70(3)(a)7, Stats.,1 when it refused to deduct fair share fees2 and union dues retroactively after a contract hiatus. We agree with the commission's conclusion and reverse the order.
*39The facts are not in dispute. Sauk County and the union had a collective bargaining agreement for 1983-84 which required the county to deduct union dues from the paychecks of all employees electing such a deduction and to deduct equivalent "fair-share" amounts from all other employees. The contract expired at the end of 1984, before agreement was reached on a replacement contract for 1985. When the 1983-84 agreement expired, the county discontinued deducting fair share fees and union dues from employee paychecks.
The commission certified that the parties had reached a bargaining impasse with respect to the 1985 agreement and the dispute was submitted to an arbitrator. In October, 1985, the arbitrator adopted the union's final offer which (with certain exceptions not material here) included all provisions of the 1983-84 labor agreement. The agreement embodied in the union's offer and accepted by the arbitrator was for the period January 1, 1985, to December 31,1985.
The union then requested the county to make fair share fees and union dues deductions retroactive to January 1, 1985. When the county refused to do so, the union filed a prohibited practices complaint under sec. 111.70(3)(a)7, Stats., alleging that the county had refused to implement the arbitration award. As indicated, the commission held that the county had unlawfully refused to implement the award and the trial court reversed.
The commission, citing Milwaukee v. WERC, 71 Wis. 2d 709, 716, 239 N.W.2d 63, 67 (1976), argues that its interpretation of sec. 111.70, Stats., should be affirmed if it is reasonable and consistent with the purposes of the Municipal Employment Relations Act. In a recent case, however, the Wisconsin Supreme Court *40rejected the use of the "sustain-if-reasonable" standard in the absence of evidence that the agency's interpretation or application of a particular law "is long continued, substantially uniform and without challenge by governmental authorities and courts." Local No. 695 v. LIRC, 154 Wis. 2d 75, 83, 452 N.W.2d 368, 372 (1990), quoting Beloit Education Ass'n v. WERC, 73 Wis. 2d 43, 67-68, 242 N.W.2d 231, 242-43 (1976). The court, emphasizing the " 'black-letter rule . . . that a court is not bound by an agency's conclusions of law,' " stated that where there is no evidence that the agency has had experience in interpreting the applicable law "as measured against" the facts of the case — where "there is no evidence of any special expertise or experience" in the particular area — "the weight to be afforded an agency's interpretation is no weight at all." Local No. 695 at 82-84, 452 N.W.2d at 371-72 (citation omitted).
The union contends that the "rational basis test" is nonetheless appropriate in this case because of the commission's experience in interpreting the Municipal Employment Relations Act and its "substantial expertise in the interpretation of collective bargaining agreements and fair share issues . . ..'' Neither the union nor the commission argue, however, that the commission has particular expertise or a position of long-standing on the precise issüe: whether it is a violation of sec. 111.70(3)(a)7, Stats., to refuse to retroactively deduct fair share fees and union dues. Moreover, the commission itself noted in its decision that "[t]he question of whether a violation of a provision [of the collective bargaining agreement] not in issue in the interest arbitration can be a basis of a finding of a violation of [s]ec. 111.70(3)(a)7, Stats., is one that has not been addressed before ... by the Commission." Under these circum*41stances, Local 695 requires that we review the commission's decision de novo..
The union and the commission argue that the collective bargaining agreement adopted by the arbitrator for 1985 required the county to deduct fair share fees and union dues retroactively. They refer us to the portion of the arbitrator's decision incorporating the union's final offer and to language in that offer stating that "[a]ll provisions of the Labor Agreement of 1983-84" were to continue in effect — including the provision requiring the county to deduct fair share fees and union dues "once each month."
Given the provisions of the arbitrator's award and the terms of the 1985 agreement specifying that the adopted contract "shall be effective as of the first day of January, 1985, and shall remain in full force and effect through the 31st day of December, 1985," we conclude that the county was plainly required to deduct fair share fees and union dues for all of 1985 — that is, for each month of the year. And where, as here, it did not do so, the commission could properly order it to make retroactive deductions for any months missed.3
Relying on Cutler-Hammer, Inc. v. Industrial Comm., 13 Wis. 2d 618, 109 N.W.2d 468 (1961), the county asserts that retroactive deductions are contrary to the parties' "past practices." It claims that it "has never made fair share deductions either retroactively or during a contract hiatus for at least five years," and that the union acquiesced in that practice.
*42In Cutler-Hammer, the court stated that "[w]hen the language of a contract is not clear and unambiguous the practical construction given to it by the acts of the parties is . . . 'entitled to great weight.'" Id. at 632, 109 N.W.2d at 475 (citation omitted). Under such circumstances, the court concluded that " [n]either the commission nor [the] court should substitute its own construction of the contract provisions for that which the parties through practical interpretation have placed thereon." Id. at 634, 109 N.W.2d at 475.
In this case, we have determined that the collective bargaining agreement provisions and the arbitrator's award are plain and unambiguous in their requirement that the county make monthly fee and dues deductions during calendar year 1985. As a result, the parties' past practices are immaterial. Even if we were to consider the county's failure to implement retroactive deductions in the preceding five years, we do not believe it has established that that "practice" was "acquiesce[d]" in by the union.
After the parties' 1982 collective bargaining agreement expired, and before an agreement for 1983-84 had been reached, the county discontinued deducting fair share fees and union dues. The union filed a prohibited practice complaint with the commission; and while it later withdrew the complaint because it feared that deduction of all past fees and dues in one lump sum would pose a hardship for the employees, it specifically stated at the time that its action was not to be considered an "admission that the county's failure to deduct dues following certification of impasse was lawful." Thus the union has not "consistently acquiesced" in the county's refusal to deduct the fees and dues retroactively.
*43The county also argues that because the union's final offer stated that wages should be paid retroactively, the rule of inclusio unius est exclusio alterius should be applied to prohibit the fair share provision from being similarly applied. This is so, according to the county, because the specific mention of a matter in a contract is "considered to exclude other matters of the same nature or class not expressly mentioned . . Goebel v. First Fed. Savings & Loan Ass'n, 83 Wis. 2d 668, 673, 266 N.W.2d 352, 355 (1978). We have noted in the past that the "exclusio" rule is not a mandatory rule of statutory construction,4 and we decline to apply it in a contract case such as this where the parties' 1985 agreement and the arbitrator’s award, read together, plainly require the county to deduct fair share fees and union dues for each month of 1985.
Finally, the county argues that where, as here, the contract provides for arbitration, all disputes "involving . . . substantive interpretation of th[e] contract" must be submitted to the arbitrator. As a result, the county maintains that the commission erred when it considered the merits of the union's complaint, rather than dismissing it outright. We are not persuaded.
We agree with the commission that sec. 111.70(3)(a)7, Stats., authorizes the commission to adjudicate a prohibited practice complaint alleging that a municipal employer has "refuse[d] or otherwise fail[ed] to implement an arbitration decision lawfully made." We agree, too, that sec. 111.70(3)(a)7 was not intended to provide an additional or alternative forum to arbitration *44for all disputes concerning a collective bargaining agreement which results from an arbitration award.
Here, however, the 1985 collective bargaining agreement resulted from a mediation-arbitration award, and where the effect of the award was to mandate retroactive dues/fees deduction, we believe disputes regarding retroactive implementation of that award are properly adjudicated in prohibited practice proceedings under sec. 111.70(3)(a)7, Stats.
By the Court. — Order reversed.
Section 111.70(3)(a)7, Stats., provides that it is a prohibited practice for Sauk County to "refuse or otherwise fail to implement an arbitration decision lawfully made . . .."
” 'Fair share agreement' means an agreement between a municipal employer and a labor organization under which . . . employes in the collective bargaining unit are required to pay their proportionate share of the cost of the collective bargaining process and contract administration measured by the amount of dues uniformly required of all members." Sec 111.70(l)(f), Stats.
The supreme court has "recognized that retroactivity is a way of life in labor negotiations" and that retroactive application of fair share agreements in particular is not improper. Berns v. Wis. Employment Relations Comm., 99 Wis. 2d 252, 266-67, 299 N.W.2d 248, 256 (1980).
See Manly v. State Farm Fire & Cas. Co., 139 Wis. 2d 249, 254, 407 N.W.2d 306, 307 (Ct. App. 1987).