No.
Fluctuations in the supply of base commodities should be accounted for using their market price sold with a fixed supply of money. You imply that 1 grain of wheat during good harvests is equivalent to one grain of wheat from a bad harvest. Not correct. Fixed supply of money takes account of the differing value of differing rates of supply, as it should.
"…people would agree on a standard set of wholesale prices of commodities to treat as the standard value in which they would prefer to have their currencies kept constant." - F. A. Hayek
"each Hayek is really just a derivative asset. How then would the issuing firm get the public to start treating the Hayeks as money? On the night of the initial auction, after the market price of the Hayeks had been ascertained, the issuing firm would specify a commodity basket (consisting of bread, eggs, milk, and other goods relevant to consumers) that cost, say, $60 at Wal-Mart. Then the firm would announce to the public the following non-binding pledge: "We will use our firm's assets to adjust the outstanding supply of Hayeks such that 5 Hayeks will always (insofar as it is humanly possible) have the purchasing power to buy this specified commodity basket." -https://mises.org/library/hayeks-plan-private-money
The objection you bring up was addressed by Hayek, and his solution was "constant but not fixed value":
"It would be expedient that the issuing institution (preferable a DAO) should announce precisely the collection of commodities in terms of which it would aim to keep the value of the [Hayek] constant. But it would be neither necessary nor desirable that it tie itself legally to a particular standard. Experience of the response of the public to competing offers would gradually show which combination of commodities constituted the most desired standard at any time and place. Changes in the importance of the commodities, the volume in which they were traded, and the relative stability or sensitivity of their prices might suggest alterations to make the currency more popular." - F. A. Hayek
The answer then is that if the price of wheat fluctuates too frequently because of good and bad harvests, then wheat would not be something included in the basket of commodities on which the currency is pegged.