Opening valuation is the value at the beginning of each accounting period, of livestock, crops, fodder, seeds, fertilizers, pesticides and cultivations and newly sown or growing crops. Livestock, crops and produce are valued at the cost of production or market value after taking marketing costs in to account.
Closing valuation is the value of the same items at the end of the accounting period.
Cost of sales is opening valuation plus purchases price minus closing valuation.

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