Nitrogen conversion efficiency (NCE) is calculated by dividing the sum of N in products (meat, milk, wool, exported feed, exported farm effluent, N in farm-grown supplements placed in storage) by the sum of N inputs (fertiliser, clover fixation, imported feed). This provides a proportion of N converted to product.
Nitrogen conversion efficiency (NCE) = N in Product / N in Inputs
High NCE typically occur when a lot of N is exported as effluent, supplements or crops. High NCE, probably those in excess of 50%-60%, may indicate that N in the system is being run down.
NCE can be highly variable depending on the mix of different products removed from farm and shouldn't be used as a relative indicator of performance between farms.