Long-term ecosystem carbon losses from silage maize-based forage cropping systems

•We calculated field-scale net ecosystem C balances for forage and grain rotations.•C loss was threefold greater from continuous silage maize than soybean-grain maize.•Alfalfa reduced C loss by 23% relative to continuous silage maize.•Winter rye cover copping and liquid dairy manure improved C-balances marginally.•Every Mg of crop residue C left in-field improved C balances by +0.9 Mg C ha−1.