Genetic improvement of maize has resulted in large increases in total seasonal dry matter accumulation (DMA), especially under conditions of abiotic stress, and there is generally strong heterosis for DMA when comparing F1 hybrid varieties to their inbred parents.  This improvement in DMA has been attributed to increased canopy photosynthesis, especially during the grain filling stage.  However, decreased crop respiration could also contribute substantially to increased DMA, since it is estimated that approximately 50% of the carbon fixed through photosynthesis over the growing season is eventually lost again to crop respiration.   In this talk we review methods we have employed in measuring leaf and whole-plant respiration of field grown maize, and provide experimental evidence that i) modern maize hybrids have reduced rates of leaf respiration, compared to older hybrids, ii) abiotic stresses such as N fertility stress, drought stress and shade stress reduce shoot respiration rates, iii) respiratory C efflux per unit existing crop dry matter declines over the entire growing season, and iv) integrated over the time from emergence to harvest maturity, F1 hybrids have lower respiratory C losses per unit DMA than do their inbred parents.