The net CO 2 assimilation rate of a maize canopy ( A c) was estimated by vertically integrating leaf chamber CO 2 exchange rates, taken between 6 and 11 weeks after emergence, using four methods: (i) the CUPRAD model based on five sunlit inclination angle classes; (ii) the CUPRAD model based on a mean leaf angle following the spherical distribution; (iii) the three-point integration method of Gauss with five sunlit inclination angle classes; (iv) the Gaussian method with leaves in a horizontal position. Canopy assimilation rates were measured using the eddy correlation method combined with soil respiration measurements obtained with a portable gas exchange system. They were used as a reference to test the vertical integration approaches. The Gaussian method and the CUPRAD model with several inclination angle classes provided similar estimates. CUPRAD based on a mean angle overestimated the crop assimilation rate by 5% compared with estimates based on several inclination angle classes. Leaves in a horizontal position integrated with the Gaussian method gave estimates 5–15% higher than those estimated by CUPRAD. CUPRAD based on several leaf angle classes overestimated the reference by 15–25% in the morning and by 30–40% in the afternoon. The Gaussian method, with five sunlit inclination angle classes, gave the best estimates of A c if the leaf chamber measurements were corrected to account for the leaf boundary layer conductances of the canopy. Morning estimates remained within the range of precision of the reference, i.e. 7%, but afternoon estimates were 10–30% higher than the reference. The afternoon discrepancy could not be explained by diurnal water stress or other physiological causes. The discrepancy is more likely to be a result of the fact that the boundary layer conductance equation used in this approach neglected the shelter effects. The upper layer of the canopy, which represented 28% of leaf area index (LAI) according to Gauss partition, contributed 38–44% of canopy assimilation rate around noon. The middle part, which accounted for 44% of LAI had a similar contribution, whereas the lower part, which accounted for 28% of LAI, contributed only 16% of canopy assimilation rate.