Photosynthetic and Crop Growth Rate Responses in Maize Hybrids at Multiple Plant Densities and N Rates.

The research was conducted at the Agronomy Center for Research and Education (ACRE) near West Lafayette, IN and Pinney Purdue Agricultural Center (PPAC) near Wanatah, IN. The experiment was a split-split plot design with six replications. The main treatment was hybrid. Mycogen 2T780 (non-RW) and 2T787 (RW) were planted at ACRE, and Mycogen 2M749 (non-RW) and 2M750 (RW) were planted at PPAC. The sub-treatment was plant density at three levels: 54,000 (low), 79,000 (medium), and 104,000(high) plants/ha. The sub-sub-treatment was nitrogen (N) fertilizer rate. All plots received approximately 28 kg/ha of starter N at planting. In addition to the common starter N, there were three side-dress N rates: 0 (low), 170 kg/ha (medium), and 340 kg/ha (high).

Photosynthetic rate were measured four times at different growth stages (V14, R1, R3, and R4) at both locations. The machine used was a CIRAS-2 portable photosynthesis system, a product of PP SYSTEMS. The paired hybrids did not have significant differences in photosynthetic rate at any of the measurement times at either location. However, the effects of plant density and N fertilizer on photosynthetic rates were almost always significant (P<0.05). The hybrid x density and hybrid x density x nitrogen interactions were rarely significant. The low density treatment always had significantly higher photosynthesis rate than the high density treatment, and the overall gain in photosynthesis for the low density compared to the high density treatment averaged 4.5 μmol m^-2 s^-1 higher (about 27 %) at ACRE and about 43 % higher at PPAC. The overall gain in photosynthesis for the medium/high N compared to the low N treatment averaged 4.5 μmol m^-2 s^-1 higher (about 42 %) at ACRE and about 36 % higher at PPAC. The gain in photosynthetic rates for corn at medium/high N (about 39 %) was particularly apparent in the fourth measurement date at both locations. 

Above ground biomass was fractionated and were measured at four developmental stages (V14, R1, R3 and R6). The dry matter content increased during the growing season as expected. There were substantial yield gains (mean of 5500 kg/ha across 3 plant densities) for all hybrids when the side-dress N rate was increased from 0 to 170 kg/ha. Further corn yield gains with an N rate increase to 340 kg/ha was significant at PPAC (hybrids 2M749 and 2M750) but not at ACRE (hybrids 2T780 and 2T787).  When side-dress N fertilizer was applied, grain yields were highest at the lowest plant density for the 2 hybrids (2T780 and 2T787) at ACRE and at the intermediate plant density for the 2 hybrids (2M749 and 2M750) at PPAC.