Light interception and utilization in relay intercrops of wheat and cotton

In China, a large acreage of cultivated land is devoted to relay intercropping of winter wheat and cotton. Wheat is sown in strips with interspersed bare soil in October and harvested in June of the next year, while cotton is sown in the interspersed paths in the wheat crop in April and harvested before the next wheat sowing in October. This paper addresses the question how strip width and number of plant rows per strip of wheat or cotton affect light interception (LI) and light use efficiency (LUE) of both component crops. Field experiments were carried out in three consecutive years: 2002, 2003 and 2004. Light interception and productivity were estimated in monocultures of wheat and cotton and four intercropping designs differing in strip and path width as well as number of rows per strip. The intercrop systems were identified by the number of rows per strip of wheat and cotton, respectively, as 3:1, 3:2, 4:2 and 6:2. Total light interception over a season was calculated from LAI measurements, using a model for light interception in a row crop. The spatial distribution and diurnal course of light in intercrops were also measured with sensors. Wheat monocrops intercepted 618MJm⁻² photosynthetically active radiation (PAR) from 18 March to harvest in 2002, 337MJm⁻² from 29 April to harvest in 2003, and 457MJm⁻² from 13 April to harvest in 2004. Averaged over 3 years, wheat in the four intercrops (3:1, 3:2, 4:2 and 6:2, respectively) intercepted 83, 71, 73 and 75% as much PAR as the sole wheat. From sowing to harvest, cotton monocrops intercepted 491MJm⁻² PAR in 2002, 426MJm⁻² in 2003, and 415MJm⁻² in 2004. Cotton in the four intercrops (3:1, 3:2, 4:2 and 6:2, respectively) intercepted 73, 93, 86 and 67% as much PAR as the sole cotton. LUE of wheat was 2.12±0.14gtotaldrymatterMJ⁻¹ PAR during the reproductive period, while that of cotton was 1.33±0.02gdrymatterMJ⁻¹ PAR over the whole growing period. No differences in LUE of wheat or cotton were found between systems. The analysis indicates that the high productivity of intercrops, compared to monocultures, can be fully explained by an increase in accumulated light interception per unit cultivated area. The component crops are thus complementary in their interception of light over space and time. The model results indicate that light interception can be modified by choice of the number of crop rows per strip and strip width. The best distribution of light is attained in systems with narrow strips, a high proportion of border rows, and high planting densities of cotton. Suggestions for system improvement are given.