The potential for relay intercropping and double cropping was assessed in field trials over three consecutive years at Gatton, Queensland. The rationale was to use controlled traffic technology to facilitate relay and double cropping and thus research a cropping system that could exploit late winter crop rainfall. In Field Trial I, grain sorghum and sunflower, broadacre crops already grown within the Southern and Darling Downs regions of Queensland and New South Wales were intercropped into wheat; sunflower was intercropped with wheat in Field Trial II. Sole summer plantings were made at the same time as intercrops were planted. The wheat crop was cut and stubble removed to facilitate this. Various planting dates (three for Field Trial I; four for Field Trial II) for the relayed summer crops were used to determine if an optimum planting time existed. Plant height, tiller number, light interception, grain yield, soil moisture and economic return were used as parameters to compare the intercrop with sole plantings in Field Trial I. Grain yield, soil moisture, rainfall infiltration and economic return were measured in Field Trial II. Research also involved the modification and testing of a tractor to carry out the sowing of the intercrop. In Field Trial I, light interception was shown to vary at different stages of the wheat crop and the use of these stages to determine optimum planting dates of the relay crop is suggested. In both trials, no differences were recorded in the grain yield between intercropped and sole cropped wheat treatments suggesting the trafficking of the plot did not affect the wheat. As neither sorghum or sunflower established as intercrops, competition was not a factor in affecting wheat yields. Moisture readings in both trials showed little change below a depth of 100 cm; however some treatment differences were present at shallower depths. In Field ii Trial I, sole summer sorghum, especially the first planting date, showed reduced water capture/ higher soil evaporation due to wheat removal initially and later transpiration loss due to crop growth and increased weed pressure. Sole wheat treatments showed increased moisture storage after harvest due to lack of water use by the crop and increased infiltration/reduced runoff due to stubble retention. Improved soil moisture recharge after rainfall events was apparent in double cropped treatments suggesting not only improved water utilisation but also improved capture and storage is possible within this system. Sorghum, commonly used throughout south eastern Queensland as a summer crop option, proved unsuitable for relay intercropping in Field Trial I for Planting Dates 1 and 2. Minimum soil temperatures for these plantings were marginal as they were close to the 15o Celsius level recommended for sorghum. However, even though establishment was poor for the intercropped plantings, it was higher for sole sorghum plantings. Wheat allelopathic effects may be involved. To avoid the temperature limitations of sorghum, sunflower was selected as an alternative intercrop in the later planting dates of Field Trial I and all dates for Field Trial II. Reasons for the poor establishment and yield of sunflowers in the earlier intercrop planting dates compared to sole plantings remain unknown but also may be related to allelopathic effects from intercropped wheat. Low soil temperature was not a factor affecting establishment Yields for planting dates were recorded in the intercropped sunflower treatments for Field Trial II and the optimal planting time for sunflowers in a wheat/sunflower relay intercrop was identified as when physiological maturity of the wheat had occurred. This may relate to the wheat crop stage. In Field Trial II, no significant differences in soil moisture were recorded between treatments from overall water use for the trial period. There were differences in water use between intercropped and sole cropped treatments for iii some rainfall events. Three rainfall events were chosen for closer study in each of the field trials conducted. Each event varied in the length and time as well as the duration and intensity of the rain that fell for the period. For the first rainfall period the moisture content of the first planting date of the sole summer treatment and to a lesser extent the second planting date of the same treatment increased, most likely due to wheat removal. In the third rainfall period the double cropped sunflower treatment with stubble tended to store less moisture and this may be due to the active crop growth at this time. It was evident in both field trials of the need for an effective weed control program in the intercrop plots. Weeds were controlled in wheel tracks by glyphosate sprays. Cultural methods may help but a herbicide suitable for both components of the intercrop would be very useful. A tractor was successfully modified to a 3 metre wheelspace and a clearance of 70 cm. This proved sufficient for planting the relay intercrop in Field Trial II without negatively affecting the yield of the standing crop. The row spacing of 18 cm for wheat in a 3 metre fixed bed and wheeltrack configuration assisted with guidance and interplanting of the relay crop. The relay crop was sown as single alternating rows.

The potential for relay intercropping and double cropping was assessed in field trials over three consecutive years at Gatton, Queensland. The rationale was to use controlled traffic technology to facilitate relay and double cropping and thus research a cropping system that could exploit late winter crop rainfall. In Field Trial I, grain sorghum and sunflower, broadacre crops already grown within the Southern and Darling Downs regions of Queensland and New South Wales were intercropped into wheat; sunflower was intercropped with wheat in Field Trial II. Sole summer plantings were made at the same time as intercrops were planted. The wheat crop was cut and stubble removed to facilitate this. Various planting dates (three for Field Trial I; four for Field Trial II) for the relayed summer crops were used to determine if an optimum planting time existed. Plant height, tiller number, light interception, grain yield, soil moisture and economic return were used as parameters to compare the intercrop with sole plantings in Field Trial I. Grain yield, soil moisture, rainfall infiltration and economic return were measured in Field Trial II. Research also involved the modification and testing of a tractor to carry out the sowing of the intercrop. In Field Trial I, light interception was shown to vary at different stages of the wheat crop and the use of these stages to determine optimum planting dates of the relay crop is suggested. In both trials, no differences were recorded in the grain yield between intercropped and sole cropped wheat treatments suggesting the trafficking of the plot did not affect the wheat. As neither sorghum or sunflower established as intercrops, competition was not a factor in affecting wheat yields. Moisture readings in both trials showed little change below a depth of 100 cm; however some treatment differences were present at shallower depths. In Field ii Trial I, sole summer sorghum, especially the first planting date, showed reduced water capture/ higher soil evaporation due to wheat removal initially and later transpiration loss due to crop growth and increased weed pressure. Sole wheat treatments showed increased moisture storage after harvest due to lack of water use by the crop and increased infiltration/reduced runoff due to stubble retention. Improved soil moisture recharge after rainfall events was apparent in double cropped treatments suggesting not only improved water utilisation but also improved capture and storage is possible within this system. Sorghum, commonly used throughout south eastern Queensland as a summer crop option, proved unsuitable for relay intercropping in Field Trial I for Planting Dates 1 and 2. Minimum soil temperatures for these plantings were marginal as they were close to the 15o Celsius level recommended for sorghum. However, even though establishment was poor for the intercropped plantings, it was higher for sole sorghum plantings. Wheat allelopathic effects may be involved. To avoid the temperature limitations of sorghum, sunflower was selected as an alternative intercrop in the later planting dates of Field Trial I and all dates for Field Trial II. Reasons for the poor establishment and yield of sunflowers in the earlier intercrop planting dates compared to sole plantings remain unknown but also may be related to allelopathic effects from intercropped wheat. Low soil temperature was not a factor affecting establishment Yields for planting dates were recorded in the intercropped sunflower treatments for Field Trial II and the optimal planting time for sunflowers in a wheat/sunflower relay intercrop was identified as when physiological maturity of the wheat had occurred. This may relate to the wheat crop stage. In Field Trial II, no significant differences in soil moisture were recorded between treatments from overall water use for the trial period. There were differences in water use between intercropped and sole cropped treatments for iii some rainfall events. Three rainfall events were chosen for closer study in each of the field trials conducted. Each event varied in the length and time as well as the duration and intensity of the rain that fell for the period. For the first rainfall period the moisture content of the first planting date of the sole summer treatment and to a lesser extent the second planting date of the same treatment increased, most likely due to wheat removal. In the third rainfall period the double cropped sunflower treatment with stubble tended to store less moisture and this may be due to the active crop growth at this time. It was evident in both field trials of the need for an effective weed control program in the intercrop plots. Weeds were controlled in wheel tracks by glyphosate sprays. Cultural methods may help but a herbicide suitable for both components of the intercrop would be very useful. A tractor was successfully modified to a 3 metre wheelspace and a clearance of 70 cm. This proved sufficient for planting the relay intercrop in Field Trial II without negatively affecting the yield of the standing crop. The row spacing of 18 cm for wheat in a 3 metre fixed bed and wheeltrack configuration assisted with guidance and interplanting of the relay crop. The relay crop was sown as single alternating rows.

La productivité et la qualité des céréales biologiques sont soumises à deux principaux facteurs limitants dans les systèmes sans élevage : des déficits chroniques en azote (N) du sol et des infestations par les adventices. Des légumineuses telles que les trèfles ou les luzernes peuvent servir à la fois de plantes de couverture et d'engrais verts grâce à leur fixation symbiotique d'N atmosphérique. Cependant, leur substitution aux céréales présente un moindre intérêt économique dans les systèmes de grandes cultures en l'absence d'animaux pour les valoriser. L'association relais de couverts de légumineuses dans un blé d'hiver nous a semblé être une option intéressante pour à la fois enrichir le système sol-plante en N, couvrir le sol dès la récolte du blé associé et limiter le risque de compétition avec le blé en décalant au printemps la date de semis des légumineuses sous couvert de blé. Pour évaluer l'efficacité de ces associations, quatre espèces de légumineuses (Medicago lupulina L., M. sativa L., Trifolium pratense L. et T. repens L.) ont été semées au tallage du blé d'hiver sur huit parcelles réparties dans la région Rhône-Alpes. Leurs effets sur la maîtrise des adventices, l'enrichissement, la préservation et la restitution d'N au système sol-plante et les performances des cultures ont été observés, durant une succession blé d'hiver-culture de printemps. Les résidus des couverts ont été enfouis à la fin de l'hiver, 9 à 12 semaines avant le semis d'une culture de printemps. Nos travaux ont montré l'absence d'effet des couverts associés sur le rendement en grains du blé d'hiver. Mais des diminutions du taux protéique des grains sont apparues dans un tiers des situations d'association notamment avec M. lupulina et T. pratense, les espèces les plus développées à la récolte du blé. Notre suivi de la disponibilité des ressources trophiques principales (eau, N, lumière) nous a permis d'identifier une compétition pour l'eau et l'N du sol. Nous avons également noté l'efficacité des couverts de légumineuses dans le contrôle de la densité des adventices dès le stade de floraison du blé et de leur biomasse durant l'interculture. Le meilleur contrôle des adventices a été permis par M. lupulina et T. pratense, à la récolte du blé, et T. pratense et T. repens, à la fin de l'automne, associé aux biomasses aériennes observées les plus importantes. Enfin, nous avons observé une forte proportion d'N issu de la fixation symbiotique dans la biomasse aérienne des légumineuses à la fin de l'automne (80 à 94%), représentant un apport d'N exogène au système sol-plante évalué entre 37 et 77 kg N ha-1. Cet enrichissement en N n'a pas entrainé d'aggravation de la lixiviation d'N durant l'hiver. Les couverts de légumineuses n'ont pas non plus diminué la lixiviation comparativement à l'absence de couvert. Après leur destruction, les résidus des couverts ont restitué une partie de l'N accumulé (+28 à +42 kg ha-1 d'N minéral sur les 90 premiers centimètres de sol par rapport au témoin à l'émergence de la culture de printemps, 12 semaines après leur destruction). Cette restitution a permis un enrichissement en N des pailles et grains de la culture de printemps et une augmentation de 30% du rendement lorsqu'il s'agissait de maïs. Finalement nous concluons sur l'intérêt des couverts de légumineuses associés en relais dans un blé d'hiver pour apporter une réponse positive aux problèmes principaux des rotations de grandes cultures biologiques (adventices, déficits d'N et diminutions des performances des cultures). Nous terminons en proposant des voies d'évolution des associations testées, notamment pour limiter les risques de compétition durant l'association. Nous évoquons également les implications scientifiques et pratiques de ce travail pour de futures études sur ce sujet. / Cereal productivity and quality are subject to two main problems in organic stockless systems: chronic soil nitrogen (N) deficiencies and weed infestation. Legume species as clovers or alfalfas can be used as cover crops and green manures due to their natural ability to fix atmospheric N. Nevertheless, their substitution to cereals in crop rotations is less economically viable without animals to use it. Relay intercropping of legume cover crops (RIL) in winter wheat was expected to simultaneously enrich the soil-plant system in N, cover the soil from the wheat harvest onwards and limit the risk of competition with wheat by delaying the undersowing of legumes in spring. To evaluate the efficiency of RIL, four legume species (Medicago lupulina L., M. sativa L., Trifolium pratense L. and T. repens L.) were undersown at wheat tillering stage, in eight fields organically managed in the Rhône-Alpes region, France. Their effects on weed control, on N enrichment, preservation and restitution in the soil-plant system and on crop performance were observed during the succession of the winter wheat and a spring crop. RIL residues were incorporated in soil in late winter, 9 to 12 weeks before the sowing of spring crops. Our work illustrated the absence of detrimental effect of RIL on wheat grain yield despite the decrease of the grain protein content in one third of the situations. This decrease was mainly observed with M. lupulina and T. pratense as they were the most developed legume species at wheat harvest. Our monitoring of trophic resources (water, N and light) enlightened the competition for soil water and N during the intercropping period. We also showed the efficiency of RIL in the control of weed density from wheat flowering stage onwards and of weed aerial biomass in late autumn. The best weed control was observed with M. lupulina and T. pratense, at wheat harvest, and with T. pratense and T. repens in late autumn, in relation to the highest aerial biomasses observed with these species. Finally, we noted an important proportion of N derived from atmosphere (Ndfa) in legumes' shoots in late autumn (80 to 94%), representing an input of exogenous N in the soil-plant system ranging from 37 to 77 kg Ndfa ha-1. The N enrichment of the system did not increase mineral N lixiviation during winter. However, legume cover crops did not significantly prevent any lixiviation of mineral N relative to the control without cover crop. After their ploughing in, legumes residues returned a part of the accumulated N (+28 to +42 kg ha-1 of mineral N in the first 90 cm of soil compared to the control at the emergence of the spring crop, 12 weeks after ploughing). This restitution of mineral N allowed a significant N enrichment of straw and grains of the spring crop with all previous legumes cover crop species. With maize as spring crop, the grain yield increased by 30%, on average, compared to the control treatment. We finally conclude on the interest of RIL in winter wheat to bring a positive response to the problems of weed control, N deficiency and crop performances in organic crop rotations. We then suggest possible improvements of the RIL system, especially against the competition for trophic resources during the relay intercropping period. We also mention scientific and practical implications of this work for future studies on this subject.