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The responses of wheat to various rates of N fertiliser were compared following field peas (PW) or wheat (WW) in the previous year. Seventeen trials were carried out at 5 sites between 1986 and 1991. The trials were on medium- and fine-textured soils (clay loams or shallow duplex soils). The overall grain yield of PW appeared greater than WW in 11 trials [was significantly greater in 9 (P<0.05)], and did not appear different in 6 trials. When no N was applied the yield advantage of PW was 41% (PW 1.91 t/ha cf. WW 1.37 t/ha). Quadratic response curves were fitted to all yield data. Rotation x N rate interaction was significant (P<0.05) in 10 comparisons. In 5 trials, while there was a yield increase to N fertiliser with WW, the yields decreased with PW. In 3 trials while there was an increase with WW there was no response with PW or a reduction at higher rates of N. In the remaining 2 trials there were responses with both PW and WW, but this was greater for WW. The response curves in these 10 trials either converged and met, indicating that the difference between rotations was due to N availability, or converged but did not meet, indicating that N was important but did not explain the whole difference. Where there was no interaction between rotation and N rate the response curves were parallel. The type of response could not be predicted. It was not profitable to apply N fertiliser to wheat in the PW rotation in 11 of the 17 trials. The average yield advantage of PW over WW, in the absence of N was 540 kg/ha, while there was an average increase of 1.7% grain protein.

The responses of wheat to various rates of application of nitrogen fertiliser were compared following lupins (WL) or wheat (WW) in the previous year. Results covered 10 sites and trials were carried out between 1979 and 1984. giving 26 site-year comparisons. The trials were on sandy or duplex (sand over clay) soils. The overall yield of WL was greater than WW on 21 occasions (significant in 10 cases, P < 0.05), less than WW on 2 occasions (both significantly) and there was no difference on 3 occasions. When no nitrogen was applied the advantage of WL was 41% (WL 1.20 t/ha and WW 0.85 t/ha). The response to nitrogen differed between trials; over all trials a quadratic model best described the responses. There were significant interactions between rotation and nitrogen rate in 10 comparisons. In 4 of these cases, response to nitrogen with WW was greater than with WL and these response curves approached I another but did not meet, indicating that both residual nitrogen from the lupins and some other benefit from the lupins were involved. In 5 cases the greater response on WW resulted in convergence with the WL response curve, suggesting that residual nitrogen explained all of the benefit of lupins. In these cases the amount ofnitrogen fertiliser required to bring the yield of WW to that of the WL without nitrogen ranged from 20 to 47 kg N/ha, with a mean of 37 kg N/ha. Parallel response curves were observed In 12 cases, indicating involvement of factors other than residual nitrogen in the response to lupins, e.g. disease cleaning effects or slow release of nitrogen throughout the season. The type of response could not be related to particular site characteristics. When quadratic coefficients were averaged to give 'average response curves', similar rates of applied nitrogen were required for maximum yields in both WW and WL, and the maximum yields were 1.23 t/ha for WW and 1.41 t/ha for WL. The average advantage of WL over WW, in the absence of nitrogen fertiliser. was 350 kg/ha.

Field trials at Beverley (19911, Salmon Gums (1991; 2 sites) and Merredin (1992; 2 sites), each with 5 rates of nitrogen (N) and 3 levels of weed control, were used to investigate the effect of weeds and N on wheat grain yield and protein concentration during 1991 and 1992. Weeds in the study were grasses (G) and broadleaf (BL). Weeds reduced both vegetative dry matter yield and grain yield of wheat at all sites except for dry matter at Merredin (BL). Nitrogen fertiliser increased wheat dry matter yield at all sites. Nitrogen increased wheat grain yield at Beverley and Merredin (BL), but decreased yield at both Salmon Gums sites in 1991. Nitrogen fertiliser increased grain protein concentration at all 5 sites-at all rates for 3 sites [Salmon Gums (G) and (BL) and Merredin (G)] and at rates of 69 kg N/ha or more at the other 2 sites [Beverley and Merredin (BL)]. However, the effect of weeds on grain protein varied across sites. At Merredin (G) protein concentration was higher where there was no weed control, possibly due to competition for soil moisture by the greater weed burden. At Salmon Gums (G), grain protein concentration was greater when weeds were controlled than in the presence of weeds, probably due to competition for N between crop and weeds. In the other 3 trials, there was no effect of weeds on grain protein. The effect of weeds on grain protein appears complex and depends on competition between crop and weeds for N and for water at the end of the season, and the interaction between the two.