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Crop yields

The spreading of OWPs increases crop yields over time. The effects are cumulative and add up over the long term. In the case of maize, after 7 spreadings the OWPs achieve the same yields as in plots receiving optimal nitrogen fertilisation. These results indicate that OWPs could potentially act as a substitute for mineral fertilisation while also guaranteeing the same yields. The effects are less pronounced on wheat. The yields, which are on the rise, have not yet exceeded those of mineral fertilisation.

Crop yields after 12 years

The average crop yield at the start of the test was 101.5 and 84.8 t of maize per ha for grain and 104.5 and 88.8 t/ha for stalks.
The yields in 2010, after 7 spreadings, are shown in Figures 1 and 2.
The spreading of OWPs has a significant positive effect on yield compared with the control in the absence of nitrogen fertilisation (around 52% higher). These effects are less pronounced in the case of additional optimal nitrogen fertilisation. In this case, only the manure treatment shows a significantly higher yield than the control plot receiving nitrogen. The other treatments show higher yields, but not significantly so.
Meanwhile, the input of ROPs without nitrogen fertilisation achieves yields comparable to those obtained in the control plots receiving nitrogen. The cumulative effects of the 7 spreadings potentially enable nitrogen fertilisation to be abandoned.

Figure 1. Grain yields expressed in quintals of DM/ha for maize in 2010 (solid shading for crops receiving optimal nigrogen fertilisation, hatching for crops not receiving additional fertilisation). The differences observed between the control treatment and the OWP treatments are significant in the absence of nitrogen fertilisation; with nitrogen fertilisation, only the manue treatment shows a significant difference compared with the control (Mann-Whitney non-parametric test).

Figure 2. Maize stem and leaf yields expressed in quintals of DM/ha for maize in 2010 (solid shading for crops receiving optimal nitrogen fertilisation, hatching for crops not receiving additional fertilisation). The differences observed between the control treatment and the OWP treatments are significant in the absence of nitrogen fertilisation; with nitrogen fertilisation, only the manure treatment shows a significant difference compared with the control (Mann-Whitney non-parametric test).

Change in the crop yield following successive spreadings

The positive effect of spreadings on the yield is gradual over time. It results from the effects of successive spreadings, as shown by the comparison of yields between plots only receiving ROPs and the control plots receiving nitrogen (Figure 3).
The figure represents the relative yield from OWP treatments alone, compared with the control + optimal nitrogen fertilisation treatment. Until 2005, this relative yield is below 1, which means that the input of OWPs alone does not equal the yields obtained with nitrogen fertilisation alone. By contrast, from 2005 in the case of maize, yields with OWPs are higher than those with nitrogen fertilisation. For wheat and barley (2007), the relative yields remain below 1. However, they have shown a continued increase since the start of the test, which may indicate that over the long term, yields with OWPs will exceed yields with mineral fertilisation alone.
These results also show the study's importance over the long term in the analysis of OWP spreading.

Figure 3. Change in relative yields from OWP treatments without nitrogen fertilisation compared with the control treatment with optimal nitrogen fertilisation.