Development of N and P flows at soil level

results of the soil component are shown in Fig. 1e, f. Soil N surplus on the pilot farms decreased from 200 kg per ha in 1998 to 125 kg per ha in 2004, while in the remainder of the period it remained fairly constant. In the ‘national average’ soil N surplus decreased till 2006, and in the remainder of the period it remained fairly constant. On average, levels of N surpluses on pilot farms and ‘national aver- age’ were similar, except the first two years, resulting in similar N use efficiency. however, despite equal average soil N surplus and soil N use efficiency, yields of

grassland (dry matter and N) and N use efficiency of grassland on the pilot farms

were higher than the ‘national average’ (chapter 4). Moreover, the level of soil N surplus on the pilot farms was realized with higher N input to the soil (organic manure, chemical fertilizers, N fixation, atmospheric deposition) and higher N output (crop N yield) than in the ‘national average’. roberts (2008) and powell et al. (2010) pointed out that N use efficiencies in dairy farming systems follow the ‘law of diminishing return’ in low input systems, i.e. N use efficiency is the highest at the lowest N input. Despite this ‘law’ soil N use efficiencies on the pilot farms were similar than those on the ‘national average’ but with a higher N input to the soil. the equal soil N use efficiency and higher N use efficiency on grassland on the pilot farms means that N use efficiency from other crops on the farms was lower than the ‘national average’. For p, soil surplus on the pilot farms was on average lower and p use efficiency higher than the ‘national average’. From 2015, the Netherlands promised the eU a p-equilibrium strategy on soils. Since 2004, the levels of soil p surplus on the pilot farms are already low and close to equilibrium.

2.5 Implemented measures

the development in N and p flows on the pilot farms is the result of adapted and implemented measures. the transition in nutrient management on the pilot farms is the result of the prototyping process described in chapter 2. It is attrac- tive because it allows active participation of farmers and other stakeholders in the whole process from analysis to monitoring and evaluation (Fig. 2, chapter 2). ‘cows & Opportunities’ is the practice-oriented follow-up of experimental dairy farm ‘De Marke. ‘De Marke’ demonstrates, among other things, that by taking a coherent set of simple measures at farm level, the input of nutrients can be drastically reduced (aarts, 2000; hilhorst et al., 2001; Verloop, 2013). an impor- tant step in the prototyping process was the construction of a Farm Development plan (FDp), with suggested measures to realize the targets and improved nutrient management (see table 4, chapter 2). almost all suggested measures were already tested on ‘De Marke’, but each measure had a farm-specific interpretation and a specific effect because of the differences in agro-ecological conditions among

farms (e.g. soil type). So, not ‘copy and paste’ but adjusted measures were used which fit on a farm taking into account the local circumstances but also the profes- sional skills and entrepreneurship of the farmer(s). Decisions of farmers to adapt to changing conditions are not only governed by economic considerations, but also by their social and psychological characteristics (chapter 3). From the suggested measures to reduce nutrient losses, the most attractive and effective adapted and implemented measures were:

1 Reducing the use of chemical fertilizers.

an easily and quickly implemented measure was to lower the nutrient applica- tion rate to crops by reducing the use of chemical fertilizers, both for N and p (Figs 2 and 3, chapter 3). reducing the use of chemical fertilizer contributed most to the reduction in nutrient surpluses in the first five project years, the period in which legislation was controlled with permitted nutrient surpluses (chapter 4). On most farms, use of chemical p fertilizer was less than 5 kg per ha. In 2002, N use of chemical fertilizer was the lowest (chapter 3 and 4), followed by an increase till 2004 and then it was stable in the remainder of the period (chapter 4). From 2006 onwards, the use of chemical fertilizers was con- trolled by the crop-specific application standards.

2 Less purchased feed by lowering crude protein (CP) and P-content of the ration. the second-best measure to reduce nutrient surpluses in the first five project years was the reduction of nutrients in imported feed (chapter 3). Lowering the N import with feed was possible due to a decrease of the cp content in the ration, especially during the summer by shortening grazing time (see next meas- ure) and supplementary stall-feeding to balance the indigested protein/energy ratio (chapter 3). the reduction of the cp content in the ration was especially realized in the first part of the project, lowering the p-content of the ration was an important issue during the last few years (see Fig. 1d and the explanation). Initially, the project had the ability to be more independent from imported feed and fertilizers by using less chemical fertilizers and by a better use of home- produced manure and higher crop yields, to be more self-sufficient in feeding animals. a reduction in the use of chemical fertilizers was realized but not a sig- nificant increase of crop yields, especially from grassland (chapter 4). the yields of grassland were still higher than on the ‘national average’ but the expected increase was not realized. therefore, the research in ‘cows & Opportunities’ should focused on analyzing the crop production to find ways to increase crop production with the same amount in use of fertilizers (organic, chemical and/or N fixation by clover).

3 Reducing grazing time.

the pros and cons of grazing are well described in chapters 3, 4 and 5. the grazing time on the pilot farms was reduced resulting in a better environmental

performance in nutrient surpluses (chapter 3), dry matter yields in grassland (chapter 4) and nitrate leaching (chapter 5). the last few years, grazing time was further reduced due to the increase of the farm size (table 1) which causes logistic and especially labour problems to sustain a extensive grazing regime. also the introduction of milking robots (on 4 pilot farms) reduced grazing time. 4 Reducing the relative number of young stock.

this measure was initiated because of a highly inefficient component in the nutrient balance, i.e., each additional heifer (young stock older than 1 year) increases the farm nutrient surpluses by 51 kg N and 7 kg p (Mourits et al., 2000). Young stock management is important because of selection for replace- ment. replacing a milking cow requires an ‘investment’ in nutrients and energy intake (aarts et al., 1999). a few pilot farms chose for off-farm rearing of young stock, which in case of nutrient management for that farm is very efficient. On the other hand, raising or fattening young stock on other farms is a case of shift- ing this ‘investment’ to elsewhere.

5 Optimizing use of home-produced organic manure.

at the start of the project in 1998 it was common in Dutch dairy farming to export some home-produced animal manure and/or import pig manure. export of animal manure during that time was not associated with many costs. Moreover, animal manure was not valued as a useful fertilizer. results of ‘De Marke’ showed, among other things, that through a better use of organic manure, less additional chemical fertilizer is needed to sustain the same crop yields or even to increase crop yields (hilhorst et al., 2001; aarts, 2000). therefore, optimizing the use of home-produced organic manure by keeping the manure on the farm (as much as possible), optimizing the distribution to crops, and optimizing the time and method of application was suggested. From 2006 onwards, the use of home-produced manure was restricted due to the change in legislation from permitted nutrient surpluses to crop-specific applica- tion standards. For farms with a derogation (more than 70% of the land use on a farm is grassland) maximum use of animal manure for all land was 250 kg N per ha (applied manure and excreta during grazing) instead of 170 kg N per ha (Schröder & Neeteson, 2008). hence, on average, there was no change in the total use of applied manure to grassland compared to the period with permitted nutrient surpluses (MINaS), but there was a shift from excreta during grazing to applied manure due to the reduced grazing time (chapter 4). On the other hand the pilot farms have to export more manure due to the increasing production intensity (more cows per ha). In general, most adopted changes in optimizing use of animal manure were: (1) lowering manure application to maize land and shift to grassland (chapter 4), (2) postponing the first application from February 1 to at least two weeks later (Oenema et al, 2008), and (3) a shift of a part of the manure application for the first grass cut (applied in March) to the second grass

cut (applied in May) (chapter 4). Lowering manure application to maize land was proven to be promising in reducing nitrate leaching (chapter 5). postponing the first manure application has till now not been proved as a measure to increase nutrient use efficiency and reducing the risk for nitrate leaching. Yet, it has been argued by aarts et al. (2000) and Verloop et al. (2006) that postponing the first manure application to mid-March is an effective measure in improving nutri- ent management. also, it has not shown that optimizing the distribution of the application of manure during the growing season is an important strategy to improve nutrient use efficiency and/or increasing crop yields. More research is needed on to the effect on crop yields and environmental impact of fine-tuning the distribution of manure (and chemical fertilizers) among crops and fields. 6 Catch crop after maize.

Since the beginning of ‘cows & Opportunities’, all pilot farms on sandy soil have been using a catch crop after maize. From 2006 onwards, growing a catch crop on sandy soil has been compulsory in the Netherlands for all maize land on sandy soils. Management of a catch crop on the pilot farms was diverse: using different types of catch crops (e.g. Italian rye grass, winter rye), fertilization of catch crop in February/March (yes or no), harvest of catch crop (yes or no), graz- ing of the catch crop (yes or no). Some farmers tried to adopt the system which was successfully applied on ‘De Marke’ where the catch crop is sown between the maize rows in early summer instead of after the maize harvest and is neither fertilized nor harvested in the following spring. It was argued that managing the catch crop this way will lead to a better catch crop with high biomass capable of catching N during the winter and spring (Verloop et al., 2006). however, in practice (on the pilot farms) problems occurred during the timing of sowing the catch crop (suitability of the machinery and/or contractor) and the risk of maize yield loss due to the competition between the catch crop and maize seedlings for light, nutrients, and water when seeding the catch crop too early. the applied strategies for catch crops on the pilot farms still lead to high levels of nitrate concentrations in groundwater below maize fields (chapter 5). For example, our analysis demonstrates that a fertilized catch crop increases nitrate concentration with 33 mg l-1_{. Because of the high nitrate concentrations below maize fields in}

the Netherlands (Willems et al., 2012), ‘cows & Opportunities’ initiated together with stakeholders demonstration fields, to promote the ‘De Marke’ strategy for management of catch crops. this strategy is not only efficient with respect to nitrate leaching but has also advantages for organic matter dynamics in maize fields on sandy soils. We assumed that with sowing a catch crop between maize rows in early summer instead of after the maize harvest provides higher above and below ground crop yield and increases or sustains the organic matter con- tent in the soil after destroying and plowing the (catch) grass sod before sowing a new (maize) crop.