The value of catch crops and organic manures for spring barley in organic arable farming

Field Crops Research 100 (2007) 168–178 www.elsevier.com/locate/fcr

The value of catch crops and organic manures for spring barley in organic arable farming Jørgen E. Olesen a,*, Elly M. Hansen a, Margrethe Askegaard a, Ilse A. Rasmussen b b

a Danish Institute of Agricultural Sciences, Department of Agroecology, P.O. Box 50, DK-8830 Tjele, Denmark Danish Institute of Agricultural Sciences, Department of Integrated Pest Management, Research Centre Flakkebjerg, DK-4200 Slagelse, Denmark

Received 1 November 2005; received in revised form 30 June 2006; accepted 3 July 2006

Abstract The effect of nitrogen (N) supply and weeds on grain yield of spring barley was investigated from 1997 to 2004 in an organic farming crop rotation experiment in Denmark on three different soil types varying from coarse sand to sandy loam. Two experimental factors were included in the experiment in a factorial design: (1) catch crop (with and without), and (2) manure (with and without). The crop rotation included grass-clover as a green manure crop. Animal manure was applied as slurry in rates corresponding to 40% of the N demand of the cereal crops. Application of 50 kg NH4-N ha1 in manure (slurry) increased average barley grain DM yield by 1.0–1.3 Mg DM ha1, whereas the use of catch crops (primarily perennial ryegrass) increased grain DM yield by 0.2–0.4 Mg DM ha1 with the smallest effect on the loamy sand and sandy loam soils and the greatest effect on the coarse sandy soil. Model estimations showed that the average yield reduction from weeds varied from 0.2 to 0.4 Mg DM ha1 depending on weed species and density. The yield effects of N supply were more predictable and less variable than the effects of weed infestation. The infestation level of leaf diseases was low and not a significant source of yield variation. The apparent recovery efficiency of N in grains (N use efficiency, NUE) from NH4-N in applied manure varied from 29 to 38%. The NUE of above-ground N in catch crops sampled in November prior to the spring barley varied from 16 to 52% with the largest value on the coarse sandy soil and the smallest value on the sandy loam soil. A comparison of grain yield levels obtained at the different locations with changes in soil organic matter indicated a NUE of 21–26% for soil N mineralisation, which is smaller than that for the mineral N applied in manure. However, this estimate is uncertain and further studies are needed to quantify differences in NUE from various sources of N. The proportion of perennial weeds in total biomass increased during the experiment, particularly in treatments without manure application. The results show that manure application is a key factor in maintaining good crop yields in arable organic farming on sandy soils, and in securing crops that are sufficiently competitive against perennial weeds. # 2006 Elsevier B.V. All rights reserved. Keywords: Organic farming; Nitrogen; Nitrogen use efficiency; Weeds; Grain yield; Catch crop; Cover crop; Crop residues

1. Introduction The proportion of organic farming in Denmark has increased considerably over the past decade, and the organically farmed area constituted 5.6% of the agricultural area in 2004 (Plantedirektoratet, 2005). Spring barley (Hordeum vulgare L.) was grown on 16% of the area for organic farming in 2004, and constituted 33% of the area grown with cereals. In Denmark, grass-clover pastures are usually established by undersowing the grass-clover in spring barley. This often means

* Corresponding author. Tel.: +45 89991659; fax: +45 89991619. E-mail address: [email protected] (J.E. Olesen). 0378-4290/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.fcr.2006.07.001

that no mechanical weed control is carried out in the barley crop. The productivity of arable crop rotations in organic farming is often low due to lack of nitrogen (N) (Berry et al., 2002). These systems also tend to be less competitive against annual and perennial weeds than organic crop rotations with a stock of ruminant animals due to the greater proportion of cash crops and the small proportion of grass-clover as green manure, which may have long-term consequences for crop production. Some of the problems with N availability and with weed control may be solved through the judicious use of catch crops and animal manure (Barberi, 2002; Thorup-Kristensen et al., 2003). The severity of diseases is often limited in these systems, which for some diseases may be related to a lower susceptibility to disease with decreasing N supply (Jensen and Munk, 1997).

J.E. Olesen et al. / Field Crops Research 100 (2007) 168–178

Several interactions between crop growth factors, e.g. N nutrition, weeds and diseases, are considerably more important in organic farming than in conventional systems, where commercial fertilisers and chemical plant protection agents can be used to optimise production. In organic farming, crop management relies to a great extent on preventive measures such as use of legumes in the crop rotation, catch crops and soil tillage (Lampkin, 1990), and the management of soil fertility is essential in such a system (Stockdale et al., 2002). The N supply in organic farming is provided either directly through manure application or indirectly by addition of organic matter in crop residues, green manure crops and catch crops. In the analysis of experiments with application of fertiliser N the apparent recovery efficiency of applied N is typically taken as a measure of the N use efficiency (NUE) (Cassman et al., 1998). NUE is usually calculated as the difference between fertilised plots and an unfertilised control. However, it may also be calculated as the slope of a regression on crop N uptake (either N in total above-ground biomass or in grain yield) versus applied fertiliser N. These analyses have rarely been used in systems, where organic sources and biological N fixation are the major N inputs (Mosier et al., 2004). The NUE is likely to vary with the source of N supply, with crop management and with local soil and climatic conditions. A better knowledge of NUE under organic farming conditions is a key factor for improving the overall resource use in these systems. However, credible estimates of NUE requires data from long-term field experiments, where the input factors are separated in a statistical design and where realistic approaches to crop and weed management have been applied within each experimental treatment. The long-term approach is needed to analyse transitional effects on soil fertility and influence of changes in management, e.g. from conventional to organic farming (Martini et al., 2004). Such an experiment was initiated at three locations in Denmark in 1997 (Olesen et al., 2000). The objective of the study presented here was to analyse the role of catch crop and manure application in maintaining large and stable yields in spring barley. The observed yields were related to N inputs and to observed weeds and diseases. This enabled the importance of different growth factors to be assessed, and thus provide a basis for suggesting improvements in management at the cropping system level. 2. Materials and methods Spring barley was grown as one of four crops in a crop rotation experiment established in 1997 (Olesen et al., 2000). Results are presented for three locations representing different soil types and climate regions in Denmark. Jyndevad (548540 N, 098080 E) represents a coarse sandy soil (4.5% clay < 2 mm, pH(CaCl2) 5.6) with an average annual rainfall of 964 mm. Foulum (568300 N, 098340 E) is situated on a loamy sand (8.8% clay, pH(CaCl2) 6.0) with an annual rainfall of 704 mm, and Flakkebjerg (558190 N, 118230 E) has a sandy loam (15.5% clay, pH(CaCl2) 7.5) with an annual rainfall of 626 mm. Full details on the design of the crop rotation experiment are given by Olesen et al. (2000). Here, we present results for

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spring barley undersown with grass-clover in a four-course rotation with a pulse crop, spring barley, grass-clover and winter cereal (rotation 2 in Olesen et al., 2000). The pulse crop was a mixture of pea (Pisum sativum L.) and barley in 1997– 2000, lupin (Lupinus angustifolius L.) in 2001, a mixture of lupin and barley at Foulum and Flakkebjerg in 2002–2004, and a mixture of field bean (Vicia faba L.), lupin and barley at Jyndevad in 2002–2004. The cereal and pulse crops were grown for grain harvest. The grass-clover was undersown in the spring barley in spring, and the grass-clover was subsequently managed as a green manure crop with mulching of the cuttings. The grass-clover was followed by winter wheat (Triticum aestivum L.), except for Jyndevad in 2001–2004 where it was followed by winter rye (Secale cereale L.). In 1996, the year before the experimental treatments were started, a spring barley crop undersown with grass-clover was grown at all locations. The crops during the 5 years prior to initiation of the experiment included different arable crops at Jyndevad and Flakkebjerg, and grass-clover and cereal crops at Foulum (Djurhuus and Olesen, 2000). 2.1. Experimental treatments The experimental factors were (i) catch crop, termed ‘CC’ (with (+) and without () catch crop) and (ii) manure, termed ‘M’ (with (+) and without () animal manure applied as slurry). All crops were present every year in two replicates resulting in a total of 32 plots at each location. However, the results presented in this paper are restricted to the yields of the spring barley crop in the four-course rotation, i.e. 8 plots/location and year. The timing of the catch crop and manure treatments are illustrated in Fig. 1. In the catch crop treatment, a mixture of perennial ryegrass (Lolium perenne L.) and four clover species (hop medic Medicago lupulina L., trefoil Lotus corniculatus L., serradella Ornithopus sativus Broth., and subterranean clover Trifolium subterraneum L.) was undersown in the pea/barley. The catch crop undersown in lupin, lupin/barley and bean/lupin/barley was a mixture of ryegrass, chicory (Cichorium intybus L.), black medic (Medicago lupulina L.) and kidney vetch (Anthyllis vulneraria L.). The ryegrass and chicory components dominated the catch crop stand in most years at all locations. The catch crop undersown in winter wheat and winter rye was ryegrass in 1997–2000 and a mixture of ryegrass and chicory in 2001–2004.

Fig. 1. Timing of the indicators of N supply and weeds infestation during growth of the spring barley and the previous pulse crop. Nman is ammoniacal N in manure, Nres the N in the above-ground residues of the previous crop, NNov the N in above-ground weeds and catch crop in November, and Rwgc is the weed and undersown grass-clover as percent of total above-ground dry weight at growth stage 59 in spring barley.

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The plots receiving manure were supplied with anaerobically stored slurry at rates of ammoniacal N corresponding to 40% of the N demand of the cereals. The N demand was based on a Danish national standard (Plantedirektoratet, 1997). Spring barley and winter wheat each received slurry corresponding to a target rate of 50 kg NH4-N ha1. Samples of the manure (slurry) were taken on the application day after thoroughly mixing the slurry in the tank. The total N content in the slurry was determined by the Kjeldahl method, and the content of ammonium-N in the slurry was determined spectrophotometrically (Crooke and Simpson, 1971). Across the 7 years the actual application rate to the spring barley crop was 46, 51 and 52 kg NH4-N ha1 at Jyndevad, Foulum and Flakkebjerg, respectively. The corresponding average application rates of total N were 88, 68 and 69 kg N ha1. Due to decreasing levels of soil potassium (K) at Jyndevad (Askegaard et al., 2003), 50 kg K ha1 in KCl were applied every year in 2001–2004 to spring barley and the pulse crop in the unmanured plots at Jyndevad. Potassium was applied as KCl to avoid interference with application of other nutrients. The gross size of individual plots was 378, 216 and 169 m2 at Jyndevad, Foulum and Flakkebjerg, respectively. Each plot was subdivided to allocate space for grain yield harvest and for destructive sampling during the growing season (Olesen et al., 2000).

Weed harrowing was used where possible to control weeds in cereals and legumes (Rasmussen et al., 2006). No mechanical weed control was performed on the spring barley crop after undersowing of grass-clover. Stubble cultivation (1–8 times) was carried out during autumn in plots without catch crops to control perennial weeds after the lupin or pea/barley crops in all years, except 1997, 2003 and 2004 at Jyndevad and Flakkebjerg, but only in 2002 at Foulum. At Jyndevad in 1999 and 2000, the stubble cultivation was only carried out in one of the replicates. The experiment at Jyndevad was irrigated, where the spring barley crops received 77, 0, 86, 80, 17, 36 and 92 mm of irrigation water in each of the years 1998–2004. The average harvest dates for spring barley were 5th, 19th and 22nd August at Jyndevad, Foulum and Flakkebjerg, respectively. All straw and grass-clover production was incorporated or left on the soil in all treatments. The mean temperature during the main growing season (May and June) was around or above the climatological normal for 1961–1990 in all years. The average rainfall in this period was greatest at Jyndevad and smallest at Flakkebjerg, and the average rainfall was from 9 to 20% greater than the normal for 1961–1990. The driest conditions were experienced at Flakkebjerg in 2001 with only 59 mm, which is 64% of the normal 92 mm of rainfall in May and June. The wettest growing season occurred in 1999 at all three locations with 57–79 mm more than the normal rainfall.

2.2. Crop management 2.3. Measurements and calculations The spring barley varieties used were Bartok in 1998–2000, Ferment in 2001–2003, and a mixture of Cicero, Otira and Punto in 2004. Both Bartok and Ferment are highly resistant to powdery mildew (Ml-O resistance) and barley net blotch, and Otira in the variety mixture has Ml-O resistance. Soils were ploughed in spring before growing spring barley at Jyndevad (average date: 17th March) and Foulum (average date: 2nd April), whereas autumn ploughing was used at Flakkebjerg (average date: 1st December), except for 2000 when ploughing was carried out in spring at Flakkebjerg due to wet conditions in the autumn and winter of 1999. The spring barley was sown at a row distance of 12 cm at Jyndevad and 12.5 cm at Foulum and Flakkebjerg. The average date of sowing was 31st March, 8th and 15th April at Jyndevad, Foulum and Flakkebjerg, respectively. The sowing rate was aimed at a plant density of 300 plants m2. The undersown grass-clover in the spring barley was a mixture of perennial ryegrass (Lolium perenne L., cultivar mixture, 21 kg ha1) and white clover (Trifolium repens L., cv. Milo, 5 kg ha1) at Jyndevad and Foulum in 1998–2000 and at Flakkebjerg in 2000 and 2001. In the other year-site combinations a mixture of perennial ryegrass (19 kg ha1), white clover (4 kg ha1) and red clover (Trifolium pratense L., cv. Rajah, 3 kg ha1) was used. The sowing of grass-clover was delayed, on average, 40, 24 and 4 days at Jyndevad, Foulum and Flakkebjerg, respectively, compared with the sowing of spring barley. At Jyndevad in all years and at Foulum in 2001–2004 sowing of the catch crop into the pulse crop was delayed to permit weed harrowing.

Measurements concerning spring barley are reported from 1998 to 2004. Grain yields were measured at maturity in two sub-plots in each plot using a combine harvester. The size of the harvested plots was 45, 48 and 32 m2 at Jyndevad, Foulum and Flakkebjerg, respectively. Samples of total above-ground biomass were taken in 3 and 4 sub-plots of 0.25 m2 sample areas in each plot at growth stage 59 in spring barley according to the BBCH scale (Lancashire et al., 1991). Each sample was separated into barley, grassclover and weeds for assessing weed pressure. To determine the amount of crop residues returned by the previous crops, samples of total above-ground biomass were taken in 1 m2 sample areas in each plot at growth stage 85, 1–2 weeks before yellow maturity in the pulse and barley crops. Similar samples of total above-ground biomass were taken about 1st November to measure the above-ground biomass of catch crops and weeds. The dry matter content of grains and plant samples were determined after oven drying at 80 8C for 24 h. Total N in the grains and plant samples were determined on finely milled samples from each plot by the Dumas method (Hansen, 1989). Total N was not determined in the plant samples taken at GS 59. The amount of straw and other residues left on the soil after harvest of the previous crops was estimated from the samples of above-ground plant material taken at growth stage 85 by subtracting the grain dry matter yield. The amount of dry matter incorporated by ploughing was estimated from the November sampling of above-ground plant material.

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Per cent cover of powdery mildew (Blumeria graminis (DC.) Speer), barley rust (Puccinia hordei Otth.), net blotch (Pyrenophora teres Drechs.), and scald (Rhychosporium secalis (Oudem.) J.J. Davis) on leaves was recorded 1 week after growth stage 59. Visual assessments were made individually for each of the two top leaves of 10 plants at two positions selected randomly within the plot (Anonymous, 1999). Assessments in 1998–2000 were made in all plots, whereas assessments in 2001–2004 were made in all plots of block 1 only. Soil samples were taken in autumn 1996 and 2004 for analysis of extractable K and P, and total C content. Sixteen soil samples were taken in each plot to 25 cm depth using a 12 mm diameter steel auger, and the samples were mixed, dried and sieved for each plot. The content of K was determined after extracting the soil for 30 min with a solution of 0.5 M ammonium acetate. The content of P was determined after extracting the soil for 30 min with a mixture of 0.5 M NaHCO3 and active carbon. The soil total C was determined using a LECO dry combustion system. 2.4. Statistical analyses The grain yields, grain N uptake and grain N concentrations were analysed using a linear mixed model with fixed effects of location, year, experimental treatments and their interactions. The effect of N input and weeds on grain yield and grain N uptake was analysed at each site separately for all years using the following linear mixed model: Y yb ¼ mþay þa1 N man þa2 N res þ a3 N Nov þ a4 Rwgc þ Eb þ F yb where greek symbols denote fixed effects, E, F and G denote random effects, and a1–a3 denote regression estimates. The indices y and b identify year and block. The random effects were assumed to be independent and normally distributed with zero mean and constant variance. Nman is ammoniacal N in the

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applied manure (kg N ha1), Nres the N in the above-ground residues from the previous crop (kg N ha1), NNov the N in the above-ground plant parts on 1st November (kg N ha1) prior to spring barley, and Rwgc is weed and grass-clover biomass as per cent of total above-ground dry weight at growth stage 59 in spring barley, i.e. Rwgc ¼ 100

W w þ W gc W b þ W w þ W gc

where Wb, Ww and Wgc are above-ground dry weight of spring barley, weeds and grass-clover (g DM m2), respectively. The timing of these regression variables in relation to crop growth is illustrated in Fig. 1. For the regression of grain N and total N uptake, the parameters a1, a2 and a3 can be interpreted as the N use efficiency (apparent recovery efficiency of applied N) of manure, crop residues and catch crops. The components of the models were estimated using the method of residual maximum likelihood (REML, Searle et al., 1992) using the Newton–Raphson algorithm implemented in the MIXED procedure of SAS, statistical analysis system (SAS Institute, 1996).

3. Results 3.1. Soil analyses Available P and K both decreased during the experiment at all locations (Table 1). The decreases were largest in the treatments without manure application. There were low levels of soil K at Jyndevad and low levels of soil P at Flakkebjerg. The soil total C decreased over time at Foulum, but was in general maintained at Jyndevad and Flakkebjerg. Manure application increased the soil C at Jyndevad, and the use of catch crop increased soil C at Foulum and Flakkebjerg.

Table 1 Mean values of the chemical analyses of the top 25 cm of the soils for samples taken in autumn 1996 prior to onset of the experiment and in autumn 2004 at the end of the experimental period Location

Year

Treatment

P (mg kg1 dry soil)

K (mg kg1 dry soil)

Jyndevad

1996 2004 2004 2004 2004

All M +M CC +CC

54 34** 36** 36 34

Foulum

1996 2004 2004 2004 2004

All M +M CC +CC

54 37* 41* 39 38

132 80 90 85 85

22.8 20.8 21.3 20.7* 21.5*

Flakkebjerg

1996 2004 2004 2004 2004

All M +M CC +CC

30 21* 23* 24 20

98 84 83 90 77

9.8 10.1 9.7 9.8 * 10.0*

48 33 ** 39 ** 36 36

Total C (g kg1 dry soil) 12.0 11.9* 12.4* 12.1 12.3

The values for 2004 are shown as mean values for plots without (M) and with (+M) manure application and without (CC) and with (+CC) catch crop. Significance levels for effects of manure and catch crop treatments on changes in soil properties from 1996 to 2004: * 0.05 > P > 0.01; ** 0.01 > P > 0.001; *** 0.001 > P.

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Table 2 Probability values of the statistical tests of the factors manure (M), catch crop (CC) and year (Y) on grain yield characteristics and above-ground biomass at GS 85 at Jyndevad, Foulum and Flakkebjerg, 1998–2004 Factor

Grain

Jyndevad M CC M  CC Y MY CC  Y M  CC  Y Foulum M CC M  CC Y MY CC  Y M  CC  Y Flakkebjerg M CC M  CC Y MY CC  Y M  CC  Y

Above-ground biomass

Yield

N uptake

N concentration

***

***

NS

***

***

**

***

**

*

*

NS

NS

**

NS

*

*

**

**

NS NS

**

***

*

y

*

*

*

*

*

*

NS

NS

NS

NS

NS

***

***

***

***

***

**

**

***

*

**

NS

NS

NS

NS

NS

***

**

***

*

***

NS

NS

y

NS

**

***

***

***

*

**

**

NS

NS

**

y

***

***

***

***

***

***

***

NS

**

NS

NS

**

NS

NS NS

**

***

***

***

***

**

**

***

**

**

*

NS

NS

NS NS NS

*

y NS

NS: P > 0.10; (y): 0.10 > P > 0.05; 0.01 > P > 0.001; (***): 0.001 > P.

Yield

NS (*):

N uptake

0.05 > P > 0.01,

application were 1.31, 1.12 and 0.95 Mg ha1 at Jyndevad, Foulum and Flakkebjerg, respectively. The corresponding grain yield benefits from catch crops were 0.41, 0.22 and 0.24 Mg ha1. Similar effects of manure and catch crops were obtained for grain N uptake. Thus the largest benefits of manure and catch crop were obtained at Jyndevad, whereas there was little difference in benefit between Foulum and Flakkebjerg. There was no significant interaction between manure and catch crops on grain yield and N uptake. However, the effect of manure application and catch crop on grain yield and N uptake varied between years (Table 2 and Figs. 2 and 3). The greatest yields were obtained at Foulum, where there was little change over time. The grain yields at Jyndevad and Flakkebjerg increased over time, in particular with manure application (Fig. 2). There was a positive effect of catch crops on grain yield in most years, except for 2004 at all locations (Fig. 3). The annual variation in grain yields at Foulum appears to be smaller with than without catch crops. Both manure and catch crops increased grain N concentration, but to varying extents at the different locations. There was significant interactions at Jyndevad and Flakkebjerg, such that an increase in grain N concentration was only obtained with a combined use of manure and catch crops (Table 3). 3.3. Above-ground biomass

(**):

3.2. Grain yield and yield components Both manure application and catch crops significantly increased grain yield and N uptake in the grains (Tables 2 and 3). The average grain yield benefits from manure

There were large effects of manure application on total above-ground biomass and N uptake at growth stage 85, whereas catch crops had minor and less significant effects (Tables 2 and 3). The difference between N uptake in total above-ground biomass and N in harvested grains defines the N in crop residues that will be potentially available to subsequent crops. A varying proportion of this residue N content will be present in the undersown grass-clover. The average residue N was 28, 42 and 58 kg N ha1 at Jyndevad, Foulum and Flakkebjerg. There was a large inter-annual variation in residue N (data not shown).

Table 3 Effects of manure (M) and catch crop (CC) on grain yield characteristics and above-ground biomass at growth stage 85 at Jyndevad, Foulum and Flakkebjerg, 1998–2004 Effects

Grain

Above-ground biomass 1

1

1

M

CC

Yield (Mg DM ha )

N uptake (kg N ha )

N concentration (g N kg )

Yield (Mg DM ha1)

Jyndevad   + +

 +  +

1.78 1.96 2.86 3.50

a a b b

24 26 36 49

a a b b

13.3 13.0 12.3 13.8

a a b c

4.58 4.76 6.30 7.28

a a ab b

55 48 61 77

a a b c

Foulum   + +

 +  +

3.03 3.33 4.22 4.36

a a b b

42 47 62 66

a b c d

13.6 14.1 14.6 15.3

a b c d

7.04 7.70 9.12 9.17

a a b b

85 88 101 109

a a b b

Flakkebjerg    + +  + +

2.39 2.69 3.40 3.59

a b c c

34 37 48 54

a a b c

13.9 13.4 14.0 14.9

a b a c

6.92 7.44 9.01 9.40

a a b b

93 93 106 113

a a b b

Within each grain yield component and group, means with different letters are significantly different at 0.05 significant level.

N uptake (kg N ha1)

J.E. Olesen et al. / Field Crops Research 100 (2007) 168–178

Fig. 2. Mean annual grain dry matter yield (a) and grain N yield (b) of spring barley for the three locations without manure (open symbols) and with manure (solid symbols) application. Vertical lines show standard errors.

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91%. The average amount of grass-clover varied from 5 g m2 at Jyndevad to 25 and 34 g m2 at Foulum and Flakkebjerg, respectively. The amount of grass-clover also varied over time and was considerably larger at all locations during the first course (1998–2000) compared with the last course (2001– 2004) of the rotation (Fig. 4). It should be noted that late sowing of grass-clover was used at Jyndevad in all years and in 2001– 2004 at Foulum due to weed harrowing before sowing. There was a tendency in both the first and second course of the rotation for a larger proportion of grass-clover without manure application (Fig. 4). The average amount of weeds varied from 22 g m2 at Flakkebjerg to 30 g m2 at Jyndevad. However, the proportion of weeds in total biomass changed over time in the experiment (Fig. 4). The proportion of annual weeds was thus reduced from the first to the second course of the rotation at Foulum and Flakkebjerg, and there was an increase in perennial weeds from the first to the second course at all locations. During the second course of the rotation, the average proportion of weeds of total biomass was greater without than with manure application and greater with than without catch crops at all locations. The large average amount of annual weeds at Foulum in the first course of the rotation was particularly due to large amounts in 1999 and 2000 (Rasmussen et al., 2006), and because mechanical weed control was used in 2001–2004 at Foulum.

3.4. Weeds and undersown grass-clover At all locations the above-ground DM of undersown grassclover and weeds at growth stage 59 showed large variations with a coefficient of variation (CV) that varied between 52 and

Fig. 3. Mean annual grain dry matter yield (a) and grain N yield (b) of spring barley for the three locations without catch crop (open symbols) and with catch crop (solid symbols). Vertical lines show standard errors.

Fig. 4. Mean proportion of dry weight of annual weeds, perennial weeds and grass-clover of total above-ground dry matter including spring barley at growth stage 59 for the first (1998–2000) and the second (2001–2004) course of the crop rotation at Jyndevad, Foulum and Flakkebjerg. Vertical lines show standard errors of total proportion of weeds and grass-clover in total above-ground biomass. The treatments include with (+CC) and without (CC) catch crop, and with (+M) and without (M) manure application.

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Table 4 Linear regression of DM yield and N uptake in grains and in total above-ground biomass at growth stage 85, 1998–2004, on ammoniacal N in manure (Nman), N in the above-ground residues of the previous crop (Nres), N in above-ground weeds and catch crop in November prior to spring barley (NNov) and weed and undersown grassclover as percent of total above-ground dry weight at growth stage 59 in spring barley (Rwgc) Nman (kg kg1 N)

Nres (kg kg1 N)

NNov (kg kg1 N)

Rwgc (kg ha1)

RMSE (kg ha1)

Grain DM yield (kg ha1) Jyndevad 1604 Foulum 3145 Flakkebjerg 2418

23.0 (2.7) 20.9 (1.5) 16.9 (1.9)

2.3 (2.7) 0.1 (2.4) 3.3 (1.9)

32.1 (5.5) 15.4 (2.7) 9.6 (3.4)

29 (10) 44 (14) 23 (14)

349 288 378

Above-ground DM (kg ha1) Jyndevad 3822 Foulum 6454 Flakkebjerg 6579

39.5 (5.6) 35.2 (3.2) 40.5 (4.4)

5.6 (5.5) 6.0 (5.0) 5.6 (4.5)

57.3 (11.6) 20.2 (5.5) 2.0 (8.0)

29 (20) 5 (28) 19 (33)

924 584 854

Location

Intercept (kg ha1)

Grain N uptake (kg N ha1) Jyndevad 20.7 Foulum 39.2 Flakkebjerg 29.7

0.29 (0.04) 0.38 (0.02) 0.30 (0.03)

0.04 (0.04) 0.03 (0.04) 0.05 (0.03)

0.52 (0.08) 0.33 (0.04) 0.16 (0.06)

0.49 (0.15) 0.67 (0.21) 0.07 (0.23)

5.2 4.3 6.3

Above-ground N uptake (kg N ha1) Jyndevad 45.9 Foulum 76.1 Flakkebjerg 84.3

0.31 (0.08) 0.37 (0.05) 0.36 (0.07)

0.05 (0.08) 0.03 (0.06) 0.06 (0.07)

0.50 (0.16) 0.27 (0.08) 0.03 (0.12)

0.46 (0.27) 0.03 (0.37) 0.45 (0.50)

12.8 8.6 12.0

Standard errors of the estimates are shown in parenthesis. RMSE is the residual mean squared error. The average intercept is shown, but the intercept depended on year in all models.

The dominant weed species were Chenopodium album L., Polygonum convolvulus L., Viola arvensis Murr. and Elymus repens (L.) Gould at Jyndevad, Tripleurospermum inodorum (L.) Schultz Bip., Stellaria media (L.) Vill. and V. arvensis at Foulum, and C. album, P. convolvulus, S. media, T. inodorum and Cirsium arvense (L.) Scop. at Flakkebjerg. 3.5. Leaf diseases Powdery mildew was not recorded on any of the two top leaves. The average disease severity of the two top leaves across all locations was 0.4, 0.9 and 0.2% for barley rust, net blotch and scald, respectively. The average severity of net blotch was 2.4% in 2001–2003 against an average severity of 0.4% for the years 1998–2000. This indicates a greater susceptibility of the cultivar Ferment, which was used in 2001–2003. There was no apparent relationship between disease severity and grain yield components, when these were corrected for cultivar and site differences (data not shown). 3.6. Residues from the previous crop The amount of N in crop residues left by the previous crop was significantly affected by the previous crop species (P = 0.02), whereas there were no differences between locations or between treatments. The mean amount of N in crop residues was 60 kg N ha1 after pea:barley, 71 kg N ha1 after lupin:barley or lupin:bean:barley and 86 kg N ha1 after lupins. The amount of N in above-ground catch crop and weeds measured on 1st November before spring barley was significantly affected by site and catch crop treatment. The mean amounts of N in weeds in treatments without catch crops were 8, 22 and 8 kg N ha1 at Jyndevad, Foulum and

Flakkebjerg, respectively. The corresponding amounts of N in biomass in treatments with catch crops were 21, 37 and 27 kg N ha1. 3.7. Yields effects of N and weeds The regression analyses showed that grain DM yield and grain N uptake were significantly increased by manure N (Nman), above-ground N in November (NNov) and by the proportion of weeds and grass-clover in the spring barley (Rwgc) (Table 4), whereas the N in crop residues (Nres) from the previous crop did not affect grain yields significantly, except at Flakkebjerg. The total above-ground dry matter and the total above-ground N uptake was also significantly affected by Nman and NNov (except at Flakkebjerg), but not by Rwgc and by Nres. Both N supply and weeds in the crop had a clearer effect on grain yields than on total above-ground biomass resulting in an increased harvest index (HI) with greater N supply and a decline in HI with increasing proportion of weeds and grassclover in the biomass (Fig. 5). The apparent recovery efficiency in grains of manure NH4-N (NUE) can be estimated from the regression analyses as the slope of the response of grain N uptake to Nman, and this varied from 29% at Jyndevad to 38% at Foulum (Table 4). The corresponding utilisation efficiencies of NNov varied considerably between locations from 16% at Flakkebjerg to 52% at Jyndevad. The estimates of NUE based on total above-ground N uptake are unreliable, since this biomass contained a variable proportion of clover, where biological N fixation also contributed to the N uptake. Weeds and undersown grass-clover had a significant impact on grain yield and N uptake at Jyndevad and Foulum, but a more uncertain effect at Flakkebjerg (Table 4). The impact of Rwgc was approximately 50% larger at Foulum compared with

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Fig. 5. Grain yield (a, b), above-ground dry matter at growth stage 85 (c, d) and harvest index (e, f) in response to the sum of N applied as ammoniacal N in manure and the above-ground N measured in November (a, c, e) and the proportion of weed and grass-clover of total biomass at growth stage 59 (b, d, f) for Jyndevad (*), Foulum (*) and Flakkebjerg (~).

Jyndevad. This was mainly attributed to effects of weeds, as replacing weed and undersown grass-clover DM with weed DM in the regression equation resulted in similar effects, whereas a significant effect of undersown grass-clover DM on yield was obtained at Foulum only (data not shown). Rwgc was negatively correlated with NNov and with Nres at all locations (data not shown). However, the only significant correlation was obtained at Jyndevad between Rwgc and NNov (r = 0.41, P = 0.002). The residuals from the regression analyses in Table 4 were not significantly correlated with any of the soil analyses (P, K and total C). 4. Discussion 4.1. Site and year effects There was large site differences in grain DM and N yields with the largest yields being obtained at Foulum. Yields in the manured treatments at Jyndevad and Flakkebjerg increased over time, whereas there was little overall change at Foulum (Fig. 2). These differences probably reflect the differences in initial soil fertility of the different locations. The cropping history with a large proportion of grass-clover in the rotation

prior to the onset of the experiment had pre-conditioned a greater soil organic matter level at Foulum (Djurhuus and Olesen, 2000), whereas a large proportion of cereals and other cash crops at Jyndevad and Flakkebjerg had resulted in low initial soil organic matter levels (Table 1). Soils with a larger initial soil organic matter content will have a more rapid decline in soil organic matter under otherwise similar conditions (Bellamy et al., 2005), and this may deliver some of the N supply to the crops. The increase in grain N uptake at Jyndevad and Flakkebjerg over the experimental period corresponded to about 5– 15 kg N ha1 depending on site and manure application (Fig. 2). Johnston et al. (1994) found that an additional year of a grass-clover ley that was incorporated 2 years prior to winter wheat gave an increase of 10–20 kg N ha1 in grain N uptake of the winter wheat. It is thus likely that the larger grain yields during the second compared with the first course of the rotation was a result of the introduction of a grass-clover green manure crop in the rotation at these two locations. There was a greater increase in grain yields over time in the manure compared with the unmanured treatments at Jyndevad (Fig. 2). This was linked with a much larger proportion of weeds and grass-clover in the unmanured treatments (Fig. 4). However, using the regression equations in Table 4, these

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differences in proportion of weeds should only result in a yield reduction of about 0.3 Mg DM ha1, whereas the observed change in yield response to manure application over time was about 1.0 Mg DM ha1. This suggests that other mechanisms linked to manure application may be responsible for the differences. The sandy soil at Jyndevad is known to be highly prone to leaching of nutrients (Askegaard et al., 2003, 2005), and despite the addition of KCl to the unmanured treatments, other nutrients besides K and N may have reduced crop growth in the unmanured treatments. The yield benefits in spring barley from catch crops varied considerably between years, and there was a clear tendency for a smaller inter-annual yield variation in the treatments with catch crops, in particular at Foulum (Fig. 3). This may be related to a smaller variation in soil N supply, because the rainfall during winter and the associated N leaching is less important in systems with catch crops (Thorup-Kristensen et al., 2003). 4.2. N use efficiency The apparent recovery efficiency of applied NH4-N (NUE) varied from 29 to 38% (Table 4), which is similar to the NUE of 33% found for spring barley in mineral fertiliser trials in Denmark (Petersen and Djurhuus, 2004) and found for fertiliser application in global cereal production (Raun and Johnson, 1999). The slurry applied had a relatively large content of NH4N as a proportion of total N (52–75%), and the NUE of applied total N consequently varied from 15 to 29%. In organic farming stacked or composted farmyard manure is frequently used, where the mineral N content is often smaller than in slurry and where the short-term NUE therefore may also be smaller. The N in above-ground residues of the previous crop did not affect yield and N uptake of the barley crop at Jyndevad and Foulum, whereas there was a positive effect corresponding to a NUE of 12% at Flakkebjerg (Table 4). At Jyndevad and Foulum, N leaching and N uptake of the catch crops probably masked the effects of the residues of the previous crop. Due to climate and soil differences nitrate leaching is greatest at Jyndevad and smallest at Flakkebjerg, which typically reduces soil mineral N in spring to very low values at Jyndevad (Hansen and Djurhuus, 1996), whereas the greater spring soil mineral N contents at Flakkebjerg contributes to the crop N supply. It has often been observed that legumes, in contrast to cereals, have a beneficial effect on grain yield of cereal crops, which are subsequently grown on the same soil (Rowland et al., 1988; Senaratne and Hardarson, 1988), and that grain yields are larger after lupin than after pea (Francis et al., 1994; Jensen et al., 2004). However, this effect was not evident in the current experiment since there was no change in yields during the period 2001–2004, when the previous crop changed from pea/ barley (2001) to lupin (2002) and lupin/barley (2003–2004). The NUE for above-ground biomass in November prior to spring barley varied from 16 to 52%, which shows a considerably larger variation among locations than for NUE of manure. This NUE from weed and catch crop biomass is affected by both mineralisation of the residues and by the effect of the catch crop

on N leaching and thus on pre-emptive competition (ThorupKristensen and Nielsen, 1998). The pre-emptive competition is likely to be small at Jyndevad, where the rooting depth is low and winter rainfall is large. Also the good aeration of the sandy soil at Jyndevad may increase N mineralisation. The observed trends in NUE at the three locations are thus consistent with the site differences in climate and soil conditions. The large estimate of NUE from catch crops at Jyndevad may have been influenced at least partly by the fact that stubble cultivation was often used in the treatments without catch crops to control E. repens, and this stubble cultivation will increase N leaching (Askegaard et al., 2005) and consequently reduce N yield in the treatments without catch crops. The estimates of NUE of catch crops did not account for the uptake of N in roots of the weeds and catch crops, and this may reduce the NUE to a value close to the 15–20% suggested by Christensen (2004). There was very little clover in the catch crop mixture, and clover N fixation is therefore not considered to have contributed significantly to the N supply of the subsequent barley. Despite the inclusion of a grass-clover crop in the rotation, there was a decline in total soil C at Foulum during the experiment, which assuming a constant C/N ratio of 12 for the active soil organic matter corresponds to an average annual decline of 81 and 45 kg N ha1 in the top 25 cm of the soil for treatments without and with catch crops, respectively. The average annual decline at Foulum in soil organic matter thus corresponded to 63 kg N ha1, whereas the corresponding changes at Jyndevad and Flakkebjerg showed an increase of 6 and 5 kg N ha1, respectively. Over the entire experimental period, the average grain N uptake at Foulum was 22 and 11 kg N ha1 greater than the corresponding values for Jyndevad and Flakkebjerg, respectively. The average atmospheric N deposition in the agricultural areas around the experimental locations varied from about 20 kg N ha1 at Jyndevad and Foulum to about 15 kg N ha1 at Flakkebjerg (Ellermann et al., 2001). Since these differences are small and since similar crop rotations were used at the different locations, the larger grain N uptake at Foulum may be attributed to the reduction in soil organic matter. The NUE for N mineralised from soil organic matter may tentatively be calculated by taking the average difference in grain N uptake between Foulum and the two other locations and dividing by the corresponding observed annual difference in N supply, which was taken as the calculated mean annual change in soil organic N plus the mean annual N deposition. Using this methodology, the NUE for mineralised soil organic N was estimated at 21 and 26% for treatments without and with catch crops, respectively. This is lower than the estimated NUE for ammonium-N in applied manure, but consistent with the fact that mineralisation occurs over a considerably longer part of the year than crop N uptake and that cereal crops at most takes up about 50% of applied mineral N (Christensen, 2004). However, this estimate of NUE is uncertain, and there is a need for improved estimates of NUE from different sources of organic N, since this is a major source of crop N supply in organic farming.

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4.3. Weeds The weed flora changed considerably during the course of the experiment (Fig. 4). The proportion of annual weeds and grass-clover in the total biomass was reduced, whereas that of perennial weeds increased at all locations. At Foulum some of this change can by explained by a change in management from the first to the second crop rotation, where weed harrowing to control annual weeds was carried out in 2001–2004, but not in 1998–2000. The reduction in the proportion of annual weeds in the biomass over time may possibly be ascribed to the increase in soil fertility and resulting yield increases, in particular at Jyndevad and Flakkebjerg. There was no decline in the proportion of weeds at Jyndevad in the treatments without manure application, where yields remained low. This indicates that annual weeds do not constitute a major problem for spring barley in this particular organic crop rotation, provided yields can be maintained through soil fertility management and manure application. The increasing proportion of perennial weeds in the biomass emphasises the importance of considering crop and weed management in organic crop rotations in an extended time domain. There is thus a need in this particular crop rotation to focus on long-term strategies for controlling perennial weeds, in particular E. repens and C. arvense. There was particularly a large increase in weed biomass in the treatments with catch crop (Fig. 4), possibly because mechanical weed control was less frequently applied in the autumn in these treatments, where any stubble cultivation to control the perennial weeds will also damage the undersown catch crop. The yield response to weeds was greater at Foulum than at the two other locations, which is probably related to the composition of the weed flora. At Foulum there was a large population of T. inodorum, which can severely reduce yields even at low population densities (Welsh et al., 1999). 4.4. Improving grain yield and quality Manure application at the rates used in the experiment increased grain yield by 1.0–1.3 Mg ha1, and the effect of N in catch crops increased the yields by 0.2–0.4 Mg ha1. Using the regression model in Table 4, the average proportion of weeds and grass-clover measured at the three locations gave an estimated grain yield reductions of 0.2–0.4 Mg ha1 with the largest yield reductions at Foulum. However, because of the large variation in proportion of weeds and grass-clover in the biomass (Fig. 5), there was also a large variation in estimated yield reduction from weeds. This shows that the yield effects of N supply are more predictable and less variable than the effects of weed infestation (Jørnsga˚rd et al., 1996). The greatest N concentration in grain was achieved with concurrent use of manure and catch crops. The use of catch crops increased grain N concentration more than application of manure, even though the effect of catch crops on grain yield was small (Table 4). This implies a different timing of N uptake in catch crop and manure treatments. Late fertiliser application

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is effective in increasing grain N concentration (Gooding and Davies, 1992; Petersen, 2004). Therefore, the observed effect might be due to late mineralisation of N in the catch crop, because the main part of N in plant material has to be mineralised before it is available to the crop, while 52–75% of the N in the manure was in the ammonium form, which is readily available. The grain yields of spring barley were primarily affected by manure application and by the use of grass-clover green manure in the crop rotation, whereas a non-legume catch crop prior to the spring barley had only a very modest effect on barley grain yields. However, catch crops were found to significantly reduce nitrate leaching in this crop rotation (Askegaard et al., 2005), with the largest reductions in nitrate leaching at Jyndevad. The yield benefit of manure application increased over time at Jyndevad, which shows that the long-term sustainability of these arable organic cropping systems depends on external nutrient inputs, in particular on sandy soils (Wivstad et al., 2005). Catch crops significantly increased soil organic matter levels, and this may in the long-term increase the NUE at cropping system level. However, the improvement of the NUE should be seen in the context of the need to improve control of perennial weeds in this cropping system. 5. Conclusions Good and stable yields of 4 Mg DM ha1 or more could be obtained at all locations in treatments with manure application after an initial transition phase, where the grass-clover was introduced as a green-manure crop in the crop rotation at all locations. On the sandy soil at Jyndevad this increase in grain yield over time was largely due to the application of manure, which indicates that other nutrients besides N have become limiting at this site. The proportion of perennial weeds in total biomass increased during the experiment, in particular in treatments without manure application, and to some extent also in treatments with catch crops. The leaf diseases levels were low and not a significant source of yield variation. The N use efficiency (NUE) estimated for ammonium-N in the applied manure was similar to that of mineral fertilisers, whereas the tentative estimates of NUE for mineralisation of soil organic matter was about a third lower. The NUE for N in catch crops varied considerably with the greatest values on the sandy soil and the smallest values on the sandy loam soil, indicating that the largest yield effects of catch crops are obtained on soils with the smallest soil N retention. The estimates of NUE for mineralised soil organic matter and for catch crops are uncertain and further studies to quantify these are warranted, since these sources of N play an essential role in organic farming. Acknowledgements The project was funded by the Directorate for Development under the Danish Ministry of Food, Agriculture and Fisheries. The project was an integral part of the activities under Danish Research Centre for Organic Farming.

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