Arable Crop Division, Bioforsk, The Norwegian Institute for Agricultural and Environmental Research, Landvik, N-4886 Grimstad, Norway

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1 Acta Agriculturae Scandinavica, Section B Soil & Plant Science, 2014 Vol. 64, No. 6, , ORIGINAL ARTICLE Cover crop stubble and straw management in seed production of timothy (Phleum pratense L.), meadow fescue (Festuca pratensis Huds.), and red clover (Trifolium pratense L.) L.T. Havstad* Arable Crop Division, Bioforsk, The Norwegian Institute for Agricultural and Environmental Research, Landvik, N-4886 Grimstad, Norway (Received 26 March 2014; accepted 23 May 2014) The market for cereal straw has diminished in many seed-production areas due to less livestock. Seed growers, therefore, want to chop and return the straw. The objective was to investigate the effect of cover crop stubble height and straw management on vegetative growth in autumn and the following year s seed yield of timothy, meadow fescue, and red clover. The plan included two stubble heights (5 10 cm and cm) combined with four different straw and stubble managements, (A) straw removal, (B) straw chopping at combining, (C) flailchopping with tractor after combining, and (D) straw chopping in two operations (B + C). The cover crop was either spring barley or spring wheat. On average for all trials and plots where the straw was removed, by the end of the sowing year, a tendency of 9% more tillers of meadow fescue, 29% more tillers of timothy, and 27% more plants of red clover was produced on plots with short than with long stubble. In the following year, the average seed yields were not significantly affected by stubble height in meadow fescue and timothy, but 11% higher on plots with short than with long stubble in red clover. This indicates that red clover is more vulnerable to the shade created by a long stubble than the two grass species. When the stubble was short, straw chopping produced equally high seed yields as straw removal in all species. For seed growers without their own husbandry or not receiving any payment for the cover crop straw, chopping of straw is, therefore, recommended as an environmentally sound and cost-saving alternative to straw removal. The straw must be chopped with sharp knives as finely as possible and spread evenly on the soil surface to facilitate rapid decomposition and rapid penetration by the undersown seed crop. Keywords: flail-chopping; light penetration; spring barley; spring wheat Introduction Seed crops of timothy (Phleum pratense L.), meadow fescue (Festuca pratensis Huds.), and red clover (Trifolium pratense L.) are usually undersown in spring barley or spring wheat cover crops. Soon after harvest of the cover crop, most growers bale and remove the straw from the field in order to create optimal conditions for the undersown grass or clover plants. This is in concurrence with Danish experiments in meadow fescue, showing a seed yield reduction of 10 15% on plots where the cereal straw was chopped at cover crop harvest compared to plots where the straw was removed immediately after harvest (Nordestgaard 1982). Neither stubble height at combining nor straw DM yields of cover crop was reported, but the cover crop was an old variety of barley with long culms. Today s varieties of barley and spring wheat are considerably shorter, more resistant to lodging, and produce less straw than varieties used in the 1970s (e.g. Feil 1992). Also the more common use of plant growth regulators in cereal production during the last decades has contributed to shorter stems (Rademacher 2009), thus lowering the straw yields. No updated information is available on how chopping the straw of today s cultivars, either with a straw chopper at the back of the combiner during harvest or with a tractormounted flail chopper after harvest, affect the * lars.havstad@bioforsk.no 2014 Taylor & Francis

2 548 L.T. Havstad following year s seed yield of timothy, meadow fescue, or red clover. Another factor influencing the amount of straw available for chopping is the stubble height at harvest. When leaving a long stubble, less straw will be chopped and returned to the field from the combiner. A thin layer of chopped straw after leaving a long stubble may be easier for new tillers to penetrate and thus less detrimental for the subsequent seed yield than a thick layer of chopped straw after leaving a short stubble. On the other hand, leaving a long stubble may lead to more shade and reduce growth and development of the undersown grass or clover plants. The effect of cover crop stubble height on plant development and first year s seed yield of timothy, meadow fescue, and red clover has not been investigated earlier. While cereal straw can be used as animal feed or bedding, the demand for straw has declined in the main seed-producing areas in SE Norway due to a reduction in livestock density. Consequently, the growers usually receive no payment for the straw; on the contrary, they often have to pay contractors NOK ha 1 for baling and straw removal. Besides the expenditure and associated difficulties with handling and placement of the bales, straw removal takes away valuable organic matter which in the long run would have enhanced soil productivity. Although inconclusive results have been reported (e.g. reviewed by Curtin & Fraser, 2003), European long-term studies have shown incorporation of straw to increase soil organic C by between 5% and 50% (eg. Powlson et al. 1987). The objective of the present research was to investigate the effect of cover crop stubble and straw management on vegetative growth in autumn and first-year seed yields of timothy, meadow fescue, and red clover. Materials and methods During a total of 13 field trials were carried out in seed crops of timothy (four trials), meadow fescue (six trials), and red clover (three trials) at locations ranging from Landvik, Aust- Agder (58 N) on the Norwegian south coast to Ridabu, Hedmark (61 N) in the inland north of Oslo (Figure 1). Except for those located at the Bioforsk Research Stations at Landvik and Apelsvoll, all trials were laid out in commercial seed crops. Plot size at establishment was 10 m m, depending on the cutting width of the combiner used for cover crop harvest. The experiments were laid out in mid-august to late September when combining cover crops of either spring barley (four trials) or spring wheat (nine trials). The experimental plan had three replicates (blocks) and two factors. Factor 1. Stubble height at cover crop harvest (1) 5 10 cm (short stubble) (2) cm (long stubble) Factor 2. Stubble and straw management (A) No chopping. Straw removed after harvest (control). (B) Straw chopped and spread at combining. (C) Stubble and intact straw in windrows chopped at 5 10 cm height with a tractor mounted flail chopper soon after harvest. (D) Similar to Treatment B + stubble and straw chopped at 5 10 cm height with a tractormounted flail chopper soon after harvest. In most trials, the tiller or plant density of the undersown crop was also counted in mobile frames ( m 2 ), and the fresh weight of straw was weighed and a sample (ca. 1 kg) taken from each plot for dry matter (DM) determination after two days drying at 60 C. In addition, the cover crop grain yield was determined on three arbitrary plots. The straw yield and grain yield in each trial are shown in Tables 1 3. In all but one trial in meadow fescue and half of the timothy trials, plots were fertilized with 30 kg N ha 1 soon after cover crop harvest. Spring fertilization in the seed harvest year to timothy and meadow fescue seed crops amounted to kg N ha 1 in the form of compound NPK fertilizer (usually or ; Tables 1 2). The spring fertilization of timothy seed crops was usually split into an early and a late application (Table 2). No fertilizer was applied to the red clover seed crops (Table 3). At the end of the growing season, plant height was measured (cm, average of three measurements per plot) and the number of vegetative tillers (timothy and meadow fescue) or plants (red clover) were counted on all plots not treated with the flailchopper (i.e. Treatments 1A, 1B, 2A, and 2B), using the same mobile frames as after cover crop harvest. In most trials in timothy and meadow fescue recordings before seed harvest included the following:. Percent plant cover in spring (assessed in May when plants had grown to a height of about 10 cm).. Percent lodging at flowering and seed harvest.. Panicle numbers as counted in mobile frames (area varying from 0.25 m 2 to 0.4 m 2 ).. Weight per unthreshed inflorescence (based on 100 random panicles that had been cut 1 cm below the inflorescence less than one week before seed harvest).

3 Acta Agriculturae Scandinavica, Section B Soil & Plant Science 549 Figure 1. Map of south Norway showing trial sites for meadow fescue (MF), timothy (T), and red clover (RC). The meteorological stations Kise, Apelsvoll, Ramnes, Hønefoss, Rygge, and Landvik have also been indicated; see Table 6. The trials were combined directly with Nursery master field plot combines (harvest plot size 1.5 m 8 m) at 20 30% seed moisture content, usually in late July (meadow fescue), August (timothy), or late August or September (red clover; Tables 1 3). The plot yields were dried to 10 12% seed moisture content and cleaned on a laboratory airscreen machine (LALS, WESTRUP, Denmark). Representative seed sample, pooled across replicates in each trial, was taken from each of the eight treatment combinations and analyzed for purity and 1000 seed weight in the seed laboratory at Bioforsk Landvik. In one trial at Apelsvoll, the photosynthetic photon flux density (PPFD), µmol m 2 s 1 was measured using a SunScan Canopy Analysis System type SS1 (Delta-T Devices, Cambridge, UK), and the red (660 nm) to far-red (730 nm) ratio was determined using a Skye Display Meter SKR 100 (Skye Instruments Ltd, Llandrindod Wells, UK). Both measurements were accomplished at soil level on straw-free control plots with either short (1A) or long stubble (1B) between 12:00 and 15:00 on 11 September, 29 September, and 14 October Analyses of variance (PROC ANOVA, [S A S Institute, 1990]) for seed yield and seed yield components were performed both individually for each trial, and collectively for all trials in each of the three species. In the overall analyzes, each trial was always regarded as a random variable. Results from individual trials will mainly be presented for seed yield, which is considered the most important character in seed production. As the use of the flailchopper in the Factor 2-treatments C and D reduced stubble height and was, therefore, confounded with the treatments in Factor 1, results will be presented for the eight treatment combinations and not as main effects and interactions among the two factors. Exceptions was made for the analyzes of vegetative growth characters in autumn (tiller number and plant height, Table 4) and light intensity in autumn (Table 5) where no registrations on flail-chopped plots were performed. Significant differences were separated by LSD 5%. While the term significant in this paper always refer to P% < 5, exact P values in the range 5 < P% < 20 have also been given in the tables to indicate tendencies. Meteorological data Within the Norwegian seed production district, there is a climatic gradient with longer growing seasons, higher temperatures, and more rainfall in the south than in the north (Table 6). Details in relation to the individual trials will be discussed later.

4 Table 1. Growing information for six trials in meadow fescue seed crops during Hedmark Landvik Apelsvoll Östfold Vestfold Landvik Meadow fescue cultivar Fure Fure Fure Fure Fure Fure Cover crop/cultivar Spring Spring Spring Spring barley/ Spring wheat/ barley/iver wheat/avle barley/tyra Kinnan Zebra Spring wheat/ Bjarne Sowing year Seeding rate (kg ha 1 ) of cover crop/meadow fescue 200/10 180/5 120/10 200/7 210/8 190/7 Fertilizer applied in seeding year (until harvesting cover crop), kg N ha 1 Date for harvesting cover crop/trial establishment 15 August 21 August 5 September 29 September 8 September 17 August Cover crop grain yield grain, kg ha Ca Tiller density of meadow fescue at cover crop harvest ( m 2 ) Average height, short stubble, cm (Treatment 1) Average height long stubble, cm (Treatment 2) Straw yield, short stubble (kg DM ha 1 ) Straw yield, long stubble (kg DM ha 1 ) Autumn fertilizer (soon after cover crop harvest), kg N ha First seed harvest year Date for spring fertilization 8 May 7 April 3 May 14 April 16 April 13 April Fertilizer applied (kg N ha 1 ) Date for growth regulation (Moddus 250 EC) 28 May 22 May 21 May 13 May 16 May 15 May Dose, kg a.i. (trinexap-ethyl) ha Date for seed harvest 31 July 22 July 28 July 26 July 31 July 31 July 550 L.T. Havstad

5 Acta Agriculturae Scandinavica, Section B Soil & Plant Science 551 Table 2. Growing information for four trials with timothy seed crops during Hedmark Landvik Landvik Östfold Timothy cultivar Grindstad Grindstad Grindstad Grindstad Cover crop/cultivar Spring barley/ Tyra Spring wheat/ Zebra Spring wheat/ Bjarne Spring wheat/ Bjarne Sowing year Seeding rate (kg ha 1 ) of cover crop/timothy 200/12 190/5 190/5 220/5 Fertilizer applied in seeding year (until harvesting cover crop), kg N ha 1 Date for harvesting cover crop/trial establishment 6 September 8 September 17 August 30 August Cover crop grain yield ( kg ha 1 ) Tiller density of timothy at cover crop harvest (per m 2 ) Average height, short stubble, cm (Treatment 1) Average height, long stubble, cm (Treatment 2) Straw yield, short stubble (kg DM ha 1 ) Straw yield, long stubble (kg DM ha 1 ) Autumn fertilizer (soon after cover crop harvest), kg N ha 1 First seed harvest year Dates for early and late spring fertilization N rate applied in early and late spring (kg N ha 1 ) Date for growth regulation (Cycocel 750) 15 May 18 May 15 May 10 May Dose, kg a.i. (Chlormequat chloride) ha Date for seed harvest 26 Aug. 5 Aug. 10 Aug. 18 Aug. Table 3. Growing information for three trials with red clover seed crops during Buskerud Landvik Buskerud Red clover cultivar Bjursele Lea Lea Cover crop/cultivar Spring wheat/avle Spring Spring wheat/bjarne wheat/zebra Sowing year Seeding rate (kg ha 1 ) of cover crop/red clover 250/ /3 200/3.6 N rate applied in seeding year (until harvesting cover crop), kg N ha 1 Date for harvesting cover crop/trial establishment 26 August 30 August 30 August Cover crop grain yield (kg ha 1 ) Plant density of red clover at cover crop harvest (per m 2 ) Average height, short stubble, cm (Treatment 1) Average height, long stubble, cm (Treatment 2) Straw yield, short stubble (kg DM ha 1 ) Straw yield, long stubble (kg DM ha 1 ) First seed harvest year Date for growth regulation (Moddus 250 EC) No Moddus 1.6 No Moddus Dose, kg a.i. (trinexapacethyl) ha Date for seed harvest 27 September 30 August 7 September Results Plant growth in autumn At the end of the growing season, there was a tendency (p% = 10) for more vegetative tillers of timothy (27%) on plots where the cover crop had been harvested with a short than with a long stubble. For meadow fescue and red clover, the corresponding increase in tiller/ plant number was 9% and 27%, respectively, but the effects were far from significant (Table 4). Leaving a long stubble at cover crop harvest significantly increased plant height in late autumn

6 552 L.T. Havstad Table 4. Effect of cover crop stubble height at grain harvest on the number of vegetative tillers or plants per m 2 and plant height at the end of the growing season. Meadow fescue Timothy Red clover Stubble height Tiller, s/m 2 Plant height, cm Tillers/m 2 Plant height, cm Plants/m 2 Plant height, cm Short, 7 12 cm Long, cm P% > >20 >20 Note: Average for plots where straw was either removed or chopped at harvest (Treatments A and B) in three trials with meadow fescue, three trials with timothy and one trial with red clover. in both timothy (40%) and meadow fescue (54%), compared to similar plots harvested at short stubble height (Table 4). Mean values for red clover pointed in the same direction but were not significantly different. Different straw treatments (Treatments A vs. B) did not have any significant impact on either tiller/ plant number or plant height in autumn (data not shown). Influence of stubble height on light conditions in autumn In autumn 2003, the light intensity in the trial at Apelsvoll was always lower on plots with a long stubble. On average for three dates, increasing stubble height from 12 cm to 23 cm significantly reduced light interception at soil level by 40% (Table 5). Also the R/FR-ratio tended to be lower (P = 15%) on plots with a long than with a short stubble (Table 5). Seed yield Meadow fescue Stubble height did not affect seed yield significantly in any of the trials provided that the straw was removed after combining (1A vs. 2A). Numerically, three trials (Apelsvoll , Landvik , and Vestfold ) showed more than 10% increase in seed yield after leaving a short stubble, but there was also one trial (Landvik ) which showed the opposite response. On average for all trials, increasing stubble height from 5 10 (1A) cm to cm (1B) reduced seed yield with 3% (Table 7). Like stubble height, chopping the straw had inconsistent effects on seed yields in the different trials. The only case with a significantly negative effect of chopping was on plots with long stubble (2A vs. 2B) at Apelsvoll in On average for all trials, chopping and removal of straw produced the same seed yield if the stubble height at combining was short (1A vs. 1B) but 3% lower seed yield if the stubble height at combining was long (2A vs. 2B; Table 7). Timothy Seed yield was above the Norwegian five-year average for cv. Grindstad (590 kg/ha during ) in all trials, except at Östfold in , where early lodging and production of secondary tillers reduced the seed yield potential (Table 8). Of the four trials, significant seed yield differences were only found at Landvik in (Table 8). In this trial, the highest seed yield was harvested on plots with short stubble and where the straw had been chopped during cover crop harvest (Treatment 1B). Conversely, the lowest seed yield was obtained after leaving a long stubble and removing the straw (Treatment 2A) On average for all trials, only small and insignificant differences in seed yield between treatments was Table 5. Effect of cover crop stubble height (cm) on light intensity (µmol m 2 s) and red/far-red ratio measured at soil level in one trial at Apelsvoll during autumn Light intensity at soil level (µmol m 2 s 1 ) Stubble height 11 September 29 September 14 October Mean Rel. R/Fr ratio a Short (12 cm) Long (23 cm) LSD (5%) 11 > a Mean of three dates in autumn

7 Acta Agriculturae Scandinavica, Section B Soil & Plant Science 553 Table 6. Mean monthly temperature at Apelsvoll, Landvik, Kise, Rygge, Ramnes, and Hønefoss during the experimental period. The 30-year normal values for (30-year normal) are also given for each location. August September October November December January February March April May June July Apelsvoll Normal Landvik Normal Kise, Hedmark Normal Ramnes, Vestfold Normal Rygge, Östfold Normal Hønefoss, Buskerud Normal found. Numerically, the highest seed yield was found after chopping and returning the straw at combining with a short (Treatment 1B) or a long (Treatment 2B) stubble. Using the flail chopper showed no advantage compared with the other treatments (Table 8). Red clover A long stubble reduced seed yield in all three trials compared to a short stubble (2A vs. 1A, Table 6). The strongest negative impact was in Buskerud in , where 44% less seed was harvested on plots with a long than with a short stubble. On average for all trials, the seed yield reduction was significant and amounted to 11% (Table 6). Averaged over all trials, straw chopping during cover crop harvest had a slightly positive effect on seed yield if the cover crop was harvested with a short stubble (1B vs. 1A) and no effect if the cover crop was harvested with a long stubble (2B vs. 2A). The positive effect of stubble removal was confirmed by the increase in seed yield after using the flail chopper on plots where the cover crop had been combined at a long stubble (2C and 2D vs. 2A and 2B). The highest yield was harvested on short-stubble plots where intact straw had been flail-chopped after harvest (1C). Seed yield components Significant differences in generative tillers between treatments were only evident at Apelsvoll in (data not shown). In this meadow fescue trial, the highest panicle number ( /m 2 ) was found on short-stubble plots where straw either had been removed (1A) or flail-chopped after harvest (1C), while fewest panicles ( / m 2 ) where registered on long-stubble plots where straw had either been chopped during harvest (2B) or removed from the field (2A). On average, for all trials no differences between treatments were found for generative tiller number in either meadow fescue (Table 7) or timothy (Table 8). Stubble and straw managements had no significant effect on the weight per inflorescence and thousand seed weight in any of the grass species (data not shown). Other characters Neither plant cover in spring nor lodging at flowering or seed harvest was significantly affected by the various stubble and straw treatments (data not shown).

8 Table 7. Effects of stubble height (cm) after cover crop harvest and various straw treatments on generative tillers/m 2 and seed yield (kg ha 1, 12% water, 100% purity) of meadow fescue. Stubble height a and straw management Generative tillers/m 2 Hedmark Landvik Seed yield (kg ha 1, 12% water, 100% purity) Apelvoll Östfold Landvik Vestfold Mean Rel No. of trials A. Short stubble. Straw removed after harvest 1B. Short stubble. Straw chopped during comb. 1C. Short stubble. Straw chopped after combining 1D. Short stubble. Straw chopped twice b 2A. Long stubble. Straw removed after harvest 2B. Long stubble. Straw chopped during comb. 2C. Long stubble. Straw chopped after combining 2D. Long stubble. Straw chopped twice b Significance (p value) >20 >20 >20 <1 20 >20 >20 >20 LSD (5%) 63 a Short stubble height = 5 10 cm, long stubble height = cm. b Straw chopped both during and after combining. 554 L.T. Havstad

9 Acta Agriculturae Scandinavica, Section B Soil & Plant Science 555 Table 8. Effects of stubble height (cm) after cover crop harvest and various straw treatments on generative tillers/m 2 and seed yield (kg ha 1, 12% water, 100% purity) of timothy. Seed yield (kg ha 1, 12% water, 100% purity) Stubble height a and straw management Generative tillers/m 2 Hedmark Landvik Landvik Östfold Mean Rel No. of trials A. Short stubble. Straw removed after harvest 1B. Short stubble. Straw chopped during comb. 1C. Short stubble. Straw chopped after combining 1D. Short stubble. Straw chopped twice b 2A. Long stubble. Straw removed after harvest 2B. Long stubble. Straw chopped during comb. 2C. Long stubble. Straw chopped after combining 2D. Long stubble. Straw chopped twice b Significance (p value) >20 >20 >20 <1 >20 >20 LSD, 5% 39 a Short stubble height = 5 10 cm, long stubble height = cm. b Straw chopped both during and after combining. Table 9. Effects of stubble height (cm) after cover crop harvest and various straw treatments on seed yield (kg ha 1, 12% water, 100% purity) of red clover. Stubble height a and straw management Buskerud Seed yield (kg ha 1, 12% water, 100% purity) Landvik Buskerud Mean Rel No. of trials A. Short stubble. Straw removed after harvest 1B. Short stubble. Straw chopped during comb. 1C. Short stubble. Straw chopped after combining 1D. Short stubble. Straw chopped twice b A. Long stubble. Straw removed after harvest 2B. Long stubble. Straw chopped during comb. 2C. Long stubble. Straw chopped after combining 2D. Long stubble. Straw chopped twice b Significance (p-value) >20 < LSD, 5% a Short stubble height = 5 10 cm, long stubble height = cm. b Straw chopped both during and after combining. Discussion It is important for both grasses and clovers to produce a sufficient number of vigorous tillers (shoots) in autumn (potential seed heads) in the establishing year to maximize seed yield in the following year (Schöberlein 1987; Boelt 1999). This is especially important in meadow fescue which requires a long primary induction period in autumn in order to produce many inflorescences with a high number of florets (Heide 1988; Havstad & Aamlid 2007). The light intensity influences plant growth through photosynthetic activity and developmental

10 556 L.T. Havstad responses. Bula (1960) found that lowered light intensity markedly reduced dry weight accumulation and vegetative development in red clover. In cocksfoot (Dactylis glomerata), Auda et al. (1966) reported that tillering, DM production, and carbohydrates (percentage of dry weight) dropped when light intensity was reduced to 75% of normal sunlight. Also Mitchell (1953) and Patel and Cooper (1961) demonstrated similar effects of light intensity on tiller development in various grasses. In the present trials, measurement at Apelsvoll in , showed 40% lower light intensity at plant crown level when the height of stubble was increased from 12 cm to 23 cm. Thus, the higher tiller number in late autumn on plots with short vs. long stubble in timothy and meadow fescue was probably due to improved light penetration (Table 6; Meijer & Vreeke 1988). The lower R/FR-ratio measured on long-stubble plots (Table 6) may also have influenced plant growth, both with regard to reduced tillering (Deregibus et al. 1983) and increased elongation (through the phytochrome mechanism; e.g. Casal et al. 1987). As there was no clear difference in vegetative growth between straw removal and straw chopping, stubble height seems more influential than straw treatment with regard to plant development. The reason for this could be that tillers/plants, in most cases, penetrated the layer of chopped straw easily and were exposed to sunlight soon after harvest, while shading from the long stubble lasted for a longer period of time (longlasting effect). Despite this variation in vegetative growth in autumn, differences among various stubble and straw chopping strategies were not significant in most of the trials in timothy and meadow fescue. One exception was at Apelsvoll in , where the fewest panicles and lowest seed yield of meadow fescue was found on long-stubble plots where the straw had been chopped during cover crop harvest (Treatment 2B). Perhaps the combination of a long stubble and a overlying layer of chopped straw was especially unfortunate during the cold autumn of With a mean temperature 2.0 C below the 30- years normal in October 2003 (Table 6), the growing period after cover crop harvest on 5 September might have been too short for tillers to fully penetrate the straw layer before winter began. In addition, this crop was also exposed to drought and very high temperatures in the following spring (3.5 C and 1.9 C above normal in April and May, respectively, Table 6), which may have resulted in a high mortality of tiller than on plots where the plants were weakened by the straw layer. In timothy, a significant reduction in seed yield with increasing stubble height from 8 9 cm to 31 cm (1A vs. 2A) was found at Landvik in Clearly, the long stubble created too much shade for optimal plant development to occur. However, in another trial (Östfold) in the same season, a nearly similar increase in stubble height, from 9 cm to 31 cm, had no negative effect on seed yield (Tables 2 and 8). The reason for this difference in response to stubble height between locations is not clear. However, the grain yield was 20% higher and the straw yield almost twice as high at Landvik than in Östfold, which suggests that the cover crop stubble was more dense and created more shade at the former location. Thus, a stronger reduction in light intensity, due to a denser stand of long stubble, might have contributed to shading from stubble being more negative in the Landvik-trial. In the red clover trials, where short and long stubble varied between locations from 9 cm to 10 cm and from 23 cm to 26 cm (Table 3), respectively, the highest seed yield was always harvested on shortstubble plots. This suggests that plant development of red clover is vulnerable to shading (Bula 1960) and this has later been confirmed by practical experiences by Norwegian seed growers (Valand & Aamlid 2014). Also in white clover, Aamlid (2011) found the highest seed yield on plots harvested with a short cover crop stubble. Of the three species, red clover was more negatively affected by long stubble than the two grass species. Flail chopping of intact straw in windrows after combining at a short (1C) or at a long stubble (2C) was usually not beneficial compared to straw chopping at combining (1B). In some cases, the flail chopper failed to chop the intact straw, as it was more or less blown through the machinery, especially at higher tractor speed. As slow driving is timeconsuming, and thus not compatible with efficient flail chopping in practical seed production, such a practice is too uncertain to recommend. Overall, activating the combiner s straw chopper when harvesting the cover crop at a short stubble (Treatment 1B) seems to be an efficient straw management method which does not reduce seed yield compared to straw removal. Thus, for seed growers without their own husbandry or not receiving any payment for the cover crop straw, this method can be regarded as an environmentally sound and cost-saving alternative when harvesting barley and spring wheat cover crops. However, in order to succeed, it is important to chop the straw as finely as possible (sharp knives) and to spread it evenly on the soil surface for rapid straw decomposition (Havstad et al. 2010) and for plants/tillers to easily penetrate the layer of chopped straw. If the straw had been chopped when combining at a short stubble height, additional chopping using the

11 Acta Agriculturae Scandinavica, Section B Soil & Plant Science 557 flail chopper did not usually have any beneficial effects on seed yield (Treatment 1D vs. 1B, Tables 7 9). However, in cases where the spreading of chopped straw has failed, e.g. the straw is clumped in heaps or the seed grower has left unfavorably long stubble, a final flail-chopping treatment can still be recommended. Some growers will probably prefer this alternative as leaving a long stubble may speed up combing or make it easier to avoid weeds or succulent, green material of red clover or other undersown crops which have become too vigorous and grown into cover crop canopy. Up to now, when practicing straw removal, an application rate of 30 kg N ha 1 soon after cover crop harvest has been recommended for stimulation of tiller growth in autumn in both timothy and meadow fescue (Aamlid 2013). Since chopped straw of both barley and spring wheat have a relatively high C/N ratio, such an input of nitrogen will also be required to stimulate the microbial decomposition of straw. Autumn fertilization was not a subject in this study, but incubation studies by Havstad et al. (2010) suggest that 30 kg N ha 1 is sufficient to support both tiller stimulation and cover crop straw decomposition. Acknowledgments Thanks are extended to Trygve S. Aamlid for valuable comment on the manuscript and to the staff at Bioforsk Landvik and Bioforsk Apelsvoll and the farmers experimental and advisory groups in Vestfold, Buskerud, Östfold, and Hedmark for skillful implementation of these experiments. Funding This research was funded by a grant from the Norwegian Seed Growers Union (Norsk frøavlerlag) and [grant number /110] from the Norwegian Research Council. 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