In a five-course rotation with organic vegetables (white cabbage, carrot and onion) the weed seed bank was reduced the year after two continuous years with red clover, mainly because of mowing of the second year red clover. The year after the weedy ribbed meliot the weed seed bank increased

Brandsæter, Lars Olav

Seljåsen, Randi

Sjursen, Helge

English

Organic Eprints

16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008 Archived at http://orgprints.org/view/projects/conference.html   Change in the weed seed bank during the first four years of a five-course crop rotation with organically grown vegetables Sjursen, H.1, Brandsæter, L.O.1  Seljåsen, R.2 Keywords: weed seed bank, red clover, yellow sweetclover , vegetables, crop rotation Abstract In a five-course rotation with organic vegetables (white cabbage, carrot and onion) the weed seed bank was reduced the year after two continuous years with red clover, mainly because of mowing and no soil cultivation the second year red clover. The year after the weedy yellow sweetclover the weed seed bank increased. Introduction The weed observations were an integrated part of the project ‘Optimum crop rotation for secure organic vegetable production’, started in 2002 at Bioforsk Arable Crops – Landvik,  in  the  southern  part  of  Norway.  The  project  has  focused  on  different  N-sufficient crop rotation systems on a clay rich soil. Of five production years two have been used for green manure fertilisation with legumes. Our results have shown that the  systems  provide  abundant  levels  of  nitrogen  for  white  cabbage,  carrots  and onions. The soil seed bank is recognized as the primary source of annual weeds in arable land. The majority of seeds entering the seed bank come from annual weeds growing in the fields. The size of the seed bank reflects past and present field management (Cavers  &  Benoit,  1989).  Albrect  (2005)  found  that  the  number  of  weed  seeds increased at sites with low crop  cover and high density of  weed  plants at the soil surface. Winter cereals, sunflowers and lupins increased for example the weed seed bank by 30-40%. Grass-clover mixtures, however, reduced the seed bank by 39%. In an  organic  farmed  six-course  rotation  investigation  in  Norway,  the  seed  bank  was reduced from a maxiumum of 17600 m-2 to a minimum of 7200 seeds m-2 after three years with perennial grass-clover ley (Sjursen, 2001). The main objective of the present study was to evaluate the effect of two years with a N-fixing  clover  crop  on  the  weed  seed  bank,  either two  continuous  years  with  red clover or two alternating years with yellow sweetclover.  Materials and methods The investigated area had four different fields with three replicates. Two of them had this rotation: Red clover, red clover, white cabbage, onion, followed by a last year with carrot. Two other fields had this rotation: yellow sweet clover (ribbed meliot), white cabbage, yellow sweet clover, onion and carrot. The plot size was 8 x 15 m. Soil                                                  1 Norwegian Institute for Agricultural and Environmental Research (Bioforsk), Plant Health and Plant Protection Division, Høgskolevn. 7, N-1432 Ås, E-mail: helge.sjursen@bioforsk.no 2 Norwegian Institute for Agricultural and Environmental Research (Bioforsk), Arable Crops – Landvik, Reddalsvn. 215, N-4886 Grimstad 16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008 Archived at http://orgprints.org/view/projects/conference.html   samples down to 0.20 m for weed seed investigations were taken each year before the growing season started (around 25th of April). The seed number analysis were carried out by the seedling emergence method described by Sjursen (2001). Number of emerged weeds on the soil surface were counted in a frame 0,5 x 0,5 m (clover crops) or 0,1 x 1,0 m (vegetable crops) during the end of June – beginning of July. The  fields  were  weed  managed  by  common  organic  methods,  like  flaming  before planting/sowing, inter-row hoing/ploughing,  and manual weeding in vegetables, and two-three times mowing in clover leys. Results The weed seed bank in the whole investigated area increased the first three years by 11% from about 28000 to 31000 seeds m-2, and was redused by 7% to 29000 seeds m-2 the fourth year. 23 weed species occurred in the seed bank. The most frequent species were Spergula arvensis L., Filaginella uliginosum (L.) Opiz., Poa annua L., Capsella  burs-pastoris  (L.)  Medicus,  Chamomilla  suaveolens  (Pursch)  Rydb.  and Stellaria media (L.) Vill. The same species were most frequent among the emerged species. The seed bank increased after one year with red clover, but was reduced after the second year of clover (figure 1 and 2). The seed bank increased the year after yellow sweetclover (figure 3 and 4). The density of emerged weed plants were high in the first year with red clover (figure 1 and 2) and in yellow sweetclover (figure 3 and 4). Figure 1. Weed seed bank in the soil (left hand y-axis) and emerged weeds (right hand y-axis) during two continuous years with red clover (RC1 and RC2) in rotation with carrot and white cabbage. Onion is the fifth year. Values with the same letter (lower case or upper case) are not significantly different at p ≤ ≤ ≤ ≤ 0.05 (n = 6).   01000020000300004000050000600002002 2003 2004 2005Carrot RC1 RC2 CabbageYear / cultureSeed bank: number of seeds m-2  020040060080010001200Emerged weeds:  number of plants m-2Seed bankEmerged weedsabbbaBCABCA16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008 Archived at http://orgprints.org/view/projects/conference.html    Figure 2. Weed seed bank in the soil (left hand y-axis) and emerged weeds (right hand y-axis) during two continuous years with red clover (RC1 and RC2) in rotation with carrot and onion. Carrot is the fifth year. See figure 1 for statistics.  Figure 3. Weed seed bank in the soil (left hand y-axis) and emerged weeds (right hand y-axis) during two alternating years with yellow sweetclover (RM1 and RM2) in rotation with carrot and onion. Onion is the fifth year. See figure 1 for statistics. 01000020000300004000050000600002002 2003 2004 2005RC1 RC2 Cabbage OnionYear / cultureSeed bank: number of seeds m-2020040060080010001200Emerged weeds: number of plants m-2Seed bankEmerged weedsaaaaABACB01000020000300004000050000600002002 2003 2004 2005Carrot RM1 Cabbage RM2Year / cultureSeed bank: number of seeds m-2020040060080010001200Emerged weeds: number of plants m-2Seed bankEmerged weedsa aaaBB BA16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008 Archived at http://orgprints.org/view/projects/conference.html   Figure 4. Weed seed bank in the soil (left hand y-axis) and emerged weeds (right hand y-axis) during two split-up years with yellow sweetclover (RM1 and RM2) in rotation with white cabbage and onion. Carrot is the fifth year. See figure 1 for statistics. Discussion and conclusions The results show that there are a close relation between the emerged weed plants at the soil surface and the seed bank the following year. The red clover was mowed two-three times the second year. This resulted in low emergence of annual weeds, and reduced  seed  bank  the  following  year  (figure  1  and  2),  in  accordance  with  the investigations  by  Sjursen  (2001)  and  Albrecht  (2005).  This  effect  was  not  seen  in yellow sweetclover. There was a tendency of weed seed bank reduction the year after white cabbage, which can be explained by the strong competition by the fast-growing cabbage  leaves  and  the manual  weeding.  This  effect  was  not seen  the  year  after carrot and onion, which is known as low-competitive crops. Teasdale et al. (2004) force the importance for organic farmers to minimize opportunities for rapid buildup of the seed bank.  References Albrecht, H. 2005. Development of arable weed seedbanks during the 6 years after the change from conventional to organic farming. Weed Reasearch 45 (5): 339-350. Cavers, P.B. & Benoit, D.L. (1989). Seed banks in arable land. In: Ecology of soil seed banks (Leck, M.A., Parker, V.T. & Simpson, R., eds.), pp. 309-328. Academic Press, San Diego, U.S.A. Sjursen, H. 2001. Change of the weed seed bank during the first complete six-course crop rotation after conversion from conventional to organic farming. Biological agriculture & horticulture 19(1): 71-90. Teasdale, J.R., Magnum, R.W., Radhakrisnan, J. and Cavigelli, M.A. Weed seed bank in three organic farming crop rotations. Agromomy Journal 96 (5): 1429-1435. 01000020000300004000050000600002002 2003 2004 2005RM1 Cabbage RM2 OnionÅr / kulturerSeed bank: number of seeds m-2020040060080010001200Emerged weeds: number of plants m-2Seed bankEmergedweedsacbc abA ABB

Change of the weed seed bank during the first four years of a five-course crop rotation with organic vegetables

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