THE CONSEQUENT INFLUENCE OF CROP ROTATION AND SIX-YEAR-LONG SPRING BARLEY MONOCULTURE ON YIELDS AND WEED INFESTATION OF WHITE MUSTARD AND OATS

The present study was conducted in the years 20072008, after 6-year-long experiments in the cultivation of spring barley in a crop rotation system and in monoculture. The other experimental factor was the spring barley protection method. Intensive protection involved comprehensive treatment of barley (in-crop harrowing, seed dressing, application of herbicides, fungicides, a retardant and an insecticide). Extensive protection consisted only in in-crop harrowing, without the application of crop protection agents, except for seed dressing. The above mentioned factors formed the background for the study on the cultivation of white mustard and oats, as phytosanitary species, in successive years. In the test plants, no mineral fertilization and crop protection were applied. Such agricultural method enabled an objective assessment of the consequent effect of monoculture, crop rotation and crop treatments. A hypothesis was made that the cultivation of the phytosanitary plants in the stand after 6-year-long barley monoculture would allow obtaining the level of yields and weed infestation similar to those of the crop rotation treatments. It was also assumed that the cultivation of white mustard and oats would eliminate differences in plant productivity caused by the negative influence of extensive protection. It was proved that the cultivation of the phytosanitary plants eliminated the negative influence of monoculture on the level of their yields and weed infestation. However, the test plants did not compensate negative consequences of extensive protection. In spite of this, white mustard and oats effectively competed with weeds, and the number and weight of weeds in a crop canopy did not cause a dramatic decline in yields. In the test plant canopy, the following short-lived weeds were predominant: Chenopodium album, Galinsoga parviflora, Echinochloa crus-galli. The absence of herbicide application resulted in the compensation of perennial species: Elymus repens and Cirsium arvense.


INTRODUCTION
In crop rotations with large cereal proportions, in particular in monoculture, there occur a number of negative phenomena resulting, among other things, in a decline in competitiveness of crop plants relative to weed flora.Increased weed infestation of a crop canopy is in turn one of the main reasons for their re-  2005).Proper crop sequencing, based on environmental factors, coupled with appropriate agricultural practices applied with it, ensures to a large extent a successive increase in yields and a reduction in weed infestation (Z a w i ś l a k , 1997; A d a m i a k , 2007).In the opinion of some authors (K w i a t k o w s k i , 2004a; S t u p n i c k a -R o d z y n k i e w i c z et al. 2004), comprehensive mechanical and chemical protection of spring barley only partially compensates the effects of inappropriate crop sequencing.The cultivation of phytosanitary plants (mustard, oats) can be included among factors which reverse the negative effects of barley or wheat monoculture.As shown by the studies of O l e s z e k (1994) and M u r a w a et al. (2004), white mustard, thanks to certain peculiar allelopathic substances, may inhibit germination and even growth of some weed species in a natural way.Oats, compared to other cereals, reacts weaker to monoculture and its cultivation after other grain plants.In strongly cerealbased crop rotations, it may perform the role of a phytosanitary and regenerating plant (J e l i n o w s k i , 1979; A d a m i a k , 1992; A d a m i a k and A d am i a k , 1994).
The above arguments allowed us to make a hypothesis that the cultivation of white mustard and oats in successive years in the stand after 6-year-long spring barley monoculture would enable the regeneration of this stand (the level of yields and weed infestation) similar to that obtained in treatments with the consequent influence of crop rotation.An assumption was also made that the cultivation of the phytosanitary plants would eliminate differences in plant productivity and weed infestation, associated with the consequent effect of two protection levels.
The aim of the present study was to assess the condition of weed flora in a white mustard and barley canopy as well as to analyse yields of these species grown for seeds, under different conditions of longterm crop succession and under different crop protection methods.

MATERIALS AND METHODS
Field investigations were conducted in the years 2007-2008 after 6-year-long (2001-2006) experiments in the cultivation of spring barley in a crop rotation system and in monoculture.A strict field experiment was established at the Czesławice Experimental Farm, belonging to the University of Life Sciences in Lublin, Poland, using the split-plot method in 3 replicates in plots with an area of 27 m 2 .The experiment was set up on grey-brown podzolic soil formed from loess (soil quality class II).
All crop protection agents were used on dates and at doses in accordance with recommendations of the Plant Protection Institute in Poznań, Poland.Mineral fertilization of spring barley, both in crop rotation and monoculture, was identical and it was as follows: N -60, P 2 O 5 -70, K 2 O -90 kg ha -1 .Organic fertilization (manure) in crop rotation was applied for potato (35 t ha -1 ), whereas under barley monoculture it was applied once every 3 years (35 t ha -1 ).With respect to the other crop plants grown in crop rotation (potato, horse bean, winter wheat), mechanical treatments and crop protection agents specific for every plant species were also applied.Soil tillage for barley and the other plant species grown in crop rotation followed agricultural practice rules.
The test plants in this experiment were as follows: white mustard in 2007 (cv.Borowska -20 kg ha -1 ) and oats in 2008 (cv.Kasztelan -200 kg ha -1 ).In the investigated plants, no mineral fertilization or plant protection were used, except for seed dressing (Raxil 069 FS) and intervention treatment with an insecticide in white mustard (Decis 0.25 EC).Such agricultural method (no fertilization and chemical protection) used with respect to the test plants under assessment was designed to enable an objective assessment of the consequent effect of the investigated factors, i.e. crop sequence and crop treatments.
Soil tillage, the quantity and date of sowing of the test plants (white mustard, oats) were within proper agricultural practice standards.The assessment of weed infestation was made using the botanical gravimetric method, during the full growing season of the test plants (mustard -flowering stage, oats -heading stage), thus, during the period of greatest accumulation of biomass by weeds in a canopy.Before weighing, mustard and oats seed yields were brought to the same moisture content level -14%.The research results were statistically verified, determining the significance of differences using Tukey's test.

AND DISCUSSION
White mustard seed yield was significantly differentiated by the investigated experimental factors (Tab.1).The cultivation of this plant in the stand after spring barley under the Norfolk crop rotation system resulted in seed yield higher by 51% than that found in the treatments with the consequent influence of monoculture.The influence of protection method in the spring barley canopy proved to be even more evident.Intensive protection, irrespective of the forecrop, provided mustard seed yield larger by 51% compared to the extensive protection treatments.The stand after spring barley grown in crop rotation and intensively protected guaranteed the largest white mustard seed yield (1.82 t ha -1 ).The lowest mustard productivity was found in the stand after extensively protected spring barley in monoculture (only 0.71 t ha -1 ).
Oats, as the other test plant, showed smaller variations in yields resulting from crop succession (Tab.1).It can be explained by the fact that white mustard, as the immediate forecrop of oats, simultaneously performed the role of a phytosanitary plant and had a modifying effect on its yielding, to which attention is drawn by some authors (P a w ł o w s k i and D e r y ł o , 1988; M u r a w a et al. 2004).In the treatments with spring barley monoculture, oats yield was smaller only by 5% compared to crop rotation.But significantly lower productivity of white mustard (by 29%) in the extensive protection treatments was noted compared to comprehensive (mechanical and chemical) protection.
The number of weeds in the test plant canopies also depended significantly on the investigated factors (Tab.2).The lowest number of weeds was recorded in the crop rotation with intensive protection, both in the mustard and oats canopy.In the stand after spring barley monoculture, the number of weeds was larger by, respectively, 15% (mustard) and 14% (oats) compared to crop rotation.The application of plant protection agents (including herbicides) in spring barley crops, irrespective of crop succession, caused the number of weeds in the intensive protection treatments to be smaller by 22% (mustard) and 31% (oats) compared to the extensive protection treatments.
The crop sequence system did not result in any great differences in air-dry weight of weeds in the white mustard and oats canopy (Tab.3).A tendency towards larger weight of weeds (by 7% -mustard, 3% -oats) in monoculture compared to crop rotation was only noted.However, the introduction of the phytosanitary test plants did not compensate the negative consequent influence of extensive protection of the crop canopy.Air-dry weight of weeds determined under such conditions was larger by 14% (mustard) and 16% (oats) compared to the treatments in which intensive protection of spring barley was applied.However, it should be noted that the cultivation of phytosanitary  crops in successive years eliminated the negative effect of monoculture and extensive protection of the canopy, in particular in the case of weight of weeds in the oats canopy.As a result, the effect of air-dry weight of weeds on oats productivity was similar in particular treatments, which is confirmed by the calculated correlation coefficients (Tab.4).
In the white mustard canopy, the number of weed species after various forecrops was 24-27, whereas in the oats canopy -22-24 (Tab.5).The largest amount of dominant weed species was found in the stand after Norfolk crop rotation (white mustard), whereas the lowest amount in the oats canopy grown after 6-year-long barley monoculture.Chenopodium album, Echinochloa crus-galli, Stellaria media occurred in greatest numbers in the canopy of both test plants.Perennial species (Elymus repens, Cirsium arvense) did not pose great threat; nevertheless, their intensified occurrence was found in the oats canopy.Among the short-lived species, the compensation of Echinochloa crus-galli and Stellaria media was also noted.However, the proportions of the other dominant weed species in the oats canopy were reduced: by 27% (Chenopodium album), 38% (Galinsoga parviflora) and 50% (other species not specified in the table ).
The results of the present study illustrate that the test plants (mustard, oats) responded by a larger decline in yield to the consequent influence of the absence of chemical protection of spring barley crops than to the negative influence of monoculture.White mustard shows similar yields regardless of the forecrop, due to its resistance to fungal diseases and high ability to compete with weeds.For this reason, it is often sown as a stubble crop in cereal monocultures (O l e s z e k , 1994; K w i a t k o w s k i , 2004b).A d a m i a k and A d a m i a k (1994,1999) note that oats does not show significant differences in yields related to an increased proportion of cereals in a crop rotation.Lower weed infestation of oats and marginal infection by the fungal disease complex determine the great yield-protective importance of this plant.Yielding of the phytosanitary plants in question was negatively correlated with the level of their weed infestation, which was the highest in the stand after the extensively protected spring barley monoculture (Tab.4).P a w ł o w s k i and W e s o ł o w s k i (1996) demonstrate that quantitative weed infestation of spring barley and of succeeding crops cultivated is determined to a large extent by spring weeds -Chenopodium album, Stellaria media, Poa annua, Echinochloa crus-galli.The regulation of weed population size, and thereby decreasing their competitiveness relative to a crop plant, depends to a large degree on the proper crop sequence.Crop rotation and also some succeeding crops (mustard, oats), thanks to their phytosanitary effect on soil and allelopathic responses, inhibit in a natural way the development of some weeds.D e r y ł o (1994) as well as K w i a t k o w s k i and W e s o ł o w s k i ( 2004) are of opinion that the application of comprehensive crop protection (foreforecrops) has a significant effect on the limitation of weed infestation of succeeding crops.O l e s z e k (1994) proves that substances contained in mustard (glucosinolates, isothiocyanates) reduce the population size of weeds such as Chenopodium album, Galium aparine, Matricaria maritima ssp.inodora, Capsella bursa-pastoris.Nevertheless, mustard press cakes mixed with soil have the greatest allelopathic effect.Hence, it can be presumed that the weed control activity of this plant in the present study was revealed only after its harvest and the ploughing-in of post-harvest residues and it limited weed infestation of the next crop grown in this stand (oats).J ę d r u s z c z a k et al. (2005) note that in years characterised by low temperature and wet weather (May -July), late emerging weeds (Galinsoga parviflora, Echinochloa crus-galli, Gnaphalium uliginosum), being the core of dominant species, occur in greatest numbers in a white mustard canopy.It is attested by the competitiveness of white mustard grown under good soil culture conditions and proper agricultural practice, even without any weed destruction, which finds reflection in the paper in question.In addition, the authors claim that growing white mustard after cereals, on poor soils of low culture, causes increased weed infestation and requires chemical weed control.
In the present study, weeds did not dominate the white mustard canopy.In spite of the absence of chemical protection of the crop, the weight of a single weed, irrespective of the investigated factors, was ca.0.75 g, on the average, and similar tendencies were also observed in the oats crop.Observations of weed infestation conducted by A d a m i a k and A d a m i a k (1999) showed that the introduction of oats into a crop rotation limited the occurrence of weeds.Oats competed with weeds better than other spring cereal species as well as some leguminous plants (horse bean).

REFERENCES
duced productivity (D e r y ł o and P a w ł o w s k i , 1982; D e r y ł o , 1994).Spring barley is a species sensitive to monoculture and related weed pressure (W e s o ł o w s k i et al. 2003; B l e c h a r c z y k et al.

Table 1
The influence of crop rotation, monoculture and spring barley protection method on yields of succeeding crops.The influence of crop rotation, monoculture and spring barley protection method on the number of weeds (per 1 m 2 ) in a succeeding crop canopy.
* -intensive protection of barley PE ** -extensive protection of barley ns -not significant Table 2

Table 3
The influence of crop rotation, monoculture and spring barley protection method on air-dry weight of weeds (g m 2 ) in a succeeding crop canopy.

Table 4
The simple correlation coefficient (r) between air-dry weight of weeds in a canopy and yields of succeeding crops.
* significant correlation coefficient

Table 5
Dominant weed species in a test plant canopy, irrespective of the protection method.