WEED INFESTATION OF WINTER WHEAT ( Triticum aestivum L . ) UNDER THE CONDITIONS OF APPLICATION OF SOME RETARDANTS

A field study was conducted in the period 2004–2007 on grey-brown podzolic soil (sandy). This study analysed the relationship between the use of stem shortening in cereals by means of retardants with the following active substances: chlormequat chloride (Antywylegacz Płynny 675 SL), trinexapac-ethyl (Moddus 250 EC), chlormequat chloride + ethephon (Cecefon 465 SL), and weed infestation. The retardants were applied at the 1st node stage (BBCH 31 – Antywylegacz Płynny 675 SL) and the 2nd node stage of winter wheat (BBCH 32 – Moddus 250 EC and Cecefon 465 SL), together with the adjuvant Atpolan 80 EC (75% of SN 200 mineral oil) or without the adjuvant. Winter wheat, cv. ‘Muza’, was grown after vetch grown for seed. The whole experiment was sprayed with the herbicides Apyros 75 WG and Starane 250 EC at the full tillering stage (BBCH 29–30). Plots where no growth regulators were used were the control treatment. Weed density and biomass showed great variation between years. In the winter wheat crop, Veronica persica, Viola arvensis, Veronica arvensis, Capsella bursa-pastoris, and Chenopodium album dominated in the dicotyledonous class, whereas Apera spica-venti, Echinochloa crus-galli, and Elymus repens were predominant among monocotyledonous plants. The level of weed infestation of the winter wheat crop, as measured by the number and air-dry weight of weeds, was significantly differentiated by years and retardants used as well as by interactions of these factors. The adjuvant Atpolan 80 EC did not have a significant effect on the above-mentioned weed infestation parameters.


INTRODUCTION
Growth regulators (retardants) are commonly used in cereal cropping.They reduce lodging of cereals and protect grain yield produced.The action of re-tardants consists in inhibiting stem elongation growth in cereals, due to which they form lower crop canopies and modify weed communities inhabiting such cereal crops (R a j a l a and P e l t o n e n -S a i n i o , 2001; K i e r z e k and G ł o w a c k i , 2004).In order to reinforce the action of these agents, adjuvants are used more and more frequently in agricultural practice.These are additives that enhance the biological activity of agents to which they are added (G a s k i n et al. 2000).Among cereals, winter wheat is particularly exposed to weed expansion due to its slow growth in the autumn and spring until the stem elongation stage (S k r z yc z y ń s k a and P a w l o n k a , 1999; A n j u m and B a j w a , 2010).Winter wheat is also characterized by low competitive ability against weeds for nutrients, water, place in the canopy, and access to light.In the opinion of S k r z y c z y ń s k a and P a w l o n k a (2004), by shortening stems of cereal plants, retardants promote deeper penetration of light into the crop canopy and thus create more favourable conditions for emergence of weeds as well as their growth and development.In growing wheat, herbicides and growth regulators are used in agricultural practice, but the available literature lacks sufficient and well documented information how retardants, used in combination with adjuvants, affect a weed community.
The aim of the present study was to verify the thesis about the modifying effect of retardant application in a winter wheat crop on weed infestation of such a crop.

MATERIALS AND METHODS
A field study was carried out at the Czesławice Experimental Farm, belonging to the University of Life Sciences in Lublin, in the period 2004-2007.It was lo-cated on grey-brown podzolic soil (sandy), designated as PWsp, slightly acidic (pH in 1M ) and rich in phosphorus, potassium, and magnesium.The experiment was set up in a randomized block design in three replicates, in 10 m 2 plots.The experimental design included treatments without retardant (control treatment) and with retardants: chlormequat chloride (Antywylegacz Płynny 675 SL, 675 g × l -1 ), trinexapac-ethyl (Moddus 250 EC, 250 g × l -1 ), and chlormequat chloride + ethephon (Cecefon 465 SL, 310 g × l -1 + 155 g × l -1 ).The retardants were applied at a rate recommended for the 1st node stage (BBCH 31)chlormequat chloride, and the 2nd node stage of wheat (BBCH 32) -trinexapac-ethyl and chlormequat chloride + ethephon, with the adjuvant Atpolan 80 EC (75% of SN 200 mineral oil) or without the adjuvant.They were applied with a P161 field sprayer (Tee-Jet Turbo 02) at a pressure of 0.25 MPa, using 250 l of liquid per hectare.The above-mentioned adjuvant (Atpolan 80 EC) was chosen based on the recommendation of the Institute of Plant Protection as an agent intended for use in a wide group of crops.
Weed infestation of the winter wheat crop was determined based on the numbers, air-dry weight and species composition of weeds per 1m 2 during the study years.Weed infestation was evaluated by the dry-weight-rank method a week before expected harvest of the winter wheat crop.For this purpose, a 0.5 m 2 frame was used which was placed randomly in each plot twice.Names of weed species followed M i r e k et al. (2002).The crop height of winter wheat plants was determined before harvest (measuring it from the soil surface to the ear tip, excluding awns).The study results were statistically analysed by analysis of variance, while the differences between means were evaluated by Tukey's test at a significance level of =0.05.Constancy classes, which followed Braun--Blanquet (1964), were determined based on a 3-year analysis of weed infestation of winter wheat crops performed before the harvest of each crop.

RESULTS
Weed infestation of the winter wheat crop, expressed by the number of weeds per 1m 2 (Table 1) and air-dry weight produced by them (Table 2), was highly significantly differentiated by weather conditions during the study years, but it was less differentiated by growth regulators used.The year 2005 was most favourable for the occurrence of weeds in great numbers in the winter wheat crop -166.8 plant × m -2 , whereas the year 2006 was the least favourable -5.4 plant × m -2 .The situation with weed biomass production was similar, as it was the highest in 2005 -65.75 g × m -2 and the lowest in the second year of the study -1.42 g × m -2 .
The number of weeds in the winter wheat crop was also observed to be significantly dependent on retardants used.The highest number of weed individuals was recorded in the control plots (without retardant) -96.3 plant•m -2 .Among the treatments with retardants, the lowest number of weeds was found in the plots sprayed with the retardant Moddus 250 EC -64.0 plant × m -2 , followed by those where the growth regulator Antywylegacz Płynny 675 SL was used -67 plant .m -2 .The application of the retardant Cecefon 465 SL resulted in the number of weeds of 77.8 plant × m -2 .This means that this latter growth regulator did not change significantly weed density per 1 m 2 of the winter wheat plots compared to both the control plots and the plots sprayed with the retardants.
The application of the adjuvant did not change significantly the number of weeds in the winter wheat crop.Nevertheless, it should be noted that the adjuvant Atpolan 80 EC used in combination with the retardants caused a clear decreasing trend in the number and weight of weeds only in the case of Antywylegacz Płynny 675 SL and Cecefon 465 SL (Tables 1 and 2).An opposite effect was obtained in the case of the retardant Moddus 250 EC, although the adjuvant Atpolan 80 EC had a clear beneficial effect, in combination with this compound, on the crop height of winter wheat stems (Table 3).
Weeds growing in the plots without retardant (control treatment) produced a weight of 28.26 g × m -2 .This value proved to be the lowest in the experiment, while compared to the growth regulator Cecefon 465 SL it was also significantly lower.Taking into consideration only the plots with retardants, it was found that weeds in the plot sprayed with Antywylegacz Płynny 675 SL produced the lowest average weight -34.34 g × m -2 , whereas it was the highest in the treatment with the retardant Cecefon 465 SL -41.68 g × m -2 (Table 2).However, the differences in weed weight between these treatments were within the limits of the experimental error.
Winter wheat plant height was significantly differentiated by all experimental factors, but to the greatest extent by growth regulators used (Table 3).On average for the three-year study period, the longest wheat stems (92.3 cm) were recorded in the control treatment (without retardant).All the retardants used significantly shortened wheat stem length and the retardant Cecefon 465 SL shortened it most of all (by 20.7%).This fact was translated into the highest dry weight of weeds and a distinctly higher number of weeds under the conditions of application of Cecefon 465 SL compared to the other retardants.The lowest retarding effect, though it was also significant, was found in the plots where the retardant Antywylegacz Płynny 675 SL was used -76.8 cm.Winter wheat plant height changed in particular seasons of the study at a level of 13.8%.Significantly lower plants developed in each successive year of the study.The study found a statistically proven positive effect of the adjuvant on reducing winter wheat stem length.The difference between treatments in favour of the one with adjuvant was 5.5% (Table 3).In the winter wheat crop, 25 weed species were found to occur in the plots with retardants and without adjuvant (Table 4) and 4 species less in the plots with retardants and with adjuvant (Table 5).Most weed taxa reached the number less than 1 plant .m -2 in the crop row.Therefore, the efficacy of the herbicides used should be considered to be satisfactory and it should be presumed that the occurrence of these taxa did not have a major effect on yield of the crop plant in question.Annual weeds such as: Veronica persica, Apera spica-venti, and Viola arvensis, dominated by far in the control treatment (without retardant) and in the plots with retardants.This means that the herbicides Apyros 75 WG and Starane 250 EC applied at the full tillering stage of winter wheat were the least effective against these species.Additionally, Veronica arvensis, Echinochloa crus-galli, Capsella bursa-pastoris, and Chenopodium album, reached significant numbers, that is, more than 1 plant × m -2 , in the control plots and generally also in the plots with retardants.The above-mentioned taxa as well as Elymus repens and Stellaria media, which occurred in some of the plots, were found to be the dominant taxa under the conditions of the present experiment.
Analysing the effect of the retardants used on phytosociological constancy of weeds in the winter wheat crop, it was found that both in the plots without adjuvant and with adjuvant application the following 3 species were constant components of this agricultural ecosystem (class IV): Chenopodium album in the treatment with Antywylegacz Płynny 675 SL, Veronica persica and Viola arvensis in the treatment with the retardant Moddus 250 EC as well as Veronica persica in the plot treated with Cecefon 465 SL -(Tables 6  and 7).Veronica persica (class IV), Chenopodium album (class IV), and Capsella bursa-pastoris (class III) reached equally high constancy in the control plot (Table 7).A numerous group of taxa was recorded in constancy class III, i.e. a class that includes species found with a frequency of at least 40%.The following species: Veronica persica, Viola arvensis, Chenopodium album, Capsella bursa-pastoris, and Elymus repens, were included in this class in the plots with retardants but without adjuvant, while in the treatments with retardants and with adjuvant the following species were additionally noted: Echinochloa crus-galli and Matricaria maritima subsp.inodora.The other species, observed in constancy class I or II, were loosely associated with the agrocenosis in question, since they inhabited it only in about 20% of identifications at the most.The adjuvant Atpolan 80 EC used in the experiment did not affect significantly the changes in the weed community inhabiting the winter wheat crop.It only reduced the occurrence of the species inhabiting the wheat crop recorded in the lowest constancy classes (I and II).0.0 x -the species found in a number less than 0.1 plant × m -2 ; --the species did not occur 0.0 x -the species found in a number less than 0.1 plant .m -2 ; --the species did not occur --the species did not occur

DISCUSSION
Excessive weed infestation of the crop is one of the factors reducing crop plant height and yield quality (A d a m c z e w s k i , 2000; W e s o ł o w s k i and W o ź n i a k , 2001).Winter cereals, due to their longer growing period, generally create greater fre-edom of growth for the weed flora than spring cereals.This statement applies to floristic richness, phenological aspects, and overall weed infestation (H o c h ó ł , 2003; B e r e s , 2010).In the study of B u j a k (1996), the number and weight of weeds were dependent on the type and number of tillage treatments.In the opinion of B u r a c z y ń s k a and C e g l a r e k ( 2008), the previous crop had a significant effect on the number and dry weight of weeds in the winter wheat crop.In reducing weed infestation, K o r r e s and F r o u d --W i l l i a m s (2002) draw attention to plant density per unit area in addition to the morphological characters.In their opinion, increased plant density reduces the penetration of solar radiation within the crop canopy, which has an inhibiting effect on weed infestation of fields.In the present study, the number and dry weight of weeds varied most of all by year (by 96.7%).The significantly highest number of weeds in the winter wheat crop was recorded in the year 2005 which was characterized by higher than average total rainfall and mean air temperature.In the other drier years, this number was three or four times lower.The growth regulators used modified significantly weed infestation of winter wheat, at a level of 17.7%.The influence of weather conditions during the growing season on the number and dry weight of weeds in the winter wheat crop has also been proved by W e s o ł o w s k i et al. (1993) and S m a t a n a et al. (2010).
Agricultural practices used in crop cultivation can result in reduced or increased weed infestation (D a s t g h e i b et al. 1999; S z e l e ź n i a k et al. 2007).Retardants, as agents that increase the resistance of cereals to lodging, shorten cereal stems by reducing the length of their individual internodes (C o x and O t i s , 1989).The present study showed a positive effect of retardant treatments on winter wheat plant height.The last year of the study proved to be the most beneficial for the action of the retardants, since in that year plants were found to be lower by 11.6 cm compared to the first year.Similar results have been obtained by A d a m c z e w s k i and P r a c z y k (1997), Ł ę g o w i a k and W y s m u ł e k (2000) as well as P i e t r y g a and D r z e w i e c k i (2003).The above--mentioned authors obtained a significant (12-18.5%)reduction in cereal plant height as influenced by the retardants Moddus 250 EC and Antywylegacz Płynny 675 SL, but only under favourable weather conditions.
The results of the present study show that stem shortening had a positive effect on reducing the total number of weeds in the crop in each year of the study.All the growth regulators used reduced weed density in winter wheat, but this reduction was the highest in the case of Moddus 250 EC and Antywylegacz Płynny 675 SL.In turn, the greater increase in biomass compared to the number of weeds proves that a less dense cereal crop not only promoted emergence of weeds, but also supported their luxuriant growth.S k r z y c z y ń s k a and P a w l o n k a (2004) also found a reduction in weed infestation of a winter triticale crop as influenced by retardants at a lower level of N (50 kg × ha -1 ).
The knowledge of the floristic composition of weed communities and dominant species in winter wheat crops plays a key role in planning treatments designed to reduce weed infestation (S k r z y c z y ń s k a and P a w l o n k a , 1999).The weed community in the plots analysed in the present study included a total of 25 weed species in the treatments without adjuvant and 21 species in the treatments with adjuvant.The taxa that were found in greatest numbers in both above--mentioned variants were as follows: Veronica persica, Apera spica-venti, Viola arvensis, Veronica arvensis, Echinochloa crus-galli, and Chenopodium album.Moreover, the following reached high numbers: Stellaria media and Elymus repens in the treatment with Antywylegacz Płynny 675 SL, Capsella bursa-pastoris in the treatment with the retardant Moddus 250 EC, and Capsella bursa-pastoris in the plot treated with Cecefonem 465 SL.
The adjuvant Atpolan 80 EC used in the experiment only slightly affected the weed community inhabiting the winter wheat plots.As a consequence, all species maintained similar constancy in the treatments compared.However, the effect of the adjuvant on reducing the level of weed infestation, as measured by the number and dry weight of weeds in the crop row, was found to show a positive trend.Similar results were obtained in the study of K w i a t k o w s k i and W e s o ł o w s k i (2011).The above-mentioned authors observed a significant reduction in the number of weeds in the crop by respectively 45% (Atpolan 80 EC), 39% (Break Thru S 240), and 34% (ammonium sulphate).The adjuvants used also had a significant effect on reducing the weight of weeds in the crop compared to the control treatment, as this weight was reduced more than twice.
species did not occur

Table 1
Number of weeds per 1m 2 in the winter wheat crop a -without adjuvant; b -adjuvant Atpolan 80 EC; n.s.-not significant a -without adjuvant; b -adjuvant Atpolan 80 EC; n.s.-not significant

Table 4
Numbers of weed species per 1 m 2 in the winter wheat crop depending on retardants used, without adjuvant (mean for 3 years)

Table 5
Number of weed species per 1 m 2 in the winter wheat crop depending on retardants used, with adjuvant (mean for 3 years)

Table 6
Constancy of weeds in the winter wheat crop in the plots without adjuvant (mean for 3 years)

Table 7
Constancy of weeds in the winter wheat crop in the plots with adjuvant (mean for 3 years)