Competition of Weeds Dominated by Wild Oat ( Avena sterilis ) in Wheat ( Triticum durum Desf.) in Jordan

: Wheat is the main ﬁeld crop grown in Jordan. Productivity is low due to different factors, including erratic rainfall, poor soil fertility and weed competition. A ﬁeld experiment was carried out at the University of Jordan Research Station for two growing seasons to determine the effect of weed competition on growth and yield of wheat ( Triticum durum Cv. “Hourani”). Treatments consisted of either allowing weeds to infest the crop or maintaining the crop weed-free for increasing durations after emergence. Results showed that, the longer the periods of weed competition, the greater the loss in crop growth and yield. Average reductions in grain and straw yields were 41% and 37%, respectively. The highest grain yield obtained was from weed-free, and the lowest was in weed-infested plots for the entire growing season. Average grain yield was not signiﬁcantly different at 14 to 49 days of weed-infested periods, while none of the weed-free periods produced a yield similar to that of the weed-free control. However, maintaining a weed-free crop for three weeks after emergence signiﬁcantly increased grain yield compared with the weed-infested control. High rainfall in the ﬁrst season almost doubled weed growth and greatly reduced wheat growth and yield compared with the second season. To determine the critical period of weed competition and the inﬂuence of weed infestation on wheat grain yield, Gompertz and logistic equations were ﬁtted to data representing increasing duration of weed-free and weed-infested periods, respectively. Based upon an arbitrary 5% level of average grain yield loss in the two years, the critical period of weed competition occurred at 0–49 days after wheat emergence, which corresponded with the rapid increase in weed biomass.


Introduction
Wheat is the main field crop grown under rainfed conditions in Jordan. The country has not been able to meet its basic food requirements in recent years. Productivity is generally low and has not been exceeding 1200 kgha −1 in recent years [1][2][3]. Although the area planted by this crop is becoming more restricted to the marginal land of low annual rainfall, it remains the largest in terms of land cultivated. A total of 60,000 ha are planted with wheat every year; the country's wheat production meets its needs for only 10 days [4]. Though Jordan wheat production fluctuated substantially in recent years, it tended to decrease through the 1970-2019 period, ending at 25,000 tons in 2019 [5]. In 2019, wheat production for Jordan was 25 tones. Though Jordan wheat production fluctuated substantially in recent years, it tended to decrease through 1970-2019 period ending at 25 tones in 2019.The area grown by this crop, however, is also decreasing over time. In 1980, for example, wheat occupied about 133,000 ha, and the total yield was 134,000 t [6]. In 2019, wheat production for Jordan was 25 tones. Though Jordan wheat production fluctuated substantially in recent years, it tended to decrease through 1970-2019 period ending at 25 tones in 2019. At present, Jordan imports more than 96% of its wheat needs [7].
Erratic and fluctuated rainfall distribution, poor soil fertility, primitive seed bed preparation practices, use of low yielding varieties with little response to fertilizers and the In general, a limited number of studies was reported on A. sterilis competition in wheat. The literature on the determination of the critical period of weed competition including A. sterilis in this crop is lacking under local conditions. Therefore, the present work was conducted to study the competition effect of weeds dominated by wild oat (A. sterilis) on growth and yield of wheat and to determine the critical period of weed competition based on weed-infested and weed-free treatments after crop emergence. Information on this period could be used by local farmers to decide suitable weed control methods at a time that will have maximum benefit to the crop growth and productivity.

Field Experiments
A field experiment was conducted to study the effect of weed competition on wheat (Triticum durum Desf. cv. "Hourani") for two years. The "Hourani" cultivar was chosen because it is usually preferred and grown by local farmers, since it is very well adapted to the low rainfall and poor soil fertility conditions prevailing in most areas that grow cereals in the country. In both growing seasons, the experiment was carried out at the University of Jordan Research Station at the campus, Al-Jubeiha, located at an elevation of 980 m above sea level, with an average annual precipitation of about 450 mm. The soil is clay loam and contained 62.3% clay, 36.7% silt, 1.1% sand and 0.7 organic matter; it had a pH of approximately 7.6. The soil of the experimental site mostly represented the soil type used to grow wheat in the country.
The soil was tilled twice, one by chisel to a depth of 20 to 25 cm, followed by a rotary cultivator. The soil land was leveled, and rows were made-opened by using a small hand hoe. Plot size was 3 × 1.05 m 2 and consisted of three rows. Planting distance was 35 cm between rows, and 40cm was left between plots. Total number of plots per block was 14, giving a total number of 56 plots in the experimental site each year. Wheat seeds of the previous year harvest were sown at a rate of 100 kg ha −1 on November each year. During both growing seasons, NPK fertilizer (18:10:16) was added a month after sowing at 250 kgha −1 , and then another application was applied two months later, using the same rate. Wheat plants were left growing under rainfed conditions. Information on prevailing climatic conditions at the experimental site during both growing seasons is shown in Table 1. In both experiments, weeds were allowed to compete with wheat plants for 0, 14, 21, 28, 35, 42 and 49 days after crop emergence; they were then removed, and plots were kept weed-free thereafter. In other plots, weeds were continuously hand-weeded and kept weed-free for similar periods after wheat emergence. Then weeds were left to grow until harvest. Weed-removal treatments (zero time) were started on 9 and 31 December in both years, respectively.
Plants were harvested at 5 cm from the above soil level on early July each year. Data on wheat grain and straw yields, plant height, and weight of 1000 kernels were recorded. Weeds in the experimental area were also harvested and separated into broadleaf and wild oat; all species were identified and their densities determined. Panicle number of wild oat plant was also recorded in the second growing season. Weeds were oven-dried at 70 • C for 72 h, and their dry weight was determined.

Statistical Analysis
Treatments in both experiments were laid out in a randomized complete blockdesign with four replicates. Data of each experiment were first subjected to the analysis of variance (ANOVA), using SAS software SAS (r) version 9.1 [35]. Treatments means were separated and compared by using the least significant difference test (LSD) at 5% level of probability.
Data on wheat growth and grain yield and that on weed-shoot dry weight in the two experiments were combined together and averaged. All wheat data were expressed as a percentage of the weed-free plots (0 time of weed-infested treatment), while weed-shoot dry weight was presented as a percentage of their dry weight in the weed-infested plants (0 time of weed-free treatment). To determine the effects of weed duration on wheat, a nonlinear regression line [36] was compared for average wheat grain and straw yield and weed competition or free periods. Similar comparisons were also performed between these treatments and weed-shoot dry weight. The Gompertz equation [37] was used to describe the increasing duration of weed-free on wheat grain and straw yield: where Y is the yield (% of season-long weed-free wheat), A is the yield asymptote (% of season-long weed-free wheat), B and k are constants, and T is the time from emergence (days). The logistic equation [38,39] was used to describe the effect of increasing duration of weed infestation on grain and straw yields: where Y is the yield (% of season-long weed-free wheat), T is the time from emergence (days), X is the point of inflection (days), and D and F are constants. Time (T) was expressed as days after wheat emergence date so the critical period of weed competition could be compared. Using a 5% level of accepted yield loss, we determined the beginning and end of the critical period by using the Gompertz and logistic equations. Grain yield data were also regressed against weed dry weight for weed-free and weed-infested treatments.

Weed Population
The naturally occurring weed species in the experimental site and their average densities are presented in Table 2. Wild oat (Avena sterilis L.) was the dominant in the two years, with an average density of 128 plants m −2 , followed by forked mouse-ear chickweed (Cerastium dichotomum L.) at 45 plants m −2 .

Wheat Growth
Weed infestation for different periods after wheat emergence reduced crop grain and straw yields compared with weed-free control for the entire growing period (Table 3).
Differences in grain yield were not significant when weeds competed for up to 3 weeks after crop emergence. Longer weed competition periods significantly reduced wheat grain yield, with the lowest yield being obtained when weeds competed with crop plants for the entire growing season. Reduction in wheat grain yield of weed-infested plots for the whole growing season was by 62.2% of the weed-free control. Weed infestation for only 4 weeks after crop emergence resulted in 36% grain yield loss compared with weed-free control, and yield was not significantly changed at longer competition periods until harvest. On the other hand, for the weed-free period up to 4 weeks after crop emergence, although the grain yield increased compared with weed-infested control, the differences were not significant  (Table 3). Crop plants kept weed-free for 5 weeks onward resulted in a grain yield not significantly different from that of the weed-free control for the entire growing season. Almost similar trend in the effect of weeds on wheat straw yield was also obtained ( Table 3). None of the weed-free periods gave straw yield similar to that of the weed-free control for the entire growing season (Table 3). Straw yield reduced more with longer weed competition periods. Generally, a long weed-free period increased straw yield, but the highest significant values obtained were when the crop was kept weed-free for 6 weeks or longer after emergence. The highest total wheat yield (grain and straw) obtained was when weeds competed with the crop not longer than 3 weeks after crop emergence and when kept weed-free for at least 6 weeks after emergence. Total wheat yield in these treatments was similar to that of the weed-free control (data not presented).
The effect of weed competition on weight of 1000 kernels showed no significant difference between the weed-free control, weed-infested crop up to 3 weeks after emergence and weed-free wheat for 7 weeks after emergence (Table 3).
Although crop height was almost similar in all weed-free treatments but was generally lower than that in weed-infested treatments, the differences were not significant (Table 3).

Weed Growth
Total weed-shoot dry weight increased at long competition periods with the highest weight obtained from weed-infested plots for the entire growing season (Table 2). However, the weed-shoot dry weight was not significantly changed at periods from 5 to 7 weeks after crop emergence. At harvest, none of the weed-free periods gave weed growth (in terms of shoot dry weight) similar to that of the weed-infested control. However, crop left Agronomy 2021, 11, 1816 6 of 16 weed-free for short periods (2 to 3 weeks after emergence) resulted highest weed growth among the weed-free periods.
The weed-shoot dry weight produced in response to both weed-infested and weedfree periods calculated as a percentage of weed-infested control is shown in Table 3. Weed growth was greater in weed-infested than in weed-free treatments.

Wheat Growth
Both wheat grain and straw yields were reduced with weed competition. Reduction in grain yield was by 24.6% and in straw yield by 20.7% of the wheat-free control ( Table 4).
The grain yield was more reduced with weed-infested periods than straw yield. Weed infestation for 6 weeks after wheat emergence produced a grain yield that was not significantly different from that of the weed-free control for the entire growing season. A weed-free period for 4 weeks after crop emergence was enough to produce a grain yield similar to that of the weed-free control ( Table 4). The grain yield of weed-free crop up to 3 weeks after emergence was not significantly different from that of the weedinfested control.
The effect of weed competition on wheat straw yield showed an almost-similar trend to that of grain yield, but with lower responses. Only weed infestation for the entire growing season significantly reduced straw yield. However, the weed-free period for 7 weeks after crop emergence increased straw yield by 40.08% compared with the weed-infested control, and by 11.04% over that of the weed-free control (Table 4). The total (grain and straw) wheat yield was not significantly changed between weedfree control and weed-infested periods up to 7 weeks after emergence and between these and weed-free periods for 7 weeks after wheat emergence (data not presented).
Kernel weight was not significantly changed at any weed-infested or weed-free periods from that of the weed free-control, and the same was for crop height; however, wheat plants were higher in the second growing season (Table 4).

Weed Growth
The weed-shoot dry weight increased with weed-infested periods (Table 4), but it did not significantly change at periods from 2 to 4 and 5 to 7 weeks after crop emergence. The highest weed growth was when weeds competed with crop plants until harvest. Differences however, in weed dry weight were not significant between weed-infested control and weeds competed for 7 weeks after crop emergence. On the other hand, weed dry weight was reduced with weed-free periods with the highest weight at 2 to 3 weeks weed-free periods after crop emergence (Table 4). The lowest weed growth obtained was at the longest weed-free periods (6 and 7 weeks after emergence).
Considering the weed-shoot dry weight calculated as a percentage of weed-infested control, weed response to weed-infested and weed-free periods is shown in Table 4. The weed-free period from 14 to 21 days after crop emergence was the period at which weeds exerted maximum effects on crop plants and weeds attained the highest growth and shoot dry weight among weed-free treatments. Weed competition for the entire growing season reduced average grain and straw yields by 41% and 37%, respectively (Figure 1a,b). The grain yield was reduced by 9.51% when plots were kept weed-infested for 28 days after crop emergence. Meanwhile, plots kept weed-free for a similar period provided 55.56% higher grain yield than that of the weed-infested control (Figure 2a). Both wheat grain and straw yields were dramatically decreased with weed competition for 14 days or longer periods after emergence (Figure  1a,b). A strong correlation coefficient (R 2 = 0.839) was found between weed-infested periods and grain yield and between these and straw yield (R 2 = 0.89). In contrast, the grain-yield line showed slight increases (not significantly changed) at weed-free periods from 14 to 28 days after emergence, and the straw yield showed almost a similar trend (Figure 2b) at periods from 28 to 42 days after emergence. However, weed-free treatment for any period increased both parameters compared with the weed-infested control. A stronger correlation coefficient (R 2 = 0.995) between weed-free duration and average wheat grain yield was found than with straw yield (R 2 = 0.691). Regression analysis of average grain yield against weed-shoot dry weight at different weed-free and weed-infested periods resulted in a linear decrease in crop yield with competition period, with coefficient of duration R 2 = 0.978 (Y = 0.008 × 4−0.878 × 3 + 29.79 × 2−347.4 × 1925) and a linear increase with weed-free period. Wheat grain yield decreased, and weed biomass increased in weed-infested treatments, and the opposite was true in the weed-free treatments.
shoot dry weight among weed-free treatments. Weed competition for the entire growing season reduced average grain and straw yields by 41% and 37%, respectively (Figure 1a,b). The grain yield was reduced by 9.51% when plots were kept weed-infested for 28 days after crop emergence. Meanwhile, plots kept weed-free for a similar period provided 55.56% higher grain yield than that of the weed-infested control (Figure 2a). Both wheat grain and straw yields were dramatically decreased with weed competition for 14 days or longer periods after emergence ( Figure  1a,b). A strong correlation coefficient (R 2 = 0.839) was found between weed-infested periods and grain yield and between these and straw yield (R 2 = 0.89). In contrast, the grain-yield line showed slight increases (not significantly changed) at weed-free periods from 14 to 28 days after emergence, and the straw yield showed almost a similar trend (Figure 2b) at periods from 28 to 42 days after emergence. However, weed-free treatment for any period increased both parameters compared with the weed-infested control. A stronger correlation coefficient (R 2 = 0.995) between weed-free duration and average wheat grain yield was found than with straw yield (R 2 = 0.691). Regression analysis of average grain yield against weed-shoot dry weight at different weed-free and weed-infested periods resulted in a linear decrease in crop yield with competition period, with coefficient of duration R 2 = 0.978 (Y = 0.008 × 4 − 0.878 × 3+29.79 × 2 − 347.4 × 1925) and a linear increase with weed-free period. Wheat grain yield decreased, and weed biomass increased in weed-infested treatments, and the opposite was true in the weed-free treatments.  Grain yield losses decreased as the weed-free period increased an deceased, with the least weed growth (<22.2% of weed-infested cont the weed-free period (Figure 3). At this point, wheat grain yield w weed competition did not affect wheat grain yield as that at early com Grain yield losses decreased as the weed-free period increased and weed dry weight deceased, with the least weed growth (<22.2% of weed-infested control) at 36 days into the weed-free period (Figure 3). At this point, wheat grain yield was maximum. Late weed competition did not affect wheat grain yield as that at early competition. Agronomy 2021, 11, x FOR PEER REVIEW 10 of 16 Figure 3. Effect of weed-free periods on average grain yield or total weed dry weight for two growing seasons.

Effect of Weed-Infested and Weed-Free Periods on Average Grain Yield of Both Seasons
The average grain yield of wheat in weed-infested and weed-free treatments is shown in Figure 4. Trends in wheat yields and weed growth at different periods of weed-infested and weed-free treatments were similar to those shown for the effect in each season separately. Generally, average wheat grain yield was strongly and negatively correlated with weed-infested period (R 2 = 0.978) and increased with weed-free periods (R 2 = 0.995). The highest yield obtained was from crop kept weed-free for the whole growing season and the lowest from weed-infested plots for the entire growing season. However, drastic reduction in grain yield was obtained with weed-infested periods, while the response of wheat yield increases was slow in the weed-free treatments. The R 2 for the nonlinear relationship between wheat grain yield and the period of competition was 0.839. Based on the arbitrary 5% level of grain yield loss, the critical period of weed competition occurred between 0 and 49 days after wheat emergence (Figure 4).
Results showed that yield loss was progressively greater with a longer duration of weed competition. However, substantial amounts of weed dry weight were produced, particularly at longer competition periods or shorter weed-free periods ( Figure 5).

Effect of Weed-Infested and Weed-Free Periods on Average Grain Yield of Both Seasons
The average grain yield of wheat in weed-infested and weed-free treatments is shown in Figure 4. Trends in wheat yields and weed growth at different periods of weed-infested and weed-free treatments were similar to those shown for the effect in each season separately. Generally, average wheat grain yield was strongly and negatively correlated with weed-infested period (R 2 = 0.978) and increased with weed-free periods (R 2 = 0.995). The highest yield obtained was from crop kept weed-free for the whole growing season and the lowest from weed-infested plots for the entire growing season. However, drastic reduction in grain yield was obtained with weed-infested periods, while the response of wheat yield increases was slow in the weed-free treatments. The R 2 for the nonlinear relationship between wheat grain yield and the period of competition was 0.839. Based on the arbitrary 5% level of grain yield loss, the critical period of weed competition occurred between 0 and 49 days after wheat emergence (Figure 4).

Effect of Weed-Infested and Weed-Free Periods on Average Grain Yield of Both Seasons
The average grain yield of wheat in weed-infested and weed-free treatments is shown in Figure 4. Trends in wheat yields and weed growth at different periods of weed-infested and weed-free treatments were similar to those shown for the effect in each season separately. Generally, average wheat grain yield was strongly and negatively correlated with weed-infested period (R 2 = 0.978) and increased with weed-free periods (R 2 = 0.995). The highest yield obtained was from crop kept weed-free for the whole growing season and the lowest from weed-infested plots for the entire y = 0.008x 4 -0.878x 3 + 29.

Weed-free period (day)
Total weed dry weight Grain (kgha-1) Figure 4. Wheat grain yield as affected by increasing durations after emergence of the crop maintained as either weed free ( ) or weed-infested ( ). The critical period of weed competition (CPC) is the interval between time after emergence for which weed emergence reduced yields by 5% and time after emergence at which crop had to be maintained weed-free to prevent a 5% yield loss. Curves based on average combined data of two years.
Results showed that yield loss was progressively greater with a longer duration of weed competition. However, substantial amounts of weed dry weight were produced, particularly at longer competition periods or shorter weed-free periods ( Figure 5).
Agronomy 2021, 11, x FOR PEER REVIEW 11 of 16 time after emergence at which crop had to be maintained weed-free to prevent a 5% yield loss. Curves based on average combined data of two years.

Figure 5.
Weed dry weight as affected by increasing durations of weed-free (□) or weed-infested (♦). Weed dry weights were measured at the various times of weed removal for plots initially infested with weeds and at harvest for the treatments kept weed-free until a specified period. Data are average of two years.

The Critical Period of Weed Competition
Parameters for equations depicting the critical time of weed removal and the critical weed-free period in wheat are summarized in Table 5. The R 2 for nonlinear relationship between wheat grain yield and the period of competition equals 0.511 for the logistic equation and 0.745 for the Gompertz equation. Table 5. Parameter estimates and correlation coefficient for logistic equation used to fit grain-yield data for increasing weed-infested period (from time of emergence) and parameter estimates for the Gompertz equation used to fit yield data for increasing weed-free period (after emergence). In this study, the beginning of the critical period was defined as the time after crop emergence by which weed competition reduced yields by 5%. Similarly, the end of the critical period was defined as the period at which the crop must remain free of weeds to prevent grain yield loss from exceeding 5%. Based on average grain yield loss in the two growing seasons, the critical period of weed competition occurred between 0 and 49 days after wheat emergence (Table 6). Table 6. The onset and the end of the critical period of weed competition for three levels of grain yield loss in wheat from the logistic and Gompertz equations.

Logistic Gompertz Time of Indicated %Yield loss
Time of Indicated %Yield loss 2.5% 5% 10% 2.5% 5% 10% 0 0 0 49 49 49 Results showed that yield loss was progressively greater with a longer duration of weed competition. Meanwhile, the onset of competition did not vary at a 2.5-10% level of grain yield losses, and the end was the same (49 days) after wheat emergence. Weed dry weights were measured at the various times of weed removal for plots initially infested with weeds and at harvest for the treatments kept weed-free until a specified period. Data are average of two years.

The Critical Period of Weed Competition
Parameters for equations depicting the critical time of weed removal and the critical weed-free period in wheat are summarized in Table 5. The R 2 for nonlinear relationship between wheat grain yield and the period of competition equals 0.511 for the logistic equation and 0.745 for the Gompertz equation. Table 5. Parameter estimates and correlation coefficient for logistic equation used to fit grain-yield data for increasing weed-infested period (from time of emergence) and parameter estimates for the Gompertz equation used to fit yield data for increasing weed-free period (after emergence). In this study, the beginning of the critical period was defined as the time after crop emergence by which weed competition reduced yields by 5%. Similarly, the end of the critical period was defined as the period at which the crop must remain free of weeds to prevent grain yield loss from exceeding 5%. Based on average grain yield loss in the two growing seasons, the critical period of weed competition occurred between 0 and 49 days after wheat emergence (Table 6). Table 6. The onset and the end of the critical period of weed competition for three levels of grain yield loss in wheat from the logistic and Gompertz equations.

Time of Indicated %Yield Loss Time of Indicated %Yield Loss
2.5% 5% 10% 2.5% 5% 10% 0 0 0 49 49 49 Results showed that yield loss was progressively greater with a longer duration of weed competition. Meanwhile, the onset of competition did not vary at a 2.5-10% level of grain yield losses, and the end was the same (49 days) after wheat emergence.

Discussion
Wheat suffers most from weed competition that can severely reduce yield quantity and quality [1]. Different weed species are usually associated with wheat [40], but narrowleaved weeds and mainly grasses represent a real threat to crop growth and productivity. Wild-oat morphological and physiological similarities and growth requirements to wheat plants increased their dominance in absence of selective and effective herbicides, crop rotation, and difficulty in differentiating the weed from crop plants by untrained labors early in growth.
Weed competition for the entire growing season reduced average grain yield by 41% and straw yield by 37% compared with the weed-free control. Other workers reported 16-75% wheat yield reduction and low quality due to wild-oat competition [11][12][13]. However, the longer the weed competition period, the more there are negative effects of the weed on wheat grain and straw yields (Tables 3 and 4).
The critical period of weed competition in wheat is rarely determined based on weed competition duration, but general weed-competition studies based on emergence time of competing species, weed and crop densities, or weed/crop density ratios are varied [14][15][16]. Reduction in grain yield was comparable to that reported by other workers at similar weed densities [11,12]. In the present work, the dramatic grain yield reduction in weed-infested treatments and the slow response of the crop to weed-free periods indicate the strong competition effect that the weed exerted on crop plants. Higher wheat plants in weedinfested plots than in weed-free plots (Table 3) indicate a strong competition for light. In dense weed population, phenotypic plasticity is an important character that allows crop plants to elongate and trap light.
Considering the average data of two years of experiments, it is clearly demonstrated that weed competition for any period after crop emergence seriously reduced wheat grain and straw yield (Figure 1a,b), while weed-free periods longer than 14 days after emergence increased crop grain yield (Figure 2a). In most cases, early weed competition is irreversible and cannot be compensated by adding more resources or late weed control [41]. Weed removal failed to overcome competition effects after certain periods, and the crop didnot recover from weed competition. Moreover, keeping the crop weed-free for certain periods didnot eliminate the competition effects of the late emerged weeds (Figure 2a,b) except at the longest period or when weed infestation was only for a short time.
Weed shoot dry weight was almost doubled at each increment increase in weed competition period ( Figure 5) but greatly reduced after each weed-free period and not substantially changed at periods longer than 28 days after crop emergence. This indicates that wheat plants suffer from weed competition shortly after emergence but can effectively suppress weeds when maintained weed-free for even 14 days after emergence. These results were compatible with the findings of other workers emphasized the importance of weed removal early in the season [42]. Wheat has been mentioned as one of the good competing crops when establish free from weeds for a relatively short period after emergence [43]. However, results showed that delay in weed removal for 14 days after crop emergence resulted average grain yield loss by 42% (Figure 4), while crop grain yield loss was reduced with weed-free period. It seems that weeds could not tolerate the shading effects offered by wheat plants to effectively compete when the crop had an initial start. This was demonstrated from the high reduction (>58% of the weed-infested control) in weed dry weight obtained as a result of delay in weed emergence for 14 days after crop emerged. Total biomass of weeds emerged thereafter was less than 12% that of the weed-infested control ( Figure 5). The critical period of weed competition coincides with the beginning of duration increase in weed dry weight. However, this period may last longer for lower levels of grain yield loss (Table 4). The determined period, however, occurred before the crop-tillering stage, which in turn supports data obtained on crop growth and yield. At a longer weed-free period than 42 days after crop emergence, it was possible to maintain a crop yield comparable to that of the weed-free control, since competition for moisture and nutrients was lessened early in crop growth. Wheat plants after this period enter into a full tiller resting stage, with good growth and resource accumulation allowing regrowth, active cell division, and stem elongation. However, any wildoat plants left during or after the crop tillering stage were greatly influenced, and the number of panicleshedding seeds for re-infestation was reduced and can be easily and selectively removed, using other methods of weed control, including herbicides. However, the timing of weed removal is influenced by wheat cultivar, planting density, agricultural practices, dominant weed species (wild oat) and its density, prevailing environmental conditions, and weed population composition. Wheat cultivars were found different in competition with weeds (including A. sterilis) [12,44,45] and in responses to growth factors [45]. The "Hourani" wheat cultivar used in this study is not among the strong competing cultivars grown in Jordan and thus might have greatly suffered from weed competition [46]. Hucl [47] stated that the tillering capacity of wheat cultivars is extremely important in the competition against weeds. Certainly, the flush of weeds emerging after the occurrence of the critical period of weed competition would need to be controlled in order to prevent re-infestation. Wheat suppressed development of the late-emerging weeds; hence, the weed dry weight at harvest was greatly reduced by 87% when wheat was kept weed-free until 21 days after emergence. It is obviously shown that wheat can withstand late weed competition better than at early competition, but when plots were initially kept weed-free, the grain yield loss was greatly reduced, indicating that weeds that emerged after the initial weed-free period had a very low effect on crop yield. These results agreed with those of Dimson [48]. Weed-shoot dry weight proved to be a good indicator of weed competition. The differences in weed-shoot dry weight between the two lines in Figure 5 represent the suppressing effect of wheat upon the weeds. The long critical period of weed competition indicates a greater yield loss due to weed-competition durations. However, this period is likely conservative and may be extended with less competitive weed species and a more competitive crop cultivar, or with different agricultural practices.
Results indicated that weed competition for a short period early in the season is serious and causes significant yield loss, and thus it leaves no alternative to complete weed removal to avoid crop growth and yield reductions. These results agreed with the findings of Singh and Bajpai [19], who reported that 4 weeks of hand-weeding at 25-70 days after sowing resulted in the best weed control and highest wheat yield. All weed-infested and weed-free treatments failed to give straw yield similar to that of the weed-free control in the first growing season (Table 3), but generally weed-free periods in the second season gave similar or more straw yield than the weed-free control (Table 4). These results may be due to the fact that the average annual rainfall in the first growing season was almost doubled, better distributed, and conserved compared with that of the second season. High rainfall in the first season aggravated weed-competition effects on crop growth and grain yield, as compared with the second season. Total weed-shoot dry weight in the second season was only 41.2% that of the first growing season. This, however, resulted in more wheat growth and yield reductions and doubled weeds' dry-matter production. It has been reported that wild oat affected grain yield more than straw weight and wheat development, productivity, and total accumulation of N and P all effected, whereas, in the weed, the competition offered by crop plants affected straw yield more than grain yield [49]. Results of the present study may confirm the high level of water needed for the high growth and competitiveness of wild oat.
The tendency of crop plants to give high growth or yield shortly after the start of weed infestation results from the fact that crop plants tend to expand resource capture or conversion efficiency in response to early or short weed-competition periods or to low weed density. However, this crop competitive advantage is rapidly dissipated at longer competition periods or at higher weed densities.
Torner et al. [50] reported that, as the density of A. sterilis increased, barley yield decreased exponentially. This relationship between weeds and crop plants occasionally occurs, since weeds have more chances to exhaust crop growth by removing more resources and exert more smothering effects on crop plants. However, most annual and effective weed species, including wild oat, tend to complete their life cycle earlier than crop plants, shedding seeds and thus ensuring their existence in the environment. Therefore, low shoot dry weight of weeds in weed-infested control or at long weed competition periods was found since weeds matured, dried and windblown before harvested. This was well demonstrated in the first growing season, which received a high average rainfall.

Conclusions
Early weed competition harmed wheat and lowered crop grain and straw yield. Wheat must be kept weed-free for at least 42 days after emergence in order to prevent significant grain yield loss. High rainfall in the first growing season doubled wild oat growth and, thus, its competition effects. Considering average grain yield of the two growing seasons, the critical period of weed (dominated by wild oat) competition occurred at 0-49 days after crop emergence. In both growing seasons, weeds emerged after weedfree periods attained low growth in general, and their competition effects on crop plants appeared lower than the weed-infested crop early in the season. These results confirm the importance of weed control early in the season in order to avoid crop yield reduction. Future research work may test the competitive abilities of different wheat cultivars in the country and possible introduction of certain plant species, including segetal weeds, that keep pathogens away from crop plants, favor pollination of cereals and propose specific actions for the introduction/reintroduction of the other disappeared or rare segetal species at field level [51]. This approach is becoming more important, considering possible future scenarios when intensive farming is possibly changed to an extensive and organic farming with a lower environmental impact [52,53]. These crop or weed species can effectively compete with A. Sterilis and reduce its impact on wheat crop.This however, may be also achieved by following an effective crop rotation or intercropping system.