Cyperus iria Weed Growth, Survival, and Fecundity in Response to Varying Weed Emergence Times and Densities in Dry-Seeded Rice Systems

Abstract

Cyperus iria is amongst the most threatening weeds of rice in Sri Lanka, India, Pakistan, and the Philippines. Broad knowledge about the ecology and fecundity of C. iria is important for its effective management. Field studies were conducted over two seasons (wet (WS) and dry seasons (DS)) to investigate the influence of C. iria populations (40 and 80 plants m⁻²) on its growth, survival, and fertility, with four emergence times, 3, 15, 30, and 45 d after rice emergence (DARE), during 2013. We postulated that (a) higher plant density of this weed would result in more biomass production and viable seeds per unit area, (b) interference of rice would reduce the biomass production and fecundity of this weed, and (c) delaying weed emergence would lead to downsizing of its percent survival and seed bank enrichment in soil. The results indicated that rice interference decreased C. iria growth and seed production as compared with those of plants established without rice interference. A linear decrease in the percent survival of C. iria without rice and a sigmoid decrease with rice were observed during both seasons. Plant height of C. iria was moderately affected up to 30 DARE, and a significant reduction was observed at 45 DARE. There was a linear relationship between C. iria shoot dry weight and seed number plant ⁻¹, across-weed density, seeding rate of rice, and emergence time. C. iria seed production, 1000-seed weight, and seed yield were greater when seedlings emerged simultaneously with the rice crop (3 DARE) than when they emerged late. Under rice weed interference growth, the production of viable seeds was completely stopped at 45 DARE. The delay in the emergence of C. iria up to 45 DARE was unable to produce seed in both seasons. The results of the current studies advocate that the emergence, weed biomass, and seed production of C. iria can be checked by adopting suitable cultural weed management practices, which can impede the emergence of weed relative to rice. These practices, enabling the respective crops to be more competitive, will foster integrated weed management approaches, thus offering a key role in seed detection to the soil seed reservoir or pool by notorious weeds in the field.

1. Introduction

Rice is grown mostly in Asia (90%). The dramatically growing population requires more water for consumption, and this will cause severe scarcity in foreseeable future; 22 million ha of paddy fields are likely expected to face water scarcity by 2025 in South and Southeast Asia [1]. This water limitation deters the rice yield, and farmers experience a loss of more than one billion USD annually. Rice in Asia is primarily established through seedling transplanting in puddled soil; however, it requires strenuous effort and engulfs abundant precious water quantity. The shortage of resources has compelled scientists and growers to search for alternative ways for establishing rice, such dry seeding. Dry-seeded rice (DSR) has numerous benefits over transplanted rice, namely quick and in-time planting, reduced labour (almost above 50%) requirement [2], and hastened crop maturity by 8–10 days earlier than transplanted. A previous study revealed that DSR saved 73% of the crucial irrigation water required during puddling and 56% during the crop growth period [3]. DSR encompasses more water- and nutrient-efficient methods, including the development of an efficient root system to improve drought tolerance relative to that of traditional transplanted crops [4]. In short, DSR crops are quick and easy to plant, less labour-intensive, and use less water besides higher economic returns [5].

DSR confronts the issue of early substantial weed infestation owing to simultaneous weed and rice germination leading to closer proximity to each other [6]. This weed infestation tends to cause yield losses, usually ranging from 33 to 70% in DSR [7,8,9] and sometimes even reaching 99%. Therefore, efficient weed control is the main concern in DSR. For the past two decades, in the process of controlling E. crus-galli (L.) Beauv. and E. colona (L.) Link, new, troublesome weeds, such as Cyperus iria L. and Sphenochlea zeylanica Gaertn., have emerged [10]. C. iria (rice flatsedge, family Cyperaceae) is an aggressive weed of rice with the ability to multiply rapidly with abundant seed production capacity (3000–5000 seeds plant⁻¹). The seedlings’ emergence can be observed summarily after rice sowing, and the seedlings flower almost one month after emergence and can institute the second progeny phase during the same crop season [11]. C. iria is a devastating weed in rice across the tropical and subtropical rice-producing areas [12,13]. It is the most common and tyrannizing weed in direct-seeded rice, marring both quantity and quality [14]. Additionally, it acts as a host, largely to insects [15]. Because of substantial weed infestation and yield losses in DSR systems, most farmers usually depend on herbicides to manage their weeds [16]. However, nonjudicious and continuous use of herbicides has led to many weeds developing herbicide resistance [17,18]. C. iria has developed resistance to herbicides in the B/2 group, which are also called acetolactate synthase (ALS) inhibitors, auxin inhibitors, and acetohydroxyacid synthase (AHAS) inhibitors [19,20]. To curb this increasing trend of herbicide resistance, there is a dire need to explore further weed management approaches such as competitive varieties, high planting densities, and plant geometry [14,18,21,22,23,24,25]. Moreover, proper fertilizer management may accelerate the crop’s competitive ability over weeds and resist herbicide resistance in weeds; it thus needs to be investigated as a pillar of integrated weed management (IWM) programs [25,26,27]. However, IWM faces hindrance in its full adoption in DSR systems owing to limited and inadequate knowledge of basic weed biology and the ecology of noxious weeds, including C. iria [24,28].

Information about the impact of weed emergence timing in relation to rice on weed–crop competition is useful for formulating influential weed management techniques [16]. Generally, late-emerging weeds are less competitive than early-emerging weeds, depending on the weed species and prevailing environmental factors [29,30,31,32].

Previously, several researchers determined the effect of the emergence time of weeds on crop–weed competition [33,34,35,36]. Most of these studies did not describe weed mortality rates linked with delayed weed emergence [30,37]. The decreased seedling survival rate in the presence of a crop canopy has been reported in previous studies on weeds other than C. iria [16,30,38]. However, weeds that emerge after the crop generally escape postemergence weed control measures, and the persistence of these late-germinating weeds allows them to proliferate more than early-establishing weed species [26]. Traditionally, studies have aimed to quantify and document the effect of emergence time on the growth behaviour and crop yield, and eventually, weed management strategies became focused on increasing crop yield, whereas little attention was devoted to reducing the soil seed bank by limiting the seed production of the major weeds problematic for rice [16,39,40].

A better and more thorough understanding of the growth, dry biomass, and fecundity of specific weed species emerging over different time intervals would benefit farmers in managing weeds escaping from pre- and postemergence herbicide treatments [16,24,29,41]. Knowledge about the viable seed production potential of C. iria as affected by its emergence timing in relation to that of rice is crucial for its efficient and sustainable management programs. No such investigations have been carried out in the Philippines, where C. iria is an important weed in DSR. Therefore, this research was carried out to (1) investigate the effects of different weed densities and varying rice seed rates on the growth, biomass production, and fertile seed formation of C. iria weeds initiating infestation at four varying times (3, 15, 30, and 45 d after the emergence of rice) and (2) study the relationship between C. iria’s emergence time and its reproductive capacity in rice.

2. Materials and Methods

2.1. Location

Field trials for the ecological study of C. iria were carried out at the research area of the International Rice Research Institute (IRRI), Laguna, Philippines. Data on rainfall and temperature at the experimental site are described in Figure 1.

2.2. Treatment and Design

The short-duration (110-day) cultivar NSIC Rc222 (IR154) was used in the study. The treatment variables were two rice seed rates (0 and 50 kg ha⁻¹), two weed populations (40 and 80 plants m⁻²), and four weed emerging times (3, 15, 30, and 45 days after rice emergence, DARE), to investigate their interactions in regard to weed survival percentage, weed plant height, growth, dry biomass, and seed production (germinable, viable, and nonviable seed) under dry seeded rice.

Randomization was performed with two rice seed rates along with two weed populations in the main plots, and the subplot comprised four weed emerging times.

2.3. Land Preparation and Sowing

The experimental area was levelled using laser-levelling. After that, dry-cultivation was carried out with two diskings followed by two passes of a tractor-operated rotavator and sowing of the crop. The first trial was carried out in the dry season (DS) on 22 January 2013 and harvested on 14th May, while the second trial was conducted in the wet season (WS) on 28 May 2013 and harvested on 23 September 23 in the same year. Rice seeds were directly sown at a seed rate of 50 kg ha^–1 with a tractor-operated seed drill. The depth of the seed was adjusted to 2–3 cm, and row-to-row distance was 20 cm. There was no sowing of the rice in one treatment as per experimental protocol. Rice seedling emergence started at 3 DAS.

2.4. Crop Husbandry

Nitrogen was applied at the rate of 50 kg ha⁻¹ in three splits. The first dose (30%) was applied at 15 days after sowing (DAS), the second (30%) at 45 DAS, and the third (40%) at 60 DAS. A basal dose of phosphatic (P) and potash (K) fertilizers (as muriate of potash (60% K₂O) and solophos (20% P₂O₅), respectively), each at the rate of 40 kg ha⁻¹, was applied. The details and schedule of all field activities are shown in

Table 1. Plan of field activities in the dry season (DS) and wet season (WS) of 2013.

Sr. No.	Activity	DAS	2013 DS	2013 WS
1	Basal fertilizer (P2O5 and K2O) application	0	22 January	29 May
2	Sowing of C. iria in plastic trays and rice in the field	0	22 January	29 May
3	C. iria planting in the field	15	7 February	13 June
4	C. iria sowing in plastic trays	8	1 February	7 June
5	Application of fertilizer N (30%)	14	5 February	12 June
6	C. iria planting in the field	23	14 February	20 June
7	C. iria sowing in plastic trays	24	15 February	21 June
9	Hand weeding	35	25 February	3 July
10	Application of fertilizer N (30%)	36	28 February	4 July
11	C. iria planting in the field	39	1 March	7 July
12	C. iria sowing in plastic trays	38	4 March	6 July
13	C. iria planting in the field	54	14 March	24 July
14	Hand weeding	53	15 March	23 July
15	Application of fertilizer N (40%)	55	19 March	25 July
16	C. iria harvesting	90	20 April	8 September
17	Harvesting and threshing of rice	111	14 May	22 September

Abbreviations, DAS = days after sowing; DS = dry season; WS = wet season.

.

The required weed density was ensured by sowing C. iria seeds in plastic trays that were filled with sterilized soil (devoid of weed seeds). We observed germination of weed seeds 6 DAS in our preliminary experiments. Hence, seeds were sown 6 days before the emergence timing in the field. One-week-old seedlings were dug out from the plastic trays without removing the soil so that seedling roots remained undisturbed, and seedlings, along with the soil ball, were plugged in the field between the rice rows. The times of weed emergence were 3, 15, 30, and 45 DARE, which corresponded with the usual weed emergence times in DSR. C. iria was grown by maintaining two populations (40 and 80 plants m^–2). After transplantation of the weed, the leftover nursery was well maintained to replace any demised plant wherever and whenever necessary and ensure the prescribed populations as per experimental requirements for at least 15 days after planting the weed. All other weeds, except C. iria, were manually pulled out for the whole growing season.

2.5. Postharvest Observations on Weeds

For the complete collection of the weed seeds, five selected plants’ inflorescences per plot were enveloped by glassine bags well before maturity as indicated by the transition of the fluorescence from a green colour to grey-brown. These fully mature plants were harvested by clipping from ground level. Just before the harvesting, data on plant height, the number of tillers, and inflorescence per plant was taken. The inflorescences were sorted out separately by clipping from other parts of the plant, sun-dried, and weighed before the seed separation. The seeds were separated, cleaned, and counted, and afterwards, 1000-grain weight was measured. The total seed number plant⁻¹ and weed seed yield in kg ha⁻¹ were measured. The plants of weeds were individually packed in paper bags and left for oven-drying at 70 °C for constant biomass determination.

2.6. Germination Test or Fecundity of Weed Seeds

Seeds from the five randomly selected plants were mixed thoroughly for subsequent germination tests in four replications. Twenty-five seeds were placed in a Petri dish with a diameter of 9 cm that consisted of two sheets of Whatman No. 1 filter paper (Whatman International Ltd., Maidstone, U.K.) moistened with distilled water (5 mL) to calculate the germination percentage of the seed. The Petri dishes were put into an incubator at oscillating day and night temperatures (30/20 °C) in a light/dark regime. The reason for using this range of temperature was because it was found to be optimal for the germination of Cyperus species in an earlier study [42]. The seed germination counting was performed (15 DAS) when the radicle became visible, and these were removed from the Petri dish. To evaluate the viability of seeds that were not germinated, the seeds were exposed to a simple pressure test with a finger. Whitish, hard embryos were supposed to be viable, whereas brown, soft embryos were treated as nonviable [43,44]. Based on these test results, germination percentage and live and dead seeds per treatment were determined.

2.7. Statistical Analyses

Analysis of variance (ANOVA) of all experimental data was performed using GenStat 8.0, 2005. Before conducting statistical analysis, the homogeneity of data variance was visually examined by scheming residuals. Results of ANOVA showed that there were significant interactions between experimental “runs” and treatments; consequently, the data for both experimental runs were analysed separately. At a 5% level of significance, standard error of difference (SED) was used to separate the treatment means. To study the useful correlations among two or more variables, a regression analysis was conducted using Sigma Plot, and diverse models were fitted [16].

A linear model

f = a + bx

(1)

was fitted for shoot biomass and tiller number, for weeds grown with and without rice in the dry season and without rice in the wet season; seed weight and seed number, for weeds grown without rice; weed percent survival, for weeds grown without rice; and seed number by shoot biomass. In this equation, f indicates the estimated parameter per plant as a function of weed emergence time (DARE) x, a is an estimate of the regression constant, and b indicates an estimate of the regression coefficient.

An exponential decay model in the form of

f = ae^−bx

(2)

was fitted for shoot biomass and tiller number data for weeds grown with rice in the wet season, and seed weight and seed number, for weeds grown with rice in both seasons. In this equation, f is the estimated parameter, x indicates the weed emergence time (DARE), a is the maximum of the parameter, and b is the slope.

Data on 1000-seed weight (weed grown with and without rice) and percent survival (weed grown with rice) were analysed using a three-parameter sigmoid model:

g = a/{1 + e^{[−(x − d50)/b]}}

(3)

where g is the estimated 1000-seed weight and percent survival as a function of different weed emergence time (DARE) x, a is the maximum 1000-seed weight and percent survival, d50 is the time (d) required to reach 50% of the maximum value, and b is the slope around d50.

The R² of the equation of the fitted model represents the coefficient of determination, which was used to measure the goodness of the fitted model. The proportion of the total variation in the parameter can be observed from R². Standard errors were used to compare the parameter estimates.

3. Results

Rice seeding rate and weed emergence time had a significant (p 0.001) interaction on percent survival of the weeds in both seasons. The survival percentage was slightly altered by delaying emergence when the C. iria plants were grown alone, while the survival rates of weed declined (p 0.001) with delayed emergence when the plants were grown with rice interference (Figure 2a,b). The first weed cohort’s survival was 94–98% when the weed grew alone and 93–100% with rice interference. A sharp decrease (p 0.001) in survival rate was observed with every successive delayed emergence compared with that of rice in the DS. Similar behaviour was found in the WS, where 77%, 0.1%, and 0% survival rates were witnessed when the weed emerged at 15, 30, and 45 DARE, respectively (Figure 2b).

3.1. Plant Height

C. iria emerging at 3 DARE attained plant heights of 91–95 cm when growing alone and 88–91 cm when allowed to grow with rice competition (Figure 2c,d). Decreased weed plant height in both seasons was observed with rice interference, and the variation amongst emergence cohorts surged with delaying emergence. Delay in weed emergence in comparison with rice emergence had little effect on plant height at 3, 15, and 30 DARE, but after this, it decreased significantly in the DS. However, this resulted in variable responses in C. iria height in the WS; after a slight increase, a decreasing trend in plant height was observed (Figure 2c,d).

3.2. Tiller Density per Plant

The tiller density plant⁻¹ of C. iria decreased linearly in the DS and exponentially in the WS (Figure 3). Weeds growing without rice competition in the DS had 25 tillers plant⁻¹, while 20 tillers plant⁻¹ were recorded in plots suffering from rice competition. The corresponding values in the WS were 14 and 9. Rice competition reduced (p 0.001) tiller density by 58–66% when the weed had an emergence time of 15 DARE and by 70–100% when it had an emergence time of 30 DARE (Figure 3a,b). Regression analysis indicated that delaying weed emergence resulted in a linear decline (y = 25.59 − (0.37)x, p = 0.001, R²= 0.88 (weed grown individually) and y = 17.77 − (0.42)x, p = 0.001, R²= 0.93 (weed and rice grown together)) in the tiller density of C. iria in the DS (Figure 3a). A similar trend was observed for C. iria grown without rice during the WS, but an exponential (y = 8.71 e^(−0.08)x, R² = 0.98) decline was seen for C. iria growing with rice competition.

3.3. Shoot Dry Biomass per Plant

Similarly to the tiller density, shoot dry biomass per plant decreased linearly in the DS and exponentially in the WS. The earliest emerging C. iria cohort had shoot dry biomass of 9–13 g plant⁻¹ when raised alone and 3.5–6 g plant⁻¹ when reared with rice competition (Figure 3c,d). Rice competition decreased (p 0.001) weed dry biomass by 53, 64, 79, and 100% in the DS and by 60, 78, 100, and 100% in the WS with weed emergence at 3, 15, 30, and 45 DARE, respectively. The dry biomass formation by C. iria was very responsive to delayed emergence timing (p 0.001) when the plant was raised with rice. Weed dry biomass reduced by 39–64% with delayed emergence from 3 to 15 DARE. There was a reduction in dry biomass of about 88–100% with each successive delay in weed emergence (Figure 3).

3.4. Seed Weight

A linear decrease in the seed weight of C. iria was observed with delay in emergence in plots without rice competition in both the DS and the WS. Rice interference caused a pronounced effect on the seed weight of C. iria. A substantial decrease (p ≤ 0.001) in seed weight was found for the lots germinated at 15, 30, and 45 DARE in rice interference plots (Figure 4a,b). Interference with rice resulted in seed weight reductions of 30%, 63%, 87%, and 100% in the DS and 48%, 91%, 100%, and 100% in the WS when C. iria germinated at 3, 15, 30, and 45 DARE, respectively.

3.5. Number of Seeds per Plant

As with seed weight, the number of seeds per plant of C. iria decreased linearly when grown without rice during both seasons (Figure 4c,d). However, more seed production per plant of C. iria was observed in the DS than in the WS. Interference of rice reduced seed production (p 0.001). The number of weed seeds plant⁻¹ greatly declined (p 0.001) for each successive delay in weed emergence in comparison with rice emergence. During both seasons, seed production was achieved when the emergence of weed was delayed up to 30 DARE in weed–rice interference plots and up to 45 DARE where weeds were raised alone (Figure 4a,b). Overall, C. iria seed production declined exponentially with delayed weed emergence along with rice interference (y = 15879.41 e^−(0.09)x in the DS and y = 43590.89 e ^{(−0.14) x} in the WS) and linearly without rice interference (y = 21245.92 − (394.84)x in the DS and y = 7151.7 − (0.162.46)x in the WS). The maximum seed production (15,821–22,203 seeds plant⁻¹ in the DS and 3589–7269 seeds plant⁻¹ in the WS) of C. iria was observed in the cohort that emerged with rice. A significant and noticeable decline in seed production of C. iria was observed when weed emerged at 30 DARE compared with early-emerging cohorts (Figure 4c,d). However, a 100% decline in the seed production of C. iria was observed when the cohort emerged at 45 DARE in the presence and absence of rice interference. The decline in seed production owing to late emergence behaved similarly to weed biomass reduction.

Shoot dry biomass (g plant⁻¹) and production of seed (number of seeds plant⁻¹) across both C. iria populations (40 and 80 plants m⁻²), raised alone or with rice interference, had a linear relation (y = −626 + 1668x in the DS and y = 221 + 955x in the WS) (Figure 5a,b). The model revealed that C. iria seed production initiated only when its dry biomass was less than 1 g plant⁻¹ in both seasons. Each g increase in plant dry biomass led to the production of 1042–1176 seeds plant⁻¹.

3.6. Thousand-Seed Weight

Rice seed rate and weed emergence had a significant (p 0.001) interaction effect on 1000-seed weight. Generally, the 1000-seed weight of C. iria was not much altered by delayed emergence up to 30 DARE in the DS when raised without or with rice interference. Similar behaviour was noted in the WS for plots without rice competition and up to 15 DARE for plots with rice interference (Figure 6a,b). A considerable reduction (p 0.001) in 1000-seed weight was found for the cohort that emerged with rice competition at 30 DARE in the WS (Figure 6b). Rice interference reduced (p 0.001) 1000-seed weight by 5–10% in the DS and by 24–86% in the WS for delayed-emerging cohorts from 3 to 30 DARE.

3.7. Seed Yield (kg ha⁻¹)

Significant reductions (p 0.001) in the seed yield (kg ha⁻¹) of C. iria were observed with delays in weed emergence time in relation to rice emergence time during both seasons. The interaction between rice seeding rate and weed emergence times was significant (p 0.001) in both seasons. Seed yield was found when there was a delay in weed emergence of up to 30 DARE for weed–rice interference plots and up to 45 DARE for plots having weed grown alone in the DS (Figure 6c). However, decreased seed yield was observed in the WS as compared with the DS. A slightly different trend was observed in the WS. Seed yield was recorded when there was a delay in weed emergence up to 15 DARE in weed–rice interference and up to 30 DARE in the plots where weed was raised alone (Figure 6d). The plots where C. iria was grown alone had higher seed yield (609–3448 kg ha⁻¹) than plots where plants were raised with rice competition (319–2404 kg ha⁻¹) at 3 DARE. Seed yield of weed cohorts decreased with delaying emergence and, this reduction was quite high when the seedlings of weed emerged on 15 DARE (61–88%) and 30 DARE (91–100%) (Figure 6c,d). Because of rice interference, the last C. iria cohort (45 DARE) failed completely to produce seed in both seasons. The response of declining seed yield (62–100% in the DS and 47–100% in the WS) was similar to the declines in the other seed yield attributes of C. iria (seed weight, number of seeds per plant, 1000-seed weight) due to the late weed emergence when raised with rice competition.

3.8. Germinated, Viable, and Nonviable Seeds

Rice seeding rate and emergence time of weed had significant (p 0.001–0.004) interaction effects on the seed germination of C. iria produced in the plots raised alone and with rice interference. Germination tests for the first two cohorts, those germinating at 3 and 15 DARE, were almost the same for plants raised with rice competition (88–88% in the DS and 48–40% in the WS) and without rice competition (89–89% in the DS and 49–50% in the WS). Germination in the third (30 DARE) and fourth cohorts (45 DARE) growing with rice was reduced (p 0.001) by 67% and 100%, respectively, in the DS and by 100% for both in the WS (Figure 7a,b). With decreasing germination percentages with delayed emergence of weed in relation to emergence of rice, viable and nonviable seed percentages soared (p = 0.035 to 0.006) correspondingly. The production of viable and nonviable weed seeds for the third and fourth lot was greater than for the first cohort during both seasons (Figure 7).

4. Discussion

The impact of weed competition on crop yield can be determined by the three most important variables; weed emergence timing in relation to that of the crop, weed seedling density, and the different competitive ability of different weed species [45,46,47,48]. Previously, several studies on weeds other than C. iria have been conducted to investigate the effect of the emergence time of weeds on crop–weed competition [25,36,49]. Most of these studies have reported different dynamics as affected by different tillage systems, sowing times of crops, and durations of weed competition in rice and other crops, but reports on rates of weed mortality connected with delayed weed emergence are missing [25,50,51,52,53,54]. Our study results were in an accordance with studies on other crops and weeds [49,53,54,55]. C. iria plant height declined linearly with delaying of its emergence in relation to that of rice. Preceding research carried out on Echinochloa crus-galli concluded that rice competition decreased the weed plant height by 26–32% in the DS and by 6–32% in the WS [16].

Similarly, Echinochloa colona plants germinating at 45 and 55 days after crop emergence produced 42% and 51% lower weed plant heights in comparison with plants emerging simultaneously with the crop. About 40% and 50% reductions in Trianthema portulacastrum were observed after the same number of days after crop emergence [55]. Reduced plant height of late-germinating weed plants was found as compared with the height of already grown and developed rice plants (Figure 1). It was observed that reductions in weed plant height and dry biomass by the interference of rice increased with delaying emergence. Rice plants emerged faster than late-germinating weed plants and gained a competitive advantage, resulting in decreased weed plant height and growth [16,55].

In the present study, a linear decrease in shoot biomass of C. iria was observed. The weed cohort grown with rice crops emerging at 15 DARE had significantly lowered shoot dry biomass (45%) as compared with the cohort emerging at 3 DARE. Similar results were reported by [16], who observed a 50% reduction in the shoot biomass of E. cruss-galli. Research carried out on different weed species (Echinochloacolona, T. portulacastrum, Ischaemumrugosum, Rottboelliacochinchinensis, Leptochloachinensis, and E. crussgalli) under dry sowing conditions revealed that rice competition reduced weed plant height, tiller density, number of leaves, and dry biomass production [16,25,44,55]. Researchers have also observed that crop competitiveness against weeds can be increased by adopting higher cropping populations [25,41,44,56] and delayed emergence of weeds in relation to crops [16]. In the current study, both of these factors resulted in the earlier growth of rice crop and earlier closure of the canopy, which caused lower plant height and shoot biomass of C. iria in the late-emerging cohorts.

Our studies demonstrated that the height of late-emerging weeds was reduced by more than 50% from that of early-emerging weeds and that the late-emerging weeds remained shaded (Figure 2) through the rest of the growing period of the crop. Reduced seed production per plant was observed in late-germinating weeds confronting higher competition by early-emerging rice. As a result, weed seed returning to the soil seed bank was also minimized, which would be quite helpful in depleting the weed seed bank in the long run. Several studies have reported decreased weed seed production owing to delay in emergence [16,25,31,41,44,47,55].

Reduced weed seed production and fecundity of surviving weeds at higher crop densities lowered weed seed replenishment back to the soil, thus gradually depleting weed seed formation [57]. The results of the present studies were in favour of previous research conducted on E. crus-galli and revealed that production of seed decreased with delayed emergence relative to that of corn [58,59], cotton [32], corn and soybean [31], and rice [16,24]. Similarly, E. colona and T. portula castrum seed production was reduced with delayed emergence in interference with maize crop [49].

Late-emerging weeds are more likely to suffer from shading than early-emerging weeds [41]. The variation in seed production due to delayed weed emergence time might be due to the difference in shading between early- and late-germinating plants [55,60,61]. We observed higher plant height of C. iria in the cohorts that emerged simultaneously with the rice than in those that emerged late. This was because the competition of early-emerging weed seedlings for nutrition and soil moisture contents with the crop was lower than that of late-emerging seedlings [23,27,30,32]. Crop canopy closure is a crucial phenomenon for the suppression of late-emerging weeds by reducing the light transmission through the canopy [16,37,44,47,62,63]. Reduced weed seed production in the current studies was observed in the late-emerging weed cohorts. Similar results were reported by Jeschke [64], who observed more weed competitiveness when weeds emerged simultaneously with the crop than when weeds germinated at later crop stages. The present studies provided useful insights into the ecology of C. iria plants that emerge at different times, without and with rice interference.

5. Conclusions

The experimental results of the current studies suggest that delayed weed emergence relative to crop emergence could be an important aspect of the development of suitable weed management approaches. The C. iria seedlings that germinated with the crop or within a month of crop establishment produced more seeds, thus enriching the soil seed bank. No viable seed production of C. iria was recorded for plants that emerged at 45 DARE. This might owe to the high seedling mortality and reduced growth due to more competition from crop plants. The results of our experiments showed that sustainable and consistent weed checking can be achieved with delays in weed emergence relative to crop, and relentless efforts are required to control early-season-germinating weeds (3–30 DARE). Weed management practices are required that are proficient at 25–30 DARE to retard weed emergence and to prevent the renewal of the soil seed bank. These findings have practical implications for rice growers in the Philippines and other related countries, where C. iria is the major weed in rice production systems. Practical implications means when rice growers delay weed emergence, weed survival, growth, fecundity, and soil seed replenishment will reduce, and ultimately, farmers can eliminate this weed. However, more studies are required to determine vertical seed distribution, seedling recruitment, persistence, and seed decaying levels in the soil brought about by cultural weed management strategies.