Littleseed canarygrass ( Phalaris minor Retz.) is one of the most troublesome annual weeds of wheat ( Triticum aestivum L.) in northwestern India since many years ago ( Bhullar et al ., 2016 ). Globally, it has been reported in more than 60 countries, covering all the continents ( Singh et al ., 1999 ). The weed is highly competitive with wheat and can reduce yield by up to 95% ( Chhokar et al ., 2006 ). It germinates in different flushes from sowing, to well after first and second irrigations. According to the current recommendation, P. minor in wheat in northwestern India, can be controlled with pre-emergence herbicides applied within 2 days after sowing (DAS) or with post-emergence herbicides at 30 DAS; post-emergence herbicides are currently most popular (>95%) with the farmers in the region. Rice-wheat is the dominant cropping system in northwestern India, and P. minor often germinates in moist seedbed at the same time as wheat. However in some seasons, rainfall during wheat sowing can stimulate early germination of weeds. Under these situations, P. minor can reach an ideal stage for herbicide application (2-3 leaf) even though wheat crop is not at the labelled stage (Zadoks 21-Z21). At present, the consequences of spraying wheat earlier than the currently recommended stage (Z21) are unclear.

Correct timing of herbicide application plays an important role in achieving effective weed control without causing crop injury. Crop tolerance to herbicides varies with herbicide choice, application dose, application timing, and environmental conditions ( Lemerle et al ., 1986 ). Growth stage of wheat and environmental conditions at the time of herbicide application also affect herbicide uptake, translocation and plant’s ability to metabolize herbicides thus affecting the extent of herbicide injury and weed control ( Tottman, 1980 ). Sulfosulfuron, pinoxaden, pre-mixes of fenoxaprop plus metribuzin and mesosulfuron plus iodosulfuron are commonly used for P. minor control in wheat in the region. Sulfosulfuron is a sulfonylurea herbicide that inhibits the activity of acetolactate synthase (ALS), an important enzyme necessary for the biosynthesis of branched chain amino acids. It has been widely used locally for the control of P. minor . There have already been some reports of resistance to sulfosulfuron in P. minor in some areas of Punjab ( Bhullar et al ., 2014 ) and Haryana ( Dhawan et al ., 2010 ) states in northwestern India. The pre-mix of mesosulfuron-methyl and iodosulfuron-methyl-sodium (ALS inhibitor) is also widely used in wheat. GR ⁵⁰ (dose required for 50% growth reduction) values of mesosulfuron plus iodosulfuron have been reported to increase with time ( AICRPWM, 2013 ). More recently pinoxaden, a selective grass herbicide belonging to the phenlypyrazolin chemistry, has been recommended for post-emergence control of P. minor . Dhawan et al . (2010) and Kaur et al . (2016) reported development of cross resistance in P. minor to pinoxaden in Punjab and Haryana states. Metribuzin is primarily a soil-residual herbicide, which can be applied post-emergence to wheat for the control of annual grasses and dicot weeds ( Schroeder et al ., 1986 ; Shaw & Wesley, 1991 ). Although metribuzin can control P. minor effectively, careful management, including timely application is required to achieve good crop tolerance and weed control ( Runyan et al ., 1982 ; Schroeder et al ., 1986 ). The pre-mix of fenoxaprop-p-ethyl plus metribuzin can provide effective weed control but it can also cause some phytotoxic effects on the tillering and grain yield of wheat ( Singh et al ., 2005 ). Even though there have been some reports of resistance to sulfosulfuron, pinoxaden and pre-mix of mesosulfuron plus iodosulfuron in P. minor in northwestern India, these herbicides are still extensively used in the region. An understanding of the effect of growth stage on herbicide tolerance of wheat and control of P. minor will help in developing recommendations for selecting the most appropriate herbicide for different situations. The objective of this study was to evaluate the effect of timing of different post-emergence herbicides on crop tolerance and their efficacy on P. minor control.

Sulfosulfuron and fenoxaprop plus metribuzin provided effective control of P. minor when applied at Z12 and Z13 wheat stages, all four herbicides were equally effective when applied at Z21, and only pinoxaden worked effectively at Z23 wheat stage. Sulfosulfuron is absorbed both through root and foliage ( Senseman, 2007 ) and has a broad window of application in wheat and longer residual effects ( Shaner, 2014 ). P. minor plants were at 1-4 leaf stages, when herbicides were applied between Z12-21 stages of wheat, and would have been killed through both root and foliage uptake. The efficacy of sulfosulfuron against 2-3 leaf stages of P. minor has been reported earlier ( Chhokar & Malik, 2002 ; Chhokar et al ., 2008 ). In contrast, Singh et al . (2003) recorded higher control of P. minor with sulfosulfuron used at Z13 compared to at Z21 wheat stage at Pantnagar, India. The results of this study suggest that even in situations where weeds germinate early after first irrigation, the current recommendation for sulfosulfuron at Z21 stage still provides effective weed control and crop yield response.

Fenoxaprop and metribuzin have contrasting modes of entry into plants; metribuzin enters the plant mainly from the soil via roots and fenoxaprop enters entirely through the foliage; their joint action may be the reason for enhanced efficacy against P. minor , particularly when it was applied at Z12-13 wheat stages. In an earlier study, Chhokar et al . (2008) reported effective control of P. minor with metribuzin applied at Z23 stage of wheat, but in our study metribuzin plus fenoxaprop had a lower efficacy than its earlier timing, which may be attributed to the large size of P. minor plants (7-8 leaf stage). The lower efficacy of the mesosulfuron plus iodosulfuron on small plants of P. minor when applied at Z12-13 wheat stages, may be due to low leaf area of weeds as these herbicides are primarily absorbed through foliage ( Yuan et al ., 2013 ) and known for more consistent results from later applications ( Tickes, 2000 ; Crooks et al ., 2004 ). In an earlier study, mesosulfuron provided better control of Lolium perenne at 4-5 leaf than at 2-3 leaf stage ( Grey et al ., 2012 ).This was also supported by our results where the best control of P. minor (>85%) with meosulfuron plus iodosulfuron was recorded at more than 3-leaf stage. Pinoxaden is also absorbed through the foliage only ( Hofer et al ., 2006 ), hence its lower efficacy when applied at Z12 and Z13 wheat stages could be related to lower leaf area of P. minor , and its failure to prevent germination of P. minor seeds after its application. Consequently, pinoxaden worked most effectively once all target grasses had fully emerged. Therefore, the best weed control with this herbicide was observed when it was applied at Z21-23 wheat stages. Kieloch et al . (2006) indicated that pinoxaden can provide high levels of weed control even at late weed growth stage which was also supported by this study.

Wheat plants showed excellent tolerance to sulfosulfuron and pinoxaden at all growth stages. In the case of pinoxaden, the safener (cloquintocet-mexyl) present in the formulation provides very good tolerance in wheat ( Hofer et al ., 2006 ). The pre-mixes of fenoxaprop plus metribuzin and mesosulfuron plus iodosulfuron were toxic to wheat plants at Z12 and Z13 stages and significantly reduced spike density compared to weed free check ( p <0.05). These results are supported by the chlorophyll fluorescence (Fv/Fm) data, which showed reduced chlorophyll fluorescence in wheat plants compared to unsprayed plants indicating that the treated plants were under stress (data not shown). The higher crop toxicity by these herbicide mixtures when used at Z13 than at Z12 stage could be related to the short time interval between herbicide application and irrigation which increased herbicide uptake. The herbicide application at Z13 stage was followed by irrigation after 24 hours, and 10 and 42 mm rainfall was received one week after the spray during 2013 and 2014, respectively ( Fig. 1 ). The close association between metribuzin toxicity in crops and soil moisture has been reported earlier ( Blackshaw et al ., 1994 ; Kleemann & Gill, 2008 ). Crooks et al . (2003) reported toxicity to wheat plants 2-4 weeks after the treatment with mesosulfuron plus iodosulfuron and the toxicity reduced significantly when herbicide application was delayed to the tillering stage of wheat ( Grey et al ., 2012 ). These studies suggest that tolerance of wheat to this herbicide increases close to the tillering stage. In our case, the crop also showed excellent tolerance to both pre-mixes when applications were made at Z21-23 stages which represent the tillering stage of wheat, and may be attributed to the enhanced capacity of wheat plants to metabolise the herbicides at these stages ( Das, 2014 ). The differential sensitivity of wheat to metribuzin applied at its different growth stages ( Shaw & Wesley, 1991 ; Grey & Bridges, 2003 ) and different doses ( Balyan, 1999 ) has been demonstrated earlier.

The higher grain yield with sulfosulfuron, when applied at Z12 and Z13 wheat stages than later in the season, could be attributed to its excellent crop safety and lower biomass of P. minor . The selectivity of pre- and post- emergence sulfosulfuron in wheat was reported by Levy (2008) . The results of our study indicated that sulfosulfuron can be adopted for the control of P. minor at these stages (Z12-13). At those early stages of wheat, fenoxaprop plus metribuzin showed toxicity, pinoxaden had poor weed control and mesosulfuron plus iodosulfuron showed both (toxicity and poor weed control) ( Table 1 ; Fig. 2 ). The crop toxicity and/or poor weed control reduced wheat plant density, spike density and grains per spike, which lowered the grain yield by 21-44% compared to the weed free check ( p <0.05) ( Table 2 ). These results are supported by the strong positive correlation among spike density and grains per spike with grain yield ( p <0.01) ( Table 3 ). The weed biomass at 90 and 135 DAS are strongly correlated and has strong negative effects on spikes/m ² and grain number per spike as well as grain yield ( p <0.01). Hence treatments that provided poor weed control allowed large weed biomass, which resulted in lower grain yield. Further, grains/spike and grain number/m ² are strongly related to grain yield, which also explain the differential response among different weed control treatments. The yield reduction with both the pre-mixes was greater when applied at Z13 than at Z12, which could be related to greater herbicide absorption associated with higher leaf area of the crop.

All four herbicides applied at the Z21 crop stage provided effective control of P. minor and were safe to wheat, and produced wheat grain yield similar to the weed-free check ( p >0.05). When herbicide treatments were applied at Z21 stage, P. minor and wheat plants were within the recommended 3-4 leaf stage, which could be the reason for high herbicide efficacy and selectivity. Effective control of P. minor at Z21 wheat stage with these herbicides was also reported earlier ( Chhokar & Malik, 2002 ; Singh et al ., 2005 ; Chhokar et al ., 2008 ). Pinoxaden provided excellent control (>90%), even with large plants of P. minor (7-8 leaf), which was the reason why it provided wheat grain yield similar to weed free check when applied at Z23 wheat stage. Weed biomass for the other three herbicides was greater than with pinoxaden, which could be associated with the differences in wheat grain yield. Singh et al . (2011) found higher P. minor control with pinoxaden compared to the pre-mix of fenoxaprop plus metribuzin when the herbicide application was made at wheat stages beyond Z23 stage. The negative linear relationship between wheat grain yield and P. minor biomass explained about 98% of the variation in grain yield ( Fig. 3 ; Table 3 ). The application of different herbicides reduced P. minor biomass by 25-98% as compared to the weedy check (192-195 g/m ² ). This level of competition from P. minor in the weedy check plots reduced crop density, spike density (35-40%) and wheat grain yield by 50% as compared to the weed free check.

As conclusions, the post-emergence application of 25 g/ha sulfosulfuron at Z12 and Z13 stages of wheat provided effective control of P. minor and was safe to wheat. Pre-mixes of fenoxaprop plus metribuzin and mesosulfuron plus iodosulfuron were toxic to wheat plants when applied at these stages. Pinoxaden application at 50 g/ha provided effective control of large P. minor (7-8 leaf). This study provides background information required to make changes to the current recommendation of timing of post-emergence herbicides in wheat in northwestern India.

The authors are thankful to Punjab Agricultural University, Ludhiana, India for providing the necessary research facilities for the research trials.