Date of Transplanting and Integrated Weed Management Effects on Growth and Yield of Black Rice (Oryza sativa L.) under SRI

India is the world’s second largest rice producer and consumer next to China. It contributes 43 percent of total food grain production and 46 per cent of total cereal production in India. Rice is cultivated in an area of 43.78 million hectares with an annual production of 118.4 million tonnes and productivity of about 2.7 tonnes per hectare Annual Report. (2020-21), Department of Agriculture, Cooperation and Farmer’s Welfare). Black rice is a special type of rice species Oryza sativa L. which is black in colour, glutinous, packed with high level of nutrients and mainly cultivated in Asia. In India, black rice is cultivated only in few areas such as northeast, West Bengal, Jharkhand and Odisha. In northeast, it is especially grown in Manipur. In order to obtained higher paddy yields, sowing at optimum time is important. The sowing date of the rice crop is important for three major reasons. Firstly, it ensures that vegetative growth occurs during a period of satisfactory temperature and high levels of solar radiation. Secondly, the optimum sowing date for each cultivar ensures that the cold sensitive stage occurs when the minimum night temperatures are historically the warmest. Thirdly, sowing on time guarantees that grain filling occurs when milder autumn temperatures are more likely, hence good grain quality is achieved (Farrell et al., 2003). Out of several factors responsible for low yield, severe infestation of weeds in SRI cultivation is also considered to be one of the major obstacles in achieving higher yield. When rice fields are not flooded continuously and plants are widely spaced as recommended under SRI, weeds get a better chance to grow. Weeds become detrimental to crops by changing the pH of soil, decreasing the nutrient availability, which in turn reduces straw yield by 13-38% and grain yield by 25-47% (Manandhar et al., 2007, Nivetha et al., 2017, Paul et al., 2019 and Tasmin et al., 2019). Therefore, efficient weed management practices are required under SRI. Keeping the above points in view, an experiment was conducted to assess the effect of date of transplanting and integrated weed management on growth and yield black rice.

A field experiment was conducted during kharif season of 2019 and 2020 in the experimental research farm of the School of Agricultural Science and Rural Development (SASRD), Medziphema campus, Nagaland University located at 20°45’43”N latitude and 93°53’04”E longitude with an elevation of 310 m above mean sea level. The soil condition of the experimental site was clayey loam in texture with a pH of 4.85, high in organic carbon (1.21%), low in available nitrogen (253.12 kg ha^-1), low in available phosphorus (18.43 kg ha^-1) and medium in available potassium (142.62 kg ha^-1). The experiment was laid out in split plot design with three dates of transplanting viz. D₁-15^th June, D₂- 30^th June and D₃- 15^th July in the main plots and five integrated weed management treatments viz. W₁- Weedy check, W₂- Conoweeding at 20 and 40 DAT, W₃- Pretilachlor 0.75 kg/ha at 3 DAT fbhandweeding at 40 DAT, W4- Pretilachlor @ 0.75 kg ha^-1 at 3 DAT fbconoweeder @ 40 DAT and W5-Pretilachlor @ 0.75 kg ha^-1 at 3 DAT fbBispyribac-Na @ 25 g a.i ha^-1 at 20 DAT in the sub-plots with 15 treatment combinations and replicated thrice. The plot size was 4 m × 3 m. Cultivar used was Chakhao Poireiton. The recommended dose of fertilizer (RDF) at 50:30:20 kg NPK ha^-1 was applied. Half dose of N and full dose of P and K were applied as basal during transplanting while the other half of N were top-dressed during tillering and flowering in further two equal halves. Transplanting was done at one seedling hill^-1 with a spacing of 25 cm × 25 cm using 12 days old seedling. As per the treatment, pretilachlor (50% EC) @ 0.75 kg ha^-1 at three days after transplanting and bispyribac-Na (10% SL) @ 20 g a.i ha^-1 at twenty days after transplanting were applied in respective plots with the help of knapsack sprayer fitted with flat fan nozzle using spray volume of 500 L/ha. Data were statistically analysed using the technique of Analysis of Variance as described by Gomez and Gomez (2010). The significance differences were tested by ‘F’ test at 5% probability level. The weed data was subjected to square root transformation with the help of the formula,  where x is the actual weed count.

Weed population and weed dry weight
 
Black rice transplanted on 15^th June recorded significantly lowest total weed population and dry weight while highest population was recorded under 15th July transplanting date which could be due to availability of higher moisture with the onset of monsoon (Table 1).



Early transplanting of black rice might have provided better environmental condition for the crop to establish and overcome the weed that led to better growth and competitive ability of crop and ultimately reduces the utilization of resources by weeds like moisture, nutrients, light and space resulting in lower weed population. The results are in close conformity with the findings of Rajaput (2013), Ghandor et al. (2017) and Kumar et al., (2017).

Integrated weed management influenced the total weed population and dry weight of weeds significantly. Weedy check gave significantly maximum population and dry weight of weeds in both the years (Table 1). The higher total weed population and dry weight could be due to higher weed intensity and its dominance in utilizing the resources like nutrients, moisture, light etc. Similar results were reported by Mondal et al., (2019) and Parihar et al., (2020). Application of pre-emergence herbicides pretilachlor @ 0.75 kg ha^-1 at 3 DAT fbhandweeding at 40 DAT recorded significantly lowest total weed population and dry weight which was followed by pretilachlor @ 0.75 kg ha^-1 at 3 DAT fbBispyribac-Na @ 25 g a.i. ha^-1 at 20 DAT. The lower population and dry weight of weeds with these treatments could be due to inherent capability of the herbicides to affect the cell division, cell growth and hindering the germination of weeds during initial stage followed by hand removal of weeds at later stage. The results were in conformity with the findings of Jadhav et al., (2016), Phukan and Deka (2021) and Reddy and Ameena (2021).
 
Growth attributes
 
Data pertaining to plant height (cm), number of green leaves plant^-1 and dry matter accumulation (g plan^t-1) (Table 2) revealed that transplanting of black rice on 15^th June recorded maximum plant height, number of green leaves plant^-1 and dry matter accumulation which was followed by transplanting on 30^th June while significantly lowest values were observed under 15^th July.



Early date of transplanting might have ensured the vegetative growth to take place during a period of satisfactory temperature and higher level of solar radiation (Patel et al., 2019). Similar results were obtained by Tiwari et al., (2018), Singh et al., (2021) and Yumnam et al., (2021).

Significantly highest plant height, number of green leaves plant^-1 and dry matter accumulation were recorded with application of pretilachlor @ 0.75 kg ha-1 at 3 DAT fb handweeding at 40 DAT and it was followed by pretilachlor @ 0.75 kg ha^-1 at 3 DAT fb Bispyribac-Na @ 25 g a.i. ha^-1 at 20 DAT (Table 2). This may be attributed to the effective control of weeds at critical stages of crop weed management resulting in increased availability of nutrients and accumulation of photosynthates. Similar findings were reported by Ansari et al., (2017), Suseendran et al., (2020) and Sanodiya and Singh (2021). Minimum values were recorded under weedy check in both the years due to heavy competition with weed flora.
 
Yield
 
The differences in grain and straw yield among the date of transplanting were found to be significant (Table 3a).



The grain and straw yield recorded under 15^th June transplanting date was found to be significantly highest and was followed by 30^th June. The higher yield in early planted crops could be due to optimum period available for growth and development which leads to more storage of photosynthates in grain as well as better yield related traits observed under earlier planted crop than the later crop. The results are in corroboration with those of Vishwakarma et al., (2016), Dileep et al., (2018) and Kumari and Prasad (2021).

Weedy check recorded significantly lowest yield during both the years (Table 3a). Pretilachlor @ 0.75 kg ha^-1 at 3 DAT fb h and weeding at 40 DAT recorded significantly highest grain and straw yield as compared to the rest of the treatments. This was followed by pretilachlor @ 0.75 kg ha^-1 at 3 DAT fb Bispyribac-Na @ 25 g a.i. ha^-1 at 20 DAT. Application of pre-emergence herbicides pretilachlor interfered with the growth and development of weeds by inhibiting the cell division and effectively controlled the germinating weeds or emerging weeds during early stages of crop growth followed by hand weeding at later stage and maintain the crop free from crop weed competition for nutrients, space and light resulting in higher number of tillers, number of panicles, grains panicle^-1 and ultimately yield. Similar findings were reported by Paul et al., (2019), Tasmin et al., (2019), Salam et al., (2020) and Bhattacharya et al., (2022).

Interaction effect of date of transplanting and integrated weed management on grain yield was found to be significant (Table 3b).



Transplanting of black rice on 15^th June along with application of pretilachlor @ 0.75 kg ha^-1 at 3 DAT fb h and weeding at 40 DAT recorded significantly highest grain yield in both the years while transplanting on 15^th July in combination with weedy check gave the lowest yield. Similar results were reported by Mubeen et al., (2014).

From the above experiment, it can be concluded that black rice should be transplanted on 15^th June along with application of pre-emergence herbicides pretilachlor @ 0.75 kg ha^-1 at 3 DAT fb Hand weeding at 40 DAT for efficient weed management and higher productivity.

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