Effect of Seed Priming Method on Agronomic Performance and Cost Effectiveness of Rainfed, Dry-seeded NERICA Rice

MATERIALS AND METHODS

The field study was conducted at the University of Calabar, Southeastern Nigeria during the early cropping season of 2009 and 2010. A 25x13 m piece of land was cleared with machete and spade, the debris burnt and minimum tillage operation consisting of opening up the spot to dibble in the seeds using a hand hoe, adopted. The field was divided into 3 blocks, each with 6 plots of 3x3 m and separated from each other by boundary of 1.0 m. NERICA 1 seeds at the rate of 60 kg ha^-1 was used for the study. Four seeds were sown per hole at a spacing of 30x30 cm and thinned to 2 seedlings per hill, 3 Weeks after Planting (WAP). Carbofuran (Furadan) at the rate of 2.5 kg a.i. per ha was incorporated into the planting rows. Each plot received 60 kg N, 13 kg P and 25 kg K ha^-1 with basal application of P and K at sowing and top dressing with one-third Urea at beginning of tillering and the remaining two-thirds at about panicle initiation. Treatments laid out in randomized complete block design were as follows:

Primed seeds were given three washings with water and re-dried under shade to the initial moisture level.

Weeding was by hand pulling and hoeing, twice at 3 and 10 WAP, while birds were controlled by the use of ‘scare crows’ and screening with fishing nets. The crop was harvested when about 90% of the grains had turned to straw colour. The time to 50% germination (T50) was calculated according to the formula of Coolbear et al. (1984) modified by Farooq et al. (2005), while number of days to 50% emergence was computed as described by Farooq et al. (2006). Analysis of variance (ANOVA) was carried out using SAS software (SAS Institute, 2001). Excel software was used to draw figures, while means were compared by applying Least Significant Difference (LSD) at 5% probability. The cost of priming was computed based on the following: one liter of tap and distilled water cost No. 0.40 and 150.00, respectively; one gram each of ascorbic acid, sodium chloride and polyethylene glycol cost No. 25.00, 0.30 and 40.00, respectively and a kilo of unmilled rice was estimated at No. 32.00. It was estimated that it took one-man day at a cost of No. 1000.00 per man-day to prime seeds for 1 ha.

RESULTS AND DISCUSSION

Effect of seed priming on germination and seedling emergence: The effect of seed priming on germination was significant (p = 0.05). Figure 1 compares the germination time for various primed seeds to non primed ones.

Fig. 1:	Pooled effect of seed priming method on time of 50% germination of NERICA 1

The mean time for 50% germination of non-primed seeds was 46 h which was reduced to approximately 40, 38 and 30 h, respectively for Osmopriming, osmohardening, Ascorbate and on-farm priming. Hardening gave the earliest germination at 28 h, a reduction of about 18 h relative to the unprimed seeds, followed closely by hydropriming. Although, osmopriming was superior to no priming, it was statistically inferior to all other priming techniques evaluated in this study. The fastest germination was obtained with hydropriming and hardening probably because of faster water uptake and re-drying which might have led to earlier initiation of metabolism process that determines radicle protrusion. The regulation of water uptake during rice seed imbibition has been reported to play an important role in germination ability of rice seed (El-Hendawy et al., 2011). On-farm priming might have been inferior to these because the seeds were not re-dried after soaking, while soaking seeds in limited water imposed by osmotic solutions was slower and resulted in less advanced metabolic processes and slower germination. Basra et al. (2005) similarly reported that primed seeds exhibit an increased germination rate which Bray et al. (1989) attributed to metabolic repair during imbibition, build-up of germination-enhancing metabolites (Basra et al., 2005) and osmotic adjustment (Bradford, 1986). Mavi et al. (2006) who worked with tomato concluded that seed priming is rather effective through the reduction of germination time than subsequent relative growth of the plant.

At 3 days after planting, only 39% of unprimed seeds emerged, compared with 92, 86, 78, 74, 63 and 60% for hardened, hydro primed, on-farm primed, Ascorbate primed, osmohardened, osmoprimed, respectively (Fig. 2), representing 53, 47, 39, 35, 24 and 21% higher seedling emergence compared with the control. Seedling emergence percentage for seeds primed with water was higher than that with osmotic solutions. Good seedling emergence is the key to controlling stand establishment. In wheat, barley and oats, Kibite and Harker (1991) showed that seed hydration improved the uniformity of seedling emergence, while Harris et al. (1999) found that priming enhanced seedling establishment and early vigor of upland rice, maize and chickpea. Primed seeds emerge from the soil faster and often more uniformly than non-primed seeds because of limited adverse environmental exposure. This is accomplished by shortening the lag metabolic phase (i.e., phase II in the triphasic water uptake pattern, Bewley and Black (1978) in the germination process.

Fig. 2:	Pooled effect of seed priming method on seedling emergence at 3 days after plantation of NERICA 1

The metabolic phase occurs just after seeds are fully imbibed and just prior to radicle emergence. Since seeds have already gone through this phase during priming, germination time in the field can be greatly reduced upon subsequent rehydration.

Effect of seed priming on growth and yield characters: Seed priming did not significantly influence plant height at maturity and weight of 1000 grains which might have been a varietal characteristic (Table 1). However, tiller number per plant, duration to 50% heading and duration to plant maturity, as well as grain yield were all significantly affected by priming NERICA 1 seeds prior to sowing.

The number of days to 50% heading and duration to plant maturity followed a consistent pattern, being least with on-farm priming, hardening and hydropriming but although Osmopriming, osmohardening and Ascorbate priming, gave plants that matured significantly earlier than the unprimed seeds, they did not differ significantly from each other (p = 0.05). Early plant maturity may be advantageous to the farmer in that the crop might escape potentially unfavorable environmental conditions such as drought. Tiller number per plant and number of fertile panicles per plant was highest in hardening, on-farm priming and hydro priming (Table 2), followed by Ascorbate priming, osmopriming and osmohardening. Hardened seeds and on-farm primed seeds gave 36.6 and 31.7% more tillers per plant and a corresponding increase of 30.3 and 26.4% more fertile panicles per plant.

Grain yield ranged from about 3.3 t ha^-1 for unprimed seeds to 5.3 t ha^-1 for hardened seeds, a difference of about 37.7%. Significantly higher yield was obtained from hardened seeds, hydroprimed seeds or on-farm primed seeds followed by osmoprimed, ascorbate primed and osmohardened all of which did not differ significantly from each other.

Table 1:	Effect of seed priming method on days to heading, duration to maturity and plant height of NERICA 1
ns: Not significant

Table 2:	Effect of seed priming method on grain yield and yield characters of NERICA rice
ns: Not significant

Table 3:	Cost and return estimates of priming NERICA 1 rice seeds

Hardening of the rice variety Swarna (MTU 7029) also reportedly gave satisfactory results (Mondal et al., 2011). Better performance resulting from priming is attributed to early seedling emergence and vigorous growth as indicated in percent emergence, days to 50% heading and number of tillers and fertile panicles per plant. However, based on benefit/cost ratio, Okonkwo and Vanderlip (1985) and Carter et al. (1992) also reported that better yields are associated with stand establishment and vigorous early growth.

Both gross and net returns (Table 3) indicated the superiority of hardening, closely followed by on-farm- and hydropriming methods, over unprimed and other priming methods studied. Compared with unprimed seeds, these methods respectively gave 37.6, 34.1 and 34.0% higher net returns. These priming methods are known to be cheap and easy technology to adopt (Berchie et al., 2010).

CONCLUSION

Although, transplanting remains the preferred method of rice establishment in West Africa, the increasing cost of labour, scarcity of water and release and rapid adoption of high yielding upland rice varieties such as NERICA suggest that direct seeding in dry unpuddled soil could be an alternative. However, poor germination, uneven crop stand and high weed infestation all of which combine to reduce yield, are constraints that discourage its adoption. Selective (even if expensive) herbicides are now available for effective weed control but poor germination and poor crop establishment are still concerns. Results from this study suggest that seed priming has the potential to overcome this problem.