‘On-farm’ seed priming: an inexpensive technology for increased food security
Our meta-analysis showed that on-farm seed priming has a significantly positive effect on crop performance, from nascence until harvest, relative to conventional (‘control’) seed sowing (Fig. 2). Although there is substantial variation (ranging from − 36 to − 7%), on-farm seed priming significantly decreases the time to emergence by 22% compared with non-primed seeds. On average, the number of plants emerged increased by 11%. Ultimately, yields increased by 21% compared with non-primed seeds, and only six out the 65 case studies reported negative effects on yield (data not shown).
Improved crop performance following on-farm seed priming can have important implications for smallholders’ food production. Higher yield is often accompanied by higher straw biomass, which is especially remunerative in mixed crop-livestock systems. Enhanced plant density reduces costs and the labour needed for re-sowing, and can also increase the willingness of farmers to invest in fertilizers, as the risk of plant stand failure is lower. Faster emergence typically results in plants reaching flowering and harvest stages earlier (e.g. by several weeks), giving farmers more labour flexibility, for example, by facilitating more optimal sowing for the subsequent crop or including an extra crop in rotation systems, or even by allowing migration for off-season work (Harris et al., 1999a, b, 2001a; Virk et al., 2006). Furthermore, the benefits are not restricted to the traits accounted for in our data, as faster development combined with the improved vigour and more uniform emergence in crops from on-farm primed seeds may save labour allocated to weeding. Although it is tempting to suggest that these benefits may increase net incomes, additional costs such as extra fertilizer or extra costs associated with harvesting, processing and storing greater yields, together with access to markets, will determine the final return from adopting on-farm seed priming.
Relationships between early growth and yield on crops grown from ‘on-farm’ primed seeds
To further investigate the relationships between rate of emergence, crop establishment and yield, we conducted separate analyses of the effect of time to 50% emergence on final emergence and the effect of final emergence on yield of crops from on-farm primed seeds relative to non-primed. Final emergence versus time to 50% emergence showed that in general, quicker emergence conferred by on-farm seed priming relative to non-primed seeds produced a higher number of successfully emerged seedlings (Fig. 3a). Although this relationship was significant (P < 0.01), it must be interpreted with caution due to the relatively small number of case studies. Meta-regression of yield versus final emergence (relative to crops from non-primed seeds) showed a positive relationship (Fig. 3b), although this relationship was not found significant. We found no difference between the hypothesized line and the meta-regression line (P > 0.05), which demonstrates that higher yields are proportional to improvements in emergence. However, in over two-thirds of the case studies, improvements in yield were proportionally higher than the expected gain due solely to improvement in final emergence. This suggests that increments in yield due to on-farm seed priming are not only a consequence of rapid and more prolific emergence, but that additional benefits may persist long after emergence.
Rapid emergence is crucial for the vulnerable seedling to avoid abiotic and biotic stresses and ensure high crop establishment (Gardarin et al. 2016). On-farm seed priming facilitates rapid emergence by accelerating germination through two complementary mechanisms. Firstly, it ensures water availability and the successful completion of phase I (the imbibition phase) prior to sowing, rather than relying on the seed imbibing soil moisture in the field where the water supply can be restricted or discontinuous (Wojtyla et al. 2016). Throughout the imbibition phase, both mechanical and biochemical changes, e.g. embryo enlargement, respiration, protein synthesis and DNA repair, are initiated (Gallardo et al. 2001; Weitbrecht et al. 2011; Steinbrecher and Leubner-Metzger 2017). All these processes prepare the seed for cell elongation (phase II, the lag phase); therefore, on-farm primed seeds are developmentally more advanced than dried seeds, resulting in a ‘head start of germination’ (Chen and Arora 2013). Secondly, on-farm primed seeds are only externally dried so that, once in the field, seeds need to absorb less water from the soil to complete phase III (the post-germination phase) when the radicle emerges from the seed coat. Furthermore, it has been reported that seed soaking enhances the production of the enzyme α-amylase (Ashraf and Foolad 2005; Farooq et al., 2017), which plays a crucial role in starch mobilization and provides the embryo with carbohydrates for respiration during germination and seedling growth (Ashraf and Foolad 2005; Farooq et al., 2017a, b). As a result, seedlings from on-farm primed seeds have more developed roots before the common limiting factors such as declining soil moisture, crust formation and/or high salinity prevent successful emergence.
Our results suggest that the gains in yield due to on-farm seed priming can be mainly attributed to enhanced emergence, i.e. rapid emergence leads to better crop establishment, which is conducive to higher yields. However, advanced establishment may also be coupled with higher vigour of individual plants, which is translated into significantly more tillers, more fruits (cobs/panicles/pods) per plant, greater number of grain and 1000-grain weight, or straw yield (Harris 2006; Rashid et al., 2006; Harris et al., 2007; Farooq et al., 2008). In addition to these physiological benefits, other circumstantial benefits are frequently observed, for example, earlier maturation decreases crop exposure to end of season drought, disease and pest attacks (Harris et al. 2001a; Rashid et al., 2006a, b). It is also likely that seed priming exerts important metabolic changes during early plant growth that are able to persist until later stages of development (Ashraf and Foolad 2005; Chen and Arora 2013); for example, there is evidence for enhanced disease resistance (Musa et al., 2001; Rashid et al. 2004; Harris et al., 2005) or drought tolerance (Wojtyla et al. 2016).
What modulates the ‘priming’ response?
It is important not only to identify the context where on-farm seed priming can best be applied, but also understand the potential situations where it can be counterproductive. Therefore, a subgroup analysis of moderators was conducted to examine potential factors that influence the effect of seed priming.
Climate
It is clear that yield benefits are more evident under low and unpredictable rain conditions. The largest response to on-farm seed priming was seen in areas with dry climates (Fig. 4a) with significantly higher yields for both arid (22%) and semi-arid (28%) climates compared to temperate climates (11%). Variation in yield between seasons due to on-farm seed priming has been frequently attributed to rainfall profiles, with greater yield increments commonly reported during rainy seasons with limited precipitation (Rashid et al., 2006a, b; Virk et al., 2006; Ousman and Aune, 2011). Low soil moisture and high evapotranspiration can slow and interrupt imbibition, which is conducive to emergence failure (Harris 1996); however, on-farm seed priming can offset a lack of soil moisture, as seeds have already imbibed water prior to sowing.
Importantly, in crust-prone soils, if rainfall occurs before emergence, shoots from on-farm primed seeds could be mechanically impeded, whilst the later emerging non-primed seedling may find more favourable soil strength (Murungu et al. 2004a). Equally, if rainfall is considerably delayed after sowing, seedlings from on-farm primed seeds may be damaged as germination has already been initiated and a lack of water could kill the developing seedling, whereas non-primed seeds will not initiate germinate until the rain comes (Murungu et al. 2003; Rashid et al., 2006a, b). However, the occurrence of these events seems to be very rare (Murungu et al. 2003, 2004a), and our data at emergence stage is consistent with the yield subgrouping, i.e. showing the higher benefits under dry climates (data not shown).
The interaction between soil temperature and on-farm seed priming, however, is less clear. Primed maize seed is more sensitive to elevated temperature under both dry and wet soil conditions (Finch-Savage et al., 2004a, b; Murungu et al. 2004b). For the former, internal seed moisture may induce heat stress by acting as a thermal conductor in soils of higher temperatures, while wet soils may exacerbate prolonged hypoxia (Finch-Savage et al., 2004a, b). Conversely, late sown wheat and chickpea plants from on-farm primed seeds have shown increased tolerance to chilling temperatures (Farooq et al., 2008a, b, Farooq et al., 2017a, b), possibly due to enhanced carbohydrate supply to the germinating embryo, which together with an accumulation of trehalose, can protect proteins and membranes from oxidative damage under abiotic stress.
Yield-limiting factors
Figure 4b shows that crops from on-farm seed grown under ‘salinity stress’ or in nutrient deficient soils had significantly higher yields compared to crops from on-farm seed grown under non-stressed environments (approximately 16% difference). In saline environments, germination is delayed or inhibited through reduced water availability and/or accumulation of toxic Na+ and Cl−. However, primed seeds are already hydrated and therefore less subjected to these constraints (Ibrahim 2016; Savvides et al. 2016). Importantly, case studies growing crops in conditions defined as non-stressed were mostly from research-managed trials using fertilizers and pesticides, whilst case studies grouped as nutrient deficient were mainly from farmer-managed trials with limited access to fertilizers and pesticides, and therefore more accurately reflect resource-poor farming conditions in marginal areas. These data indicate that on-farm seed priming can compensate, to some extent, for low-yielding environments and the lack of inputs that would further limit yields. Under low fertility environments, the quicker development of seedlings from on-farm primed seeds allows greater uptake from fertilizers, before nutrients are leached from the soil surface or become volatized (Harris et al. 2001b; Rashid et al., 2006a, b).
Declining soil fertility together with limited access to affordable mineral fertilizers is a major constraint for achieving optimal yields in marginal areas of developing countries (Chianu et al. 2012). However, low-cost strategies that combine on-farm seed priming with low amounts of inorganic fertilizers have been carried out to alleviate nutrient deficiencies with promising results (Aune and Ousman, 2011a, b; Ousman and Aune, 2011a, b). On-farm seed priming in combination with micro-dosing, i.e. application of small amounts of fertilizer in the planting pocket, demonstrated greater fertilizer use efficiency than micro-dosing alone for all the crops tested (Aune and Ousman, 2011a, b; Ousman and Aune, 2011a, b). Small amounts of micronutrients added to the water used for on-farm seed priming, e.g. ZnSO4, can also be highly cost-effective (Harris et al., 2007a, b, Farooq et al., 2008a, b).
Plant type
On-farm seed priming of all the major tropical crops produces similar or greater yields than traditionally sown crops in almost all cases (data not shown). However, decreased performance following on-farm seed priming has also been occasionally reported for barley (Rashid et al., 2006a, b), pearl millet (Aune and Ousman, 2011a, b), rice (Rehman et al. 2011), sesame (Ousman and Aune, 2011a, b), maize (Ali et al., 2008), wheat (Islam et al., 2015), and cotton (Murungu et al. 2004b), although for each of these crops, thee are also studies showing an increased performance (e.g. Harris et al., 2007a, b; Farooq et al., 2008a, b; Rashid et al., 2006a, b). Importantly, negative results are rarely attributed to the incompatibility of priming with the crop, but rather to untimely adverse environmental conditions. The largest yield loss due to on-farm seed priming was 8% for pearl millet in a series of on-station trials; however, in this study, the farmer-managed replicates registered a 30% increase in yield (Aune and Ousman, 2011a, b). Therefore, we have found no consistent evidence of negative interactions between specific crops and on-farm seed priming, which suggests that this is therefore a safe practice for all crop species trialled so far.
The effect of categorizing case studies by plant type on total yield is shown in Fig. 4c. On average, the yield increase of cereals (monocots) was 13% less than dicots. Dicot plants, broadly represented by legumes with 18 out of 23 case studies, responded better to on-farm seed priming averaging a 28% yield increase. This is in line with our final emergence data where greater effect sizes generally belonged to dicotyledonous crops (data not shown). Cereals were commonly grown with irrigation or during the rainy season, whilst legumes were sown as a component of the rotation after cereals in the post-rainy season or in fallow lands that were unsuitable for the main crop. In these marginal contexts, the benefit of seed being hydrated prior to sowing leads to more rapid emergence and establishment.
We cannot conclude from our data whether specific crops are more responsive to on-farm seed priming than others; however, on-farm seed priming may facilitate the use of legumes into rotational and intercropping systems. Currently, in both rotational and intercropping systems, the adoption of legumes is largely discouraged due to poor establishments of the legume component. In rotation, legumes are commonly grown utilizing residual soil moisture remaining during the dry season, and with no additional fertilization, whilst in intercropping systems, the planting of a legume companion is delayed in order to avoid shading and competition (Masvaya et al. 2017). Therefore, on-farm seed priming may ameliorate these unfavourable planting conditions and boost the benefits of cereal-legume cropping systems, e.g. by improving soil fertility and providing an additional income.