Review on the Effect of Seed Source and Size on Grain Yield of Bread Wheat (Tritium Aestivum L.)

Wheat Production in Ethiopia

Ethiopia ranks second in sub-Saharan Africa in total wheat area and production. It is one of the most important cereal crops cultivated in the country Birhanu [9]. The Ethiopian highlands are considered a center of diversity for wheat and it has been cultivated in this region for several millennia with little change in farm implements and farming practices among small holder farmers. Wheat is mainly cultivated in the south eastern, central, south and north western highlands at altitudes ranging from 1500-2800 m.a.s.l. primarily under rain-fed conditions.

Wheat production in Ethiopia ranks fourth in area coverage surpassed only by tef (Eragrostis tef), maize (Zea mays), and sorghum (Sorghum bicolor) and it is the third largest crop in total production [10]. The relatively low mean yield may be partially attributed to the low level of adoption of improved varieties and improved management techniques [11]. Ethiopia imports more wheat than any other crop at 1.74 million tons annually and consumption is increasing much faster than production with an annual population growth rate of three percent [12]. Increasing wheat production is important to the economic stability and food security of Ethiopia. Utilization wise, wheat is used for the manufacture of flour for different purposes. Bread, biscuits and pasta products such as macaroni, spaghetti and noodles are some of the industrial products. Bread wheat is known to be a major source of energy and protein. Traditionally the crop is used for making Dabo, Dabokolo, Genfo, Kinche and other types of foods. The

straw is good source for animal feed and is also used for hatching roofs [13].

Seed Sources

Seed source influenced growth and yield during the growing season but the response depended on seed size and variety. The source of seed is can also be important since location influences seed nutrient content. The seed source has significant effect on seedling dry weight. Alemayehu [14] was reported that the number of tillers were significantly affected by main effect of variety, seed source and seed rate. Interaction effect of variety and seed source has highly significant effect on number of tillers per 1 m length. The influence of seed source on seed quality attributes, such as germination percentage, rate of germination, seedling root length and seedling fresh weight, was significant under laboratory conditions [14].

Farmers in Ethiopia can obtain seed from formal and informal sources. The formal seed source is a system composed of institutional and organizational arrangements consisting of all enterprises and organizations that are involved in the flow of modern varieties from the Agricultural Research Centers to farming communities, which include variety development, release and registration, seed multiplication and processing, seed marketing and distribution and seed quality control and certification. The informal seed source under Ethiopian context is defined as a system of seed production and distribution along with the different actors where there is no legal certification in the process [15]. In Ethiopia, the informal (either self-saved seed or farmer-to-farmer seed exchange, cooperative based seed multiplication and distribution, NGO based seed multiplication and distribution) seed source is the dominant for seed supply which accounts for 90% of the seed used by smallholder farmers [16].

Five key features distinguish the informal seed system from the formal seed system. The informal system is traditional, semi-structured, operate at the individual community level, uses a wide range of exchange mechanisms, and usually deal with small quantities of seeds. It is demanded by farmers and may depend upon indigenous knowledge of plant and seed selection, sourcing, retaining and management as well as local diffusion mechanism.

Seed Quality

Seed quality is defined as standard of excellence in certain characteristics and/or attributes that will determine the performance of the seed when sown or stored [17]. Seed quality describes the potential performance of a seed lot. Trueness to variety; the presence of inert matter, seed of other crops, or weed seed; germination percentage; vigor; appearance; and freedom from disease are important aspects of seed quality. High-quality seed lots should meet minimum standards for each of these characteristics. High quality seed is the key to successful agriculture [18]. For achieving optimum growth and yield production, the basic requirement of farming is to obtain quality seed [19]. Good quality seed can increase yields by 5-20%. The extent of this increase is directly proportional to the quality of seed that is being sown. According to Akbari, et al. [19] reported seed quality is very important to optimum growth and yield production in farm which influenced by many factors such as genetic characteristics, viability, germination percent, vigor, moisture content, storage conditions, survival ability and seed health, but their most important is germination percent and vigor. High quality seed is important to ensure maximum seed germination and seedling vigor, which is turn is instrumental in achieving maximum yield. Seed quality determines the optimum growth and yield production in the farm which is influenced by many factors such as genetic characteristics, viability, germination percentage, vigor, moisture content, storage conditions, survival ability and seed health [19].

The seed quality is also reflected in the final growth, maturity of plants, their uniformity and stability of yield. For seed to play a catalytic role, it should reach farmers in a good quality state, i.e. high genetic purity and identity, as well as high physical, physiological and health quality [20]. If the seed lots possess high genetic purity and high germination percentage and a minimum of inert matter, weed and other crop seeds, free from diseases and meets the certification standards, is said to have high quality.

In general, poorer quality seeds show low viability, reduced germination and emergence rates, poor tolerance to sub optimal conditions and low seedling growth rates. If seedling emergence is inadequate, crop yield will be reduced and in most situations no amount of efforts and expenses later on crop development can be compensated for these effects. There is a clear relationship existing between crop density and yield with yield increasing asymptotically as the density increases. Therefore the number of seedlings emerging does not only affect total crop yield, but the size of individual plant and the graded yields, time taken to reach maturity and uniformity of the plant at maturity [21].

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Seed Size

Seed size is an important physical indicator of seed related to yield, market grade factors and harvest efficiency and important parameter, which influences the germination, growth and biomass of the nursery seedlings and that trend leads to the future crop. Kumar and Seth [23] observed that seed size affected the seed yield significantly. A wide array of different effects of seed size has been reported for seed germination, emergence and related agronomical aspects in many crop species. However, these results varied widely between species. With increased seed size higher germination and emergence were determined.

Seed size was the trait that was most consistently associated with yields. Grain yield depended on the seed used and it decreased with an increase in the size of seeds. In wheat, seed size is positively correlated with seed vigor: larger seeds tend to produce more vigorous seedlings. Grain yield of wheat significantly increased by increasing seed size. Variation in seed size within a seed lot affected neither seedling emergence, plant fertility, nor grain yield per plot. Gan and stobbe [24] was suggested that small- seeded plants produced 2-44% less tiller grain yield than neighboring large-seeded plants. Grain yield with smallest size (2-2.2 mm) was significantly lower than larger seed sizes [25]. Stougaard and Xue [26] who reported that use of larger seed sizes improved grain yields by 18% and the use of small seeds reduced yield by 16% in wheat. In other crops, similarly by Bicer [27] reported that in chickpea and lentil found that plants from large seeds yielded 6% more than medium seeds and 10% more than mixed seeds. The large seeded plants compensated for the yield loss by producing a 10% higher tiller grain yield plants grown from the uniform large-seed wheat.

Larger seeds of spring wheat produced higher yields than smaller seeds under late-sown conditions, but not under optimum management conditions. Similarly, Rukavina, et al. [7] found that low-vigor spring wheat seed produced lower crop stand and lower grain yields. Vigorous growth associated with large seed or with varieties promoted early growth and yield.

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Figure 2: Effect of seed size on seedling vigour in wheat [28]. Effect of large seed on grain yield of wheat Larger seeds of spring wheat produced higher yields than smaller seeds under late-sown conditions but not under optimum management conditions. In wheat, seed size not only influence emergence and establishment but also affected yield components and ultimately grain yield. Simmone, et al. [29] reported that size of seed has a strong influence on germination as well as growth and biomass increment of a plant. With increasing seed size spike production and density, number of tillers, main stem length, thousand kernel mass, test mass, seed vigor and yield increased, maturity was advanced and seed moisture content at harvest decreased in Croatian spring malting barleys [7].

Use of large seed size and increased seeding rates can improve wheat competitiveness and provide an effective means to reduce wild oat biomass and seed production. Kumar and Seth [23] observed that seed size affected the seed yield significantly. Plants derived from large seed appear to have greater vigour and are able to acquire a larger share of plant growth factors relative to plants derived from small seed. In similar study was reported that use of larger seed sizes improved grain yields by 18% and the use of small seeds reduced yield by 16% in wheat [30]. Stougaard and Xue [30] opined that 18% of increased yield could be obtained by larger seeds in wheat. Large seeds have more food storage for embryo growth and development which lead to vigorous growth of the seedling before weeds can emerge and create competition. Lima, et al. [31] noted that crop growth rate at the beginning of the growth cycle was higher in plants originating from large seeds. Khurana and Singh [32] who noted that seed size variations affected leaf area, large seeds producing greater leaf area.

Stobbe, et al. [33] reported that crops grown from large kernels consistently yielded higher than crops grown from small kernels of the same cultivar, for both wheat (Triticum aestivum L.) and barley (Hordeum vulgare). Adejare [34] reported that large seed size of elite maize had higher seed quality and higher seed yield compared to other medium and small sizes. High grain yield was obtained with the 7 mm seed size seed size of 6 and 6.5 mm in corn [35]. Effect of medium seed and small size on grain yield wheat Abdul Raziq, et al. [36] determined that wheat sown with medium size of seed and small size seed followed a simultaneous decrease in wheat production and concluded that large size seed resulted in maximum grain yield (5294.30 kg ha-1) and yield decreased considerably when sowing was done with medium size and small size seed Elliott, et al. [37] noted that small seeds produced seedlings with much less vigour. Dar, et al. [38] have mentioned that small seeds to medium sized ones produced better germination and seedling vigor than those of bigger ones. Smallest seed size had lowest emergence therefore, it is assumed that plants grown from small seed had less fertile tillers than those grown from large seed. Whereby, grain yield and biological yield decreased in smallest seed size. It is obvious that increase in biological yield by increasing seed size was related to higher seedling weight and weight of 100 plants were produced by larger seed sizes in wheat [25]. Adebisi, et al.

[39] reported that in tropical soybean lots with small seed size had maximum seed germination (97%) and emergence (90%) while those with large seed size produced the highest seed (88) per plant, pods (54) per plant and seed yield (9.72 g) per plant. Effect of seed size on wheat germination and seedling vigour The crop stand is mainly dependent of good germination of seed and germination is entirely dependent on the viability and quality of seed [40]. For achieving optimum growth and yield production, the basic requirement of farming is to obtain quality seed [19]. Similarly, seed size is an indicator of seed related to yield, market grade factors and harvest efficiency. The crop sown with large size seeds showed remarkably better agronomic performance with 95.29% germination, 96.61 cm plant height, 6.40 tillers plant-1, 11.44 cm spike length, 42.65 grains spike-1, 42.80 g seed index (g), 10241 kg ha- 1 biological yield and 4857.10 kg ha-1 grain yield [36]. In wheat, seed size is positively correlated with seed vigor, larger seeds tend to produce more vigorous seedlings [41]. Germination rate and seedling vigour index values increased with the increase of seed size suggesting the selection of larger seeds for good stand establishment in rice. In wheat, seed size is positively correlated with seed vigour: larger seeds tend to produce more vigorous seedlings [42]. Generally, bigger seeds germinate quicker and would take lesser duration when compare to that of smaller ones and has better field performance than small seed.

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