Sorghum [
Sorghum bicolor (L
.) Moench] is an important cereal crop belonging to family
Poaceae. It is naturally self-pollinated monocotyledon crop with the degree of spontaneous cross-pollination, in some cases, reaching up to 30% depending on panicle type (
Poehlman and Sleper, 1995). The annual domesticated sorghums are diploid (2n=2x=20) and tropical origin C
4 crop
(Dicko et al., 2006). Sorghum is fifth most important cereal crop globally after rice, wheat, barley and maize (
FAO, 2012). It has been domesticated since approximately 3000 years B.C. in the Ethiopia region (
Ayana and Bekele, 1998). Ethiopia has a wide range of geographical adaptation and the country is a center of diversity for the crop
(Tesso et al., 2007). It is produced for its grain, which is used for food, feed and stalks for fodder and building materials in developing countries, while it is used primarily as animal feed and in sugar, syrup and molasses industry
(Dahlberg et al., 2011). It is a major food and nutritional security crop to more than 100 million people in Eastern horn of Africa
(Gudu et al., 2013) including Ethiopia, providing a principal source of energy (70% starch), proteins, vitamins and minerals
(Duodu et al., 2003).
Ethiopia is the third largest producer of sorghum in Africa behind Nigeria and Sudan, which contributed about 12% of annual production
(Wani et al., 2011) and the second after Sudan in the Common Market for Eastern and Southern Africa (COMESA) member countries (
USAID, 2010). It is the third most important crop both in sown area (ha) and becoming third primary staple food crop in Ethiopia after teff and maize (
CSA, 2015) and second most important crop for
injera (common leavened flat bread) making next to teff (
Adugna, 2012). Currently, sorghum is produced by 5 million small holders and its production is estimated to be 4.6 million metric tons from nearly 2 million hectares of land giving the national average grain yield of around 2.3 tons per hectare (CSA, 2015). It covers 16% of the total area allocated to grains (cereals, pulses and oil crops) and 14.58% of the area covered by cereals (
CSA, 2015). The crop is cultivated in all regions of Ethiopia between 400m and 2500m altitude, mostly at lower altitudes along the country’s Western, South-Western, North Eastern, Northern and Eastern peripheries (
EIAR, 2014) and staple food crop on which the lives of millions of poor Ethiopians depend (
Adugna, 2007). 44% and 30% yield loss due to weeds in maize and sorghum, respectively
(Stroud, 1989). An estimated yield loss about 10% in the less developed countries and 25% in the developing countries is caused by weeds (Akobundu, 1987). Meanwhile, weeds are also the hosts of various crop pests and pathogens (
Tao and Hu 2009).
Sorghum is susceptible to weed competition at its early stage of growth because the seedlings start weak and frail. Sorghum has also lower water requirement than most weeds. This means that weeds with higher water requirements tend to take up more water per unit of dry mater produced. Weeds interfere with the growth of crop (
SPL, 1988).
Botanical names of common weeds associated in Sorghum cultivation of the study area are
Pennisetum villosum Fresen.,
Sorghum arundinaceum (Desv.) Stapf,
Sida rhombifolia L.,
Corchorus trilocularis L.,
Cyanotis sp.,
Eragrostis pilosa (L.) P. Beauv,
Pennisetum pedicellatum Trin,
Dinebra retroflexa (Vahl) Panzer,
Abutilon figarianum Webb,
Rottboellia cochinchinensis (Lour.) Clayton,
Aristida adscensionis L.,
Rhynchosia malacophylla (Spreng.) Eoj.,
Ipomoea eriocarpa R. Br. and
Ipomoea sp. (Unpublished data). These weed have great impact on the sorghum growth and yield. Weeds compete with crops for water, nutrients and light. Being hardy and vigorous in growth habit, they grow faster than crops and consume large amount of water and nutrients, thus causing heavy losses in yields.
Chemical weed control plays a major role in increasing the efficiency of modern cropping systems
(Combellack et al., 1992). The herbicide glyphosate, N-(phosphonomethyl) glycine, is a biocide with a broad spectrum activity that was introduced for weed control in agricultural production fields in 1974 (
Benbrook, 2016). Glyphosate is taken up by the foliage of plants and transported throughout the plant resulting in plant death after several days. Glyphosate is formulated with various adjuvants
(Li et al., 2005), in particular surfactants such as polyoxyethylene amine (POEA), to enhance the uptake and translocation of the active ingredient in plants. The best known product formulated with POEA is Roundup (
Benbrook, 2016). Glyphosate products are used primarily before planting of traditional agricultural crops and after planting of genetically modified glyphosate-resistant crops (
Duke and Powles, 2009). Increasingly, they have been used for desiccation as a ‘harvest aid’ on traditional grain crops (Goffnett,
et al.,
2016;
Nelson et al., 2001; Zhang et al., 2017).
Commercial glyphosate formulations usually contain a monovalent salt of glyphosate, due to their high water solubility
(Baird et al., 1971; Franz, 1985). Glyphosate salts perform a variety of important functions. In particular, the salt portion of the formulated product may allow for greater absorption of glyphosate through its more effective penetration into the leaf
(Nordby et al., 2011). However, the salts do not have an impact on the herbicidal activity, since only the parent acid acts at the target site within the plant. When comparing different salt formulations with the same active ingredient, the acid equivalent of the formulation should be taken into consideration
(Nordby et al., 2011). Consequently, differences in the theoretical yield of the parent acid of formulated products applied could be observed under these circumstances. In addition, various types and amounts of adjuvant additives, either included in the formulated products or added in the tank mixture, have been found to improve glyphosate performance in different ways. For instance, surfactants, the most commonly used adjuvants, can activate herbicide diffusion across the cuticle by penetrating into the plant cuticle and improving herbicide uptake (
Hess and Foy, 2000). It is also worth mentioning that formulations may differ with respect to the quantity of glyphosate that can ultimately be concentrated, due to the different molecular weights of different salts and the various adjuvants that have been used by different manufacturers
(Miller et al., 2013). Outcome of research confirms that interaction among glyphosate formulations and leaf surfaces should be taken into consideration, as they may be crucial to the efficacy of the formulations (
Travlos and Chachalis, 2010;
Travlos and Chachalis, 2013). Concerning formulation of glyphosate products, the responses of various weed species vary among the different formulations. The objectives of this study were to (i) ensure efficacy of candidate chemical Glpho (Glyphosate-isopropylammonium 41% SL) on sorghum weeds non-selectively before sorghum sowing (ii) To verify the product efficacy of non-selective herbicide Glpho (Glyphosate-isopropylammonium 41% SL) in comparison with a standard check Glymax 48% SL (Glyphosate 48% EC).
