Journal of Natural Sciences Research www.iiste.org ISSN 2224-3186 (Paper) ISSN 2225-0921 (Online) Vol.4, No.9, 2014 85 To Establish the Yield Response Factor (Ky) of Different Soybean Varieties in Northern Area of Pakistan Muhammad Hameed*,Gul Daraz, Fayaz Ahmad Department of Water Management, Faculty of Crop Production Sciences, The University of Agriculture, Peshawar-Pakistan E-mail: muhammadhameed46@yahoo.com Abstract A field study was conducted on clay loam soil at the Agriculture research institute, Swat during Kharif 2012. Main objective of the study was to determine the yield response factor on maximum,optimal and minimum irrigation levels of soybean varieties, using two varieties (swat 84 and malakand 96) having four replicates and four irrigation levels. Relative yield response (Ky) were determined from relative yield decrease (1-Ya/Ym) and relative evapotranspiration deficit (1-ETa /ETm). Ky values on I 1 (I 40 ) of variety V 1 (Swat 84) were recorded from 0.38 to 1.48 and Ky values on I 1 of V 2 (Malakand 96) were recorded from 0.82 to 2.78. Ky values on I 2 (I 60 )of variety V 1 were recorded from 0.37 to 1.77, and Ky value on I 2 of V 2 were recorded from 0.79 to 3.23. Ky values on I 3 (I 80 ) of variety V 1 were recorded from 0.30 to 1.52, and Ky values on I 3 of V 2 were recorded from 0.47 to 3.14. Ky values of variety V 1 ranged from 0.30 to 1.77, for V 2 it ranged from 0.47 to 3.23.Hence it is concluded that among both the varieties V 2 performed better on irrigation two (V 2 I 2 ), thereforethis strategy is recommended for irrigated areas of Khyber Pakhtunkhwa, Pakistan INTRODUCTION The first domestication of soybean has been traced to the eastern half of North China in the eleventh century B.C. or perhaps a bit earlier. Soybean has been one of the five main plant foods of China along with rice, soybeans, wheat, barley and millet. According to early authors, soybean production was localized in China until after the Chinese-Japanese war of 1894-95, when the Japanese began to import soybean oil cake for use as fertilizer. Shipments of soybeans were made to Europe about 1908, and the soybean attracted world-wide attention. Europeans had been aware of soybeans as early as 1712 through the writing of a German botanist. Some soybean seed may have been sent from China by missionaries as early as 1740 and planted in France. The first use of the word "soybean" in U.S. literature was in 1804. However, it is thought that soybean was first introduced into the American Colonies in 1765 as "Chinese vetches" . Early authors mentioned that soybeans appeared to be well adapted to Pennsylvania soil. An 1879 report from the Rutgers Agricultural College in New Jersey is the first reference that soybeans had been tested in a scientific agricultural school in the United States. For many years, most of the references to this crop were by people working in eastern and southeastern United States where it was first popular. Most of the early U.S. soybeans were used as a forage crop rather than harvested for seed. Most of the early introductions planted in these areas were obtained from China, Japan, India, Manchuria, Korea, and Taiwan. A record 2.9 million bushel soybean crop was produced in 2001 on 74.1 million acres with an average per acre yield of 39.6 bushels. The leading soybean states are Iowa and Illinois. In 2003, Iowa had 10.6 million acres of soybeans while Illinois had 10.3 million. The highest state yield ever achieved was 50.5 bushels per acre produced by Iowa farmers in 1994. To investigate the effects of the water deficit on yield and yield components of soybean in semi-arid conditionson clay soil, the irrigation treatments of 33% (I 33 ), 67% (I 67 ), 100% (I 100 ) and 133% (I 133 ) were taken. The ratios were found from the total irrigation water applied (I W )/cumulative pan evaporation (CPE) with four days irrigation interval. The average amount of irrigation water applied to treatments (I 133, I 100 , I 67 and I 33 ) was 1058, 795, 533 and 263 mm and 1094, 823, 551 and 272 mm for Toyokomachi and Toyohomare cultivars, respectively. The maximum green pod yields were 20.6 and 29.1 t ha -1 with 997 and 922 mm water consumption for Toyohomare and Toyokomachi, respectively in I 133 treatments. Yield response factor (k y ) values of I 100 , I 67 and I 33 treatments were determined as 2.17, 0.92 and 0.59 for Toyohomare and 3.50, 0.61 and 0.61 for Toyokomachi, respectively. (Comlekciogluet al.2011).Nevertheless, most soybeans are cultivated under rain-fed conditions that are prone to drought. Water stress is detrimental to soybean growth throughout its development (Karam et al ., 2005) and causes serious reduction in seed yield at the flowering and pod elongation stages because of flower and pod abortion (Liu et al ., 2003). As the soybean plant ages from stage R1 (beginning bloom) through stage R5 (seed enlargement), its ability to compensate for stressful conditions decreases and the potentialdegree of yield reduction from stress increases (Foroud et al.1993).Moisture stress in soybean reduced the number of nodes per plant, number of pod per plant, plant weight, number of seed per pod and seed weight. Additional irrigation application increased seed yield 1000-seed weight and seed weight per plant (Kolarik, 1990).Water stress imposed