MODELING DEVELOPMENT, GROWTH AND YIELD OF WHEAT UNDER DIFFERENT SOWING DATES, PLANT POPULATIONS AND IRRIGATION LEVELS

Wheat ( Triticum aestivum L) is the most staple food of about one third of the population. It is also the principal food crop of Pakistan. It is grown on an area of 8069 thousand hectares with an annual production of 19235 million tones and an average yield of 2384 kg ha-1 (Anonymous, 2003). Wheat has always been one of the most reliable traditional food grain crop of hot semi arid and continental climates. In Pakistan, it is grown from 24o N to 37o N, from 61o E to 76o E and from sea level to about 3000 m below snowline. The main wheat growing areas, however, fall in the Indus plains.There are many components of production technology which significantly affect grain yield of wheat in a region, for example sowing date, seeding rate, irrigation etc. Sowing date affects the growth and yield of wheat by affecting its environment. Early sowing always produces higher yields than late sowing mainly, due to longer duration of growth. Each day delay in sowing of wheat from 20th November onward in Punjab (Pakistan) decreases grain yield @ 36 kg ha-1 d-1 (Hussain et al. , 1998).Similarly planting time influences the seed rate requirements of crop. Higher seed rate is used in late planting to compensate reduced components of yield. Varying responses have been reported to seed rate (khan, 1973; Khan et al. , 2001) and row spacing (Singh et al. , 1993). Increasing plant population usually gives higher yield (Nanaenko and Loktionov, 1991), and more than 6 t ha-1 and provide reasonable profit to the grower. Cultivars with higher yield potential play a vital role in increasing the yield per unit area, if they are provided with optimum husbandry management. Growth analysis of higher yielding newly evolved cultivars is needed to document their response to environmental, agronomic and physiological factors.All physiological processes of plant are directly or indirectly influenced by a water status. Water is essential for cell turgidity which is related to photosynthesis and to the growth of cells, tissues and organs ( Reddi and Reddy, 1995). Cultivar response to irrigation is critical to enhanced production.Irrigation has been shown to consistently increase wheat yield in Pakistan (Hussain et al. , 1977). However, such results were not analyzed quantitatively in relation to the prevailing weather. An understanding of the influence of weather on the response of crop yield to irrigation is needed to help plan likely demand for water resources to determine optimal irrigation strategies. To achieve this understanding, information is needed about the factors controlling water use by crops and the influence of drought on the physiological processes crucial to the yield determination. Such information may also provide useful guidance to plant breeders in the developing of genotypes more able to resist drought. Conventional methods of analysis in agronomic research usually produce results specific to the sites and seasons in which experiments are conducted. For example, yield formation in wheat and other crops is often described using yield components. Therefore, a very large number of agronomic and physiological studies have been based on this type of analysis. Thus a coherent framework for analyzing the process of yield is lacking. These old approaches of yield component analysis and growth analysis have failed to provide this framework (Monteith, 1981). Therefore, the results from such studies provide few insights into the causes of crop responses to agronomic treatments.There is a need to quantify crop growth and development relative to the potential determined by the season and site to help agronomists in analyzing the causes of variation in yield. For example, phonological development of a cereal crop is dependent upon both temperature and photoperiod ( Gallagher et al. , 1983). Growth of wheat crop is primarily dependent upon the amount of intercepted radiation (Gallagher and Biscoe, 1978) and crop transpiration (Jamieson et al. , 1984). Thus final grain yield is affected by harvest index (Donald and Hamblin, 1976). To carry the analysis of yield formation beyond above mentioned quantitative and qualitative descriptions, predictive models of crop growth and yield are required.Crop simulation models are important tools for agronomic management strategy evaluation, both under irrigated and rainfed conditions. The computer simulation modeling has been used in an attempt to bring ideas together (e.g. Weir et al. , 1984). Recently many mechanistic simulations were reported about the growth, development and yield of wheat from sowing to maturity in response to a field environment ( Porter et al. , 1983; Otter Nacke et al. , 1986; Travis and Day, 1988; Aggarwal et al. , 1994; Rickman et al. , 1996). These models simulate daily dry matter production as a function of radiation and temperature and water availability. In Pakistan no studies of crop growth simulation have been reported. The present study was therefore, conducted with the following objectives:1.To examine the effect of environment and agronomic practices such as sowing date, plant population and irrigation on growth, development and yield of wheat cultivars.2.To determine the irrigation requirements for wheat in Faisalabad, especially information regarding timing and amounts of irrigation and expected crop response.3.To develop a computer simulation model of growth, yield based of sound physiological and environmental principles.