Graduate Thesis Or Dissertation
 

Competitive effect for grain yield of four cultivars in pure and mixed populations in wheat (Triticum aestivum Vill;, Host)

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  • Concerns regarding maintaining wheat yield per hectare and extending the commercial life expectancy of cultivars grown in the Yaqui Valley of Mexico prompted this investigation. Four genetically diverse spring wheat cultivars were grown in pure stands and in all possible combinations at the Northwest Agricultural Research Center at Ciudad Obregon from 1975 through 1978. The composite populations included: 1) a base pop4lation with equal amounts of seed of each cultivar representing the various combinations which was reconstituted and grown each year, 2) a first derived population which was a random sample obtained at harvest from the previously grown base population and grown in 1976-77 and 1977-78 and 3) a second derived population composed of a random sample of seed obtained from the first derived population and grown in 1977-78. The base, first and second derived populations were also grown simultaneously during the third year of the study so that a direct measure of the competitive ability of the cultivars within the composite could be obtained. Data collected from both the pure stands and composite treatments included: 1) grain yield, 2) number of spikes per unit area, 3) number of spikelets per spike, 4) number of kernels per spike, 5) 100 kernel weight and 6) plant height. Date of flowering was obtained for the 1977-78 growing season only. General and specific competitive ability of each cultivar in the composite combinations were determined for the treatments within and across years. Also, simple phenotypic correlations and path-coefficient analyses were employed to determine the relative importance of the components of yield to the competitiveness of a cultivar when grown in a composite. It was observed in this study that those cultivars with the highest grain yield when grown in pure stands were the most competitive in the composites. There were some cultivar x year interactions for the pure stands; however, the same cultivars either ranked first or second for all three years. One cultivar was consistently the lowest each year. In 1976-77, six of the composite treatments exceeded the highest yielding pure stand treatment. This year coincided with the most adverse growing conditions including a leaf rust infection. Jupateco, a cultivar selected for its leaf rust resistance when the study was initiated, was overcome by the pathogen during the second year of this study and a valid comparison as to the selective pressure of this disease was lost as a consequence. Spikes per unit area and kernel number were the most important components of yield both in contributing to high yield and stability of yield. They were also the most important components of yield in contributing to the selective advantage of certain cultivars in the composites. When generating composites, it is important to select varieties which complement one another in terms of their components of yield. Results from this study strongly suggest that to achieve yield stability through the use of composites, a mixture of cultivars should be made each year as the genetic shift among cultivars within the composites is so rapid that the desired genetic diversity would be lost after the first harvest.
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