Introduction: As regards the decrease of underground water level and the limitation of water resources in Iran, and considering the drought tolerance of sorghum cultivars, with the production of this product, it is hoped that a step will be taken to meet the country's need for fodder. Sorghum (Sorghum bicolor L.) is considered one of the important fodder plants in arid and semi-arid regions of Africa and Asia, which has high photosynthetic capacity, high nitrogen and water efficiency, and dual use of seed and fodder. Due to the fact that it is not possible to change genetic factors in the short term, therefore, through changing environmental factors can improve the nutrients consumed by livestock from fodder plants. Among the environmental factors, the maturity stage of the plant has the greatest effect on the ability to digest forage plants. By delaying the harvest time of sorghum, the yield of fresh and dry fodder and crude fiber increased, and crude protein, ash percentage, and number of leaves decreased. The purpose of this study is to investigate the effect of seed rate and different stages of sorghum harvesting on fodder yield and agronomic characteristics of two fodder sorghum cultivars.
Materials and Methods: This study investigated the effect of seeding rate and harvest stages on forage yield and some agronomic characteristics of two sorghum cultivars during the two years 2007 and 2008 at the Zahak Agricultural and Natural Resources Research Station using split-split plots with a randomized complete block design and three replications. Two sorghum cultivars (Speedfeed and a local landrace of Zabol) were considered as the main factor, harvest stages in four stages (botting, heading, soft dough, and hard dough) as sub-factors, and seeding rate at three levels (15, 20, and 25 kg/ha) as sub-sub-factors. The planting date was considered to be August 15. Each experimental plot consisted of 6 planting lines with a length of 7 meters. Harvesting was done from the four middle lines after removing the borders with an area of 12 square meters.
Results and Discussion: The results of composite variance analysis of the data showed that there was a significant difference at the 1% probability level between the treatments related to different harvest stages in terms of plant height. The process of height increase from the botting stage to the hard dough stage was incremental, so that the highest plant height, with an average of 164.2 cm, was related to the hard dough harvesting stage. Entering the reproductive stage of the plant causes the production of leaves to stop, while the growth of the stems continues. Due to the stop of new leaf production and the fall of old leaves in the lower nodes of the plant, the share of leaves from dry matter decreased.
 The effect of cultivar, harvest stage, and cultivar at harvest stage on the number of leaves per plant had a significant difference at the probability level of 1 percent. The highest number of leaves was related to the stage of harvesting the stem of the botting and heading, with an average of 9.10 and 8.72. With the delay in harvesting, the number of green leaves decreased.
The results of the analysis of variance showed that the effect of the harvesting stage on the yield of fresh and dry fodder was significant at the probability level of 1%. The increase in the yield of fresh and dry fodder showed that it was higher in the harvesting stages of soft dough and hard dough than in other harvesting stages. The delay in harvesting increases the lignin and maintenance tissues of the plant, which increases the yield of the forage in the plant.
Conclusion: In order to produce the desired fodder, it is recommended to cultivate the speedfeed variety with a seed rate of 20 kg per hectare and harvest at the soft dough stage.