Introduction: The arid and semi-arid region occupies most of south-west Iran, and livestock farming is one of the most important activities in this region. The goals of study were to recognize the effects of different level of Nitrogen and phosphorus fertilizers on morphological characteristics, productivity and forage quality of Opuntia ficus-indica (L.) Miller. Opuntia is cultivated in both hemispheres and all continents. Its products have always been very popular among the rural populations of arid and semi-arid areas in its native region of Mexico and in the Mediterranean basin, where it was introduced at the beginning of the 16^th century. They include fruits and cladodes for human nutrition and cattle feeding. Moreover, its utilization is common in popular medicine (flowers and cladodes), and field management.
Materials and Methods:  Experiments were conducted in field of Agriculture Sciences and Natural Resources University of Khuzestan in the semi-arid of Khuzestan, Iran. The experiment was performed as a split plot in a randomized complete block design with 3 replications (plot).The objective was to study Nitrogen (0, 50, 100 and 150 kgha^-1) and Phosphorus (0, 25, 50 and 100 kg ha^-1) fertilizations on growth of Opuntia ficus-indica (L.) Mill. After two years of planting in the field, the plants were harvested and the characteristics of height, number of cladodes, fresh weight, phosphorus, nitrogen, amino acid, soluble carbohydrates, mucilage, fiber, chlorophyll were measured.
Results and Discussion: The effect of phosphorus and nitrogen fertilization on the growth and quality of Opuntia ficus-indica was investigated in this study. Phosphorus fertilization increased the dry matter content and the amino acid composition of the cladodes, which are the flattened stems of the cactus plant. Nitrogen fertilization had a positive and linear effect on the dry matter production and the yield of the plant, measured by the fresh weight of the shoots. However, no significant response to nitrogen fertilization was observed in the carbohydrate content of the cladodes, which remained relatively constant across different levels of nitrogen application. Nitrogen fertilization also increased the nitrogen concentration in the plant tissue, indicating a higher uptake and assimilation of this nutrient. The cladode area index (CAI), which is the ratio of the total cladode area to the ground area, increased with increasing nitrogen levels, reflecting a higher biomass accumulation and canopy development. The results showed that the application of 100 kg N / ha from urea source, which was the intermediate level of nitrogen treatment, resulted in an improvement of the vegetative parts of the plant and an increase in the fresh weight of the shoots, so that it was comparable to the yield obtained from the highest level of nitrogen treatment (200 kg N / ha). The results also showed that the amount of crude protein, amino acids and mucilage in the shoots, which are important indicators of the nutritional and medicinal value of the cactus plant, showed a good response to increased nitrogen consumption. However, phosphorus fertilization was able to affect some traits, especially the amount of amino acids and protein, which are two important properties in forage plants. These results suggest that phosphorus fertilization can enhance the quality of the cactus plant as a feed source for livestock.
Conclusion: In this study, it was found that Opuntia ficus-indica, which is a drought-tolerant and multipurpose cactus plant, had a significant performance even in the control treatment (without fertilization), when it was irrigated five times per year (one time in spring, three times in summer and one time in autumn). However, fertilization was necessary for optimal growth and quality of the plant. Due to the change in the amount of nitrogen and phosphorus in the cactus plant and also the affected properties, it seems that nitrogen fertilization was more effective than phosphorus fertilization in enhancing the productivity and value of the plant. In summary, increasing fertilizing with nitrogen and phosphorus resulted in greater productivity of O. ficus-indica in Khuzestan, a semi-arid region in Iran. Phosphorus fertilization involved less economical risk to the farmer, as it required lower amounts and frequencies of application, but positive responses occurred only when soil phosphorus was increasing. This work stresses the importance of using replicated field trials to look for quantitative differences in yield and fruit quality from visually indistinguishable cactus fruit clones, which may have different genetic and physiological characteristics.