• Development of Forage Sorghum Tissue Testing for Efficient Fertilization, 2009

      Ottman, Michael J.; Walworth, James; Ottman, Michael J. (College of Agriculture, University of Arizona (Tucson, AZ), 2010-09)
      A nitrogen fertilizer study was conducted in order to develop tissue testing guidelines for fertilizer application to forage sorghum. The study was conducted at the University of Arizona Maricopa Agricultural center on a sandy clay loam soil irrigated using surface flood methods. Forage sorghum was planted on 8 July 09 and fertilized with eight N rates varying from 0 to 350 lbs N/acre in 50 lb N/acre increments. The plants were sampled six times during the growing season and the lower stem, most recently developed leaf, and whole plant were analyzed for nitrogen content. Maximum yield at final harvest was obtained at 150 lbs N/acre and plant growth was highly affected by N rate. Before the initiation of rapid growth, the relationship between plant growth and N content in the various tissues was weak (R2 < 0.20), but was very strong (R2>0.50) from the initiation of rapid growth through the pre-boot stage at the time when post-plant nitrogen fertilizer application may be considered. Stem nitrate was most strongly related to yield for the tissues tested, but the relationships between plant growth and total N in the newest leaf and whole plant were also very strong. Preliminary tissue testing guidelines are suggested for nitrate in the stem tissue. The lower stem, newest leaf, and whole plant are all potential candidates for development of tissue testing guidelines for forage sorghum.
    • Water Use Efficiency of Forage Sorghum Grown with Sub-optimal Irrigation, 2009

      Ottman, Michael J.; Ottman, Michael J. (College of Agriculture, University of Arizona (Tucson, AZ), 2010-09)
      A forage sorghum irrigation study was conducted at Maricopa, AZ to determine water use and if sub-optimal irrigation increases water use efficiency and profitability. Sorghum was planted on July 10 with a row spacing of 40 inches and irrigated three times with a total of 8.7 inches of water to establish the crop. Variable amounts of irrigation water were applied commencing on Aug 12 based on 25, 50, 75, and 100% of estimated crop water use (evapotranspiration, ET). The plots were 53.3 ft wide (16 rows) and 40 ft long. ET was estimated from soil water measurements using a neutron probe. The total amount of water applied was 15.5, 19.8, 23.7, and 27.8 inches for the 25, 50, 75, and 100% ET treatments, respectively. The forage was harvested on Oct 28 near the soft dough stage. Forage yields adjusted to 70% moisture were 11.3, 16.4, 21.5, and 23.1 tons/acre for the 25, 50, 75, and 100% ET treatments, respectively. Yield produced per inch of water used by the crop (WUEET, water use efficiency of water used in ET) increased with water application. Yield produced per inch of water applied to the crop (WUEirr, water use efficiency of irrigation water applied plus rainfall) also increased with water application, but then decreased from the 75 to 100% ET treatments. Nevertheless, sub-optimal irrigation strategies are not economical using the results from this study assuming a water cost of $45 per acre-foot and a sorghum silage value of $20 per ton. For sub-optimal irrigation strategies to be economical, water costs would have to increase, sorghum silage value would have to decrease, or the differences in the irrigation efficiencies of the strategies being compared would have to be greater than measured in the present study.