CASE STUDY: Yield and quality of traditional senescent and stay-green sorghum and in situ ruminal disappearance of respective crop residues

Nutritional composition of sorghum before grain harvest and ruminal disappearance of crop residues were evaluated in traditional senescent and stay-green sorghum hybrids grown under restricted water conditions. Hybrids were seeded in a randomized complete block design (experimental unit = 2.63-ha plots; n = 12, 6 plots per treatment) in limited water conditions (330 mm/season). Plants were sampled 129 d after seeding and botanically fractioned for yield and nutrient composition (Exp. 1) and in vitro true digestibility (Exp. 2). Crop residues were baled and in situ ruminal disappearance was evaluated using a crossover design (Exp. 3). Ruminally cannulated steers (n = 6; BW = 722 ± 65 kg) were randomly assigned to treatments: hybrid (traditional senescent vs. stay-green) and supplement (0 or 0.68 kg/animal daily; cottonseed meal). Experimental periods (n = 4) included a 10-d adaptation phase before incubations of 0, 3, 6, 12, 24, 3, 48, and 72 h. Data were analyzed using GLIMMIX procedures of SAS. Greater (P ≤ 0.05) whole-plant, grain, and stalk DM yields were observed with stay-green hybrid. Stay-green stalks contained less ash (P = 0.04) and greater fiber (P ≤ 0.03) than the traditional senescent cultivar. Projected whole-plant, grain, and stalk digestible OM yield was greater (P ≤ 0.05) with stay-green than the traditional senescent cultivar. Ruminal residue OM disappearance of both hybrids was increased (P < 0.01) with supplementation beyond 12 h of incubation. Under restricted water conditions, stay-green sorghum cultivar appears to better attend agronomic parameters for forage production compared with the traditional senescent cultivar.