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Two trials were conducted to determine and evaluate the rumen degradable and escape protein (EP) value of two summer annuals for yearling cattle. In Trial 1,120 heifers (Bos taurus) (530 lb), arranged in a completely randomized design, were supplemented with 0, 0.25, 0.40, or 0.55 lb crude protein (CP)/d supplied by corn gluten meal:blood meal (CGM:BM) replacing a corn starch:molasses control, keeping the supplements equivalent in energy. Heifers grazing ‘Piper’ sudangrass [Sorghum bicolor (L.) Moench] (70 d) showed a negative linear (P < 0.02) response to supplemental protein and quadratic (P < 0.10) response (1.41,1.54,1.32, and 1.14 lb/d gain, respectively). Heifers grazing Northrup King ‘Millex’ pearl millet [Pennisetum americanum (L.) Leeke] (55 d) showed a cubic (P < 0.05) response (1.01, 1.23, 1.08, 1.25 lb/d gain, respectively) which tended to be linear (P = 0.13). Cattle on pearl millet responded to supplemental protein while cattle on sudangrass did not, which is supported by high EP values of sudangrass extrusa. In Trial 2, 60 Angus heifers and 60 Angus steers (600 lb) grazing sudangrass (56 d) were allotted to (i) molasses (ii) corn starch (iii) CGM:BM at 0.25 lb CP/d (iv) corn starch and CGM:BM at 0.25 lb CP/d, or (v) CGM:BM at 0.50 lb CP/d. All five treatments contained the same amount of molasses, and corn starch provided energy equal to CGM:BM. Supplemental protein improved gains (P = 0.10) (1.10,0.99,1.14 lb/d, respectively) over the molasses control (0.84 lb/d) but did not increase gains over the corn starch (1.17 lb/d). The lack of response to supplemental EP above the corn starch is again supported by high EP values of the sudangrass extrusa. Research QuestionSummer annuals can provide alternate pastures for grazing yearling cattle during summer slump periods of cool season perennial forages. They are also an alternative crop for cultivated land less suitable (or less profitable) for other cropping systems. Summer annuals can be grazed more intensely than perennial forages because perennial stands must be maintained for subsequent years. For the summer annuals to be a viable alternative, however, they should provide high gains per acre. This can be achieved if digestibility and protein levels are adequate. The objective of Trial 1 (1989) was to determine the escape protein value of sudangrass and pearl millet for growing calves. The objective of Trial 2 (1990) was to further evaluate the metabolizable protein value of sudangrass. Literature SummaryMetabolizable protein is the combination of microbial protein synthesized from rumen degradable protein and escape protein (not degraded in the rumen and used directly by the animal). Metabolizable protein should be adequate for optimum cattle growth before the forage is a viable option. The protein requirement of rapidly growing ruminants is not met by microbial protein synthesis alone. Dietary escape protein has increased performance in growing cattle when energy and ammonia were not limiting microbial protein synthesis. The metabolizable protein value of sudangrass and pearl millet for growing calves has not been evaluated. Study DescriptionIn Trial 1, 60 heifers grazed ‘Piper’ sudangrass (70 d) and 60 heifers grazed Northrup King ‘Millex’ pearl millet (55 d). Treatments: Corn starch:molasses control at 0 lb crude protein (CP)/d Corn gluten meal:blood meal (CGM:BM) at 0.25 lb CP/d CGM:BM at 0.40 lb CP/d CGM:BM at 0.55 lb CP/d In Trial 2, 60 heifers and 60 steers grazed Piper sudangrass (56 d). Treatments: Molasses Corn starch CGM:BM at 0.25 lb CP/d Corn starch and CGM:BM at 0.25 lb CP/d CGM:BM at 0.50 lb CP/d All five treatments contained the same amount of molasses; and corn starch provided energy equivalent to that of CGM:BM. Applied QuestionDid supplemental escape protein increase gains of cattle grazing sudangrass or pearl millet? Supplemental escape protein did not increase gains of cattle grazing sudangrass. The cattle primarily ate leaves which were highly digested. The sudangrass leaves appeared to supply enough escape protein. Supplemental escape protein slightly increased gains of cattle grazing pearl millet. The pearl millet leaves were also highly digested but did not supply adequate escape protein; however, supplementation might not improve gains enough to be economical. Is metabolizable protein limiting in sudangrass? The lack of response to supplemental protein above the corn starch control and the adequate levels of rumen ammonia measured, suggests metabolizable protein is not limiting for growing calves. |
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