Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/28/2006
Publication Date: 1/1/2007
Citation: Ganskopp, D.C., Aguilera, L., Vavra, M. 2007. Livestock forage conditioning among 6 northern great basin grasses. Rangeland Ecology & Management 60(1):71-78. Interpretive Summary: One of the potential positive aspects of having livestock on rangelands is that bunchgrasses that are grazed during the growing season may start growing again and generate new herbage that is nutritionally superior to ungrazed grasses. We studied the regrowth quantity and quality responses of 6 of the most common rangeland grasses from the Pacific northwest and northern Great Basin by applying cattle grazing when plants were leafy, when seed stalks were just beginning to rise (boot stage), and when seed stalks had emerged and were flowering (anthesis). The 6 grasses included crested wheatgrass, bluebunch wheatgrass, Idaho fescue, bottlebrush squirreltail, Thruber’s needlegrass, and basin wildrye. All grazing treatments generated nutritionally superior (more protein and more digestible) herbage in late summer and early fall than ungrazed grasses, but the latest treatment (anthesis grazing) was usually the best. Vegetative, boot stage, and anthesis grazing treatments reduced fall standing crop by about 34, 48, and 79 percent, respectively, when compared with ungrazed grasses. Grazed grasses generated more and higher quality regrowth when growing season moisture was somewhat limiting than in a year with 200 percent of average precipitation. Of the 6 grasses tested, bluebunch wheatgrass and crested wheatgrass exhibited the poorest forage quality responses. Bottlebrush squirreltail consistently generated more nutritionally superior regrowth than the other 5 grasses. Soil moisture content when grasses were grazed was a poor predictor of how much regrowth would occur. Managed cattle grazing can successfully improve forage quality for later use by nutritionally stressed livestock or wildlife.
Technical Abstract: Studies of Anderson and Scherzinger’s 1975 forage conditioning hypothesis have exhibited varied results. Our objectives were: 1) to evaluate late summer/early fall forage quality of crested wheatgrass (Agropyron desertorum (Fischer ex Link) Schultes), bluebunch wheatgrass (Agropyron spicatum [Pursh] Scribn. & Smith), Idaho fescue (Festuca idahoensis Elmer), bottlebrush squirreltail (Sitanion hystrix (Nutt.) Smith), Thurber’s needlegrass (Stipa thurberiana Piper), and basin wildrye (Elymus cinereus Scribn. & Merr) from ungrazed paddocks and paddocks grazed at vegetative, boot, and anthesis, and 2) test hypotheses that post-grazing regrowth yields were correlated with soil moisture content when grazing occurred. Crop-year precipitation in 1997 and 1998 was 134 and 205% of average. Crude protein (CP) and in vitro dry matter digestibility ( IVDMD) of ungrazed grasses displayed expected seasonal declines in quality. Among ungrazed grasses, mean late summer/early fall CP’s were 5.7% in 1997 and 3.6% in 1998. Mean IVDMD’s were 47 and 41%, respectively. Late season quality was enhanced by vegetative, boot stage, or anthesis grazing applications with the phenologically youngest regrowth always ranked highest in CP and IVDMD in late summer/early fall. Among grasses, 1997 CP and IVDMD means were 9.0 and 55% for regrowth following anthesis grazing, respectively. No regrowth followed anthesis grazing in 1998, but CP and IVDMD means from boot stage treatments were 5.5 and 47%, respectively. With CP measures, grasses responded differently to treatments in 1997, but all reacted similarly to treatments in 1998. Bluebunch wheatgrass and crested wheatgrass exhibited the poorest forage conditioning responses. Vegetative, boot stage, and anthesis grazing induced late 1997 summer/early fall standing crop reductions of 34, 42, and 58%, respectively, and 34, 54, and 100% reductions in 1998. Residual soil moisture was an ineffective predictor of subsequent regrowth yields. Managed cattle grazing can successfully elevate late season forage quality for subsequent use by stock or wildlife.