Nutritive value of herbage of five semi-irrigated pasture species across an irrigation gradient K. B. Jensen, B. L. Waldron, M. D. Peel and J. G. Robins USDA-ARS, Forage and Range Research Laboratory, Utah State University, Logan, UT, USA Abstract As water resources become limiting, the need to produce stable amounts of herbage with a high nutri- tive value increases. An understanding of how levels of irrigation affect crude protein (CP) and neutral-deter- gent fibre (NDF) concentrations and, in vitro true digestibility (IVTD), is critical in the management of pastures. Smooth bromegrass (Bromus inermis Leyss.), prairiegrass (Bromus willdenowii Kunth), hybrid-brome- grass [meadow (Bromus riparius Rehm.) · smooth bromegrass], creeping foxtail (Alopecurus arundinaceus Poir.) and RS wheatgrass (Elymus hoffmannii Jensen & Asay) were established under a line-source irrigation system to evaluate the effect of five water levels and three harvest dates on concentrations of CP and NDF, and values of IVTD. With CP concentration of herbage, there were positive linear significant relationships with decreasing water levels for all species. Rankings for mean CP concentrations across water levels and harvest dates were smooth bromegrass > RS wheatgrass > hybrid-bromegrass > creeping foxtail > prairiegrass. No consistent relationships with IVTD across water levels were detected among the species. Averaged across water levels, harvest dates and years, smooth bromegrass, hybrid-bromegrass and creeping foxtail had significantly higher IVTD values than RS wheat- grass and prairiegrass. Rankings of mean NDF concen- trations across water levels and harvest dates combined over years were prairiegrass > RS wheatgrass > hybrid- bromegrass > creeping foxtail > smooth bromegrass. Herbage of smooth bromegrass had higher IVTD values and lower NDF concentrations than hybrid-bromegrass, prairiegrass, creeping foxtail and RS-hybrid at all water levels. Keywords: smooth bromegrass, prairiegrass, hybrid- bromegrass, creeping foxtail, RS wheatgrass, irrigation, nutritive value Introduction Production of herbage in temperate regions of the world often experiences growing conditions where drought is more common than excess water (Waggoner, 1993). Drought is usually the major physical limitation to dry- matter (DM) yield of herbage resulting from reduced cell size and division (Levitt, 1980; Blum, 1993). Xeromorphic features of plants, such as thick walls, thick cuticles and highly lignified tissues, are often associated with plants adapted to hot, dry environments (Levitt, 1980) and are typically correlated with low digestibility values of herbage (Buxton et al., 1996). Limited information is available concerning how dif- ferent amounts of irrigation affect the nutritive value of herbage as characterized by crude protein (CP) and neutral-detergent fibre (NDF) concentrations, and in vitro true digestibility (IVTD), in the herbage of the grasses, smooth bromegrass (Bromus inermis Leyss.), prairiegrass (Bromus willdenowii Kunth), hybrid-brome- grass [meadow (Bromus riparius Rehm.) · smooth bromegrass], creeping foxtail (Alopecurus arundinaceus Poir.) and RS wheatgrass (Elymus hoffmannii Jensen & Asay) during the growing season. Under a line-source sprinkler design, CP concentra- tion of herbage increased as drought stress increased in perennial ryegrass (Lolium perenne L.), orchardgrass (Dactylis glomerata L.), meadow bromegrass and tall fescue (Festuca arundinacea Schreb.) (Asay et al., 2002; Jensen et al., 2003, 2008). CP concentration increased in maize (Zea mays L.) (Crasta and Cox, 1996) and alfalfa (Medicago sativa L.) (Halim et al., 1990; Deetz et al., 1996) when exposed to water stress. Buxton et al. (1996) reported that CP concentration in herbages is strongly influenced by available soil-nitrogen (N), which may be reduced in drier soils given the reduction in microbial activity and mineralization of organically bound N, suggesting that CP concentrations tend to be Correspondence to: Kevin Jensen, USDA-ARS, Forage and Range Research Laboratory, Utah State Univ., Logan, UT 84322-6300, USA. E-mail: kevin.jensen@ars.usda.gov. Received 6 May 2009; revised 21 October 2009 Published 2010. This article is an US Government work and is in the doi: 10.1111/j.1365-2494.2009.00723.x public domain in the USA. Grass and Forage Science, 65, 92–101 92