Responses to Divergent Selection for Fiber Concentration at Two Disease Potentials in Smooth Bromegrass

groups, although the difference in NDF at reproductive stage was attributed to the difference in maturity. At Neutral detergent fiber (NDF) concentration is related to intake reproductive stage, the low-NDF groups also had lower potential of forages but might also be related to plant reaction to yield per plant than the high-NDF groups; at the vegetapathogens. Three smooth bromegrass (Bromus inermis Leyss.) populations from these cycles of uniparental mass selection for low NDF, tive stage, their difference in yield was not significant. one cycle of biparental mass selection for high NDF, and the base In smooth bromegrass (Bromus inermis Leyss.), one population were evaluated at high and low disease potentials at three cycle of divergent selection for NDF produced a lowlocations for 2 yr and three harvests each year. High disease severity, NDF synthetic and a high-NDF synthetic (Carpenter measured as proportion of foliage diseased, was associated with high and Casler, 1990). The low-NDF synthetic was lower in NDF and high lignin concentration (on a dry-matter basis and a cellNDF and ADF than the high-NDF synthetic, but the wall basis). Nevertheless, disease potentials did not affect the relative two synthetics were not significantly different in sward performance of selection cycles on any traits measured. On average, dry matter yield or acid detergent lignin (ADL). Most NDF was reduced 3.5 g kg21 DM per cycle (P , 0.001) by selection recently, Casler (1999) reported on the evaluation of for low NDF, and increased 11.1 g kg21 DM in one cycle (P , 0.001) selection cycles from six recurrent selection methods by selection for high NDF. For the spring harvest, the response in for reducing NDF in smooth bromegrass. Three of these NDF could partially be attributed to a change in heading index. A positive correlated response in acid detergent lignin was found in the methods completed three cycles; the other three methspring (P , 0.001) while a negative correlated response in cell wall ods completed two cycles. The response to the selection lignin was detected in the summer and fall (P , 0.001). Selection for was significant at the vegetative and/or reproductive NDF modified the composition of NDF, as cell wall hemicellulose stage for five of the six methods. The least effective changed in the opposite direction of NDF. Lower dry matter yield method in that study was uniparental mass selection at per plant appeared to be associated with lower NDF and higher heading stage using wet-laboratory analysis for NDF inbreeding. Selection for low NDF, accompanied by slight selection determination. The three selection cycles selected by pressure on disease resistance, resulted in a reduction of NDF without that method as well as the base population of the selecan increase in disease susceptibility. tion were independently evaluated in this study with more replications and at more sites. Cell walls, especially when highly lignified, could N detergent fiber is a measure of cell wall serve as preexisting structural defenses against microorconcentration for forages (Goering and Van Soest, ganisms which are incapable of producing the appro1970). It is also an indicator of intake potential of feed priate degradative enzymes (Ride, 1983). Therefore, it is for ruminant animals. When forage alone is fed to sheep conceivable that genetic modification of NDF, of which or cattle, NDF is negatively correlated with dry matter lignin is a part, could change the reaction of plants to intake (Osbourn et al., 1974; Reid et al., 1988). When pathogens. Water soluble carbohydrates (WSC) make forage is fed together with concentrates, NDF is negaup the largest share of noncell-wall carbohydrates. tively correlated with dry matter intake if the rumen Breese and Davies (1970) reported that successful divercapacity of the animal is not reached (Conrad et al., gent selection for WSC in perennial ryegrass (Lolium 1964; Mertens, 1987; Ruiz et al., 1995). Thus, it is desirperenne L.) for three generations resulted in higher susable to reduce NDF of forages. ceptibility to crown rust infection (caused by Puccinia Genetic modification of NDF in forages has been coronata Corda) for the families with high WSC. Webb reported. In reed canarygrass (Phalaris arundinacea L.), et al. (1996) reported that plants selected for low ADL one cycle of bidirectional selection for NDF and forage in alfalfa (Medicago sativa L.) showed greater susceptiyield created the following groups: low NDF–low yield, bility to alfalfa rust (caused by Uromyces striatus J. low NDF–high yield, mean NDF–mean yield, high Schröt.) and spring blackstem (caused by Phoma medNDF–low yield, and high NDF–high yield (Surprenant icaginis Malbr. & Roum. in Roum.) in the field than et al., 1988). At both reproductive and vegetative stages, those selected for high ADL, although a further experithe two low-NDF groups, on average, had lower NDF ment in the greenhouse found little or no relationship and acid detergent fiber (ADF) than the two high-NDF between ADL and the reaction of alfalfa to the rust pathogen. It was our concern that selection for low NDF L.X. Han, Dep. 436, AP9A/2, Abbott Laboratories, 100 Abbott Park might have increased susceptibility of smooth bromeRoad, Abbott Park, IL 60064-6124; M.D. Casler, Dep. of Agronomy, grass to diseases. Univ. of Wisconsin-Madison, 1575 Linden Dr., Madison, WI 53706On the other hand, selection for disease resistance 1597; C.R. Grau, Dep. of Plant Pathology, Univ. of Wisconsin-Madihas been shown to alter NDF in some cases further son, 1630 Linden Dr., Madison, WI 53706-1598. Part of a dissertation submitted by the senior author for the partial fulfillment of the requirements of the Ph.D. degree at Univ. of Wisconsin-Madison. Received Abbreviations: ADF, acid detergent fiber; ADL, acid detergent lignin; 23 Nov. 1999. *Corresponding author ([email protected]). CW, cell wall; NDF, neutral detergent fiber; NIRS, near-infrared reflectance spectroscopy; WSC, water soluble carbohydrate. Published in Crop Sci. 41:30–39 (2001).