Divergent Selection for Two Measures of Intake Potential in Smooth Bromegrass

mine which selection criteria will have the greatest impact on animal performance without detrimental effects Voluntary intake is often the greatest limitation to animal perforon agronomic traits. Plant breeders must evaluate large mance. Two different laboratory measures of intake potential were used as selection criteria in four smooth bromegrass (Bromus inermis numbers of individual plants, often with some form of Leyss) populations. The objective of this study was to measure the replication, to obtain reliable data upon which to base direct and correlated responses to one cycle of divergent selection for selection decisions. Thus, plant breeding programs must neutral detergent fiber (NDF) concentration or particle-size reduction rely upon indirect assessments of intake, based on inexindex (PSRI). Sixteen progeny populations were created by divergent pensive, rapid, and repeatable laboratory predictors. selection (high or low) for NDF or PSRI of smooth bromegrass leaf Cell wall concentration, estimated as neutral deterblades within four populations. Divergent selection for NDF resulted gent fiber (NDF), provides a measure of fibrous bulk in changes of 4 to 8 g kg 1 (0.7–1.3% of the mean) for NDF and 2.2 which is negatively associated with voluntary intake to 3.0% (6.6–9.4% of the mean) for PSRI. Divergent selection for (Van Soest, 1994). It is not known whether forages that PSRI resulted in changes of 1.9 to 3.9% (5.4–12.9% of the mean) for PSRI and inconsistent changes for NDF. Although realized heritabilithave inherently high NDF concentration limit ruminant ies were low, there was additive genetic variation for both traits within intake by their low caloric density or their high bulk each population. Divergent selection for NDF resulted in strong volume, two characteristics that are typically conchanges in PSRI, in the opposite direction, and largely consistent founded in voluntary intake studies (Van Soest, 1994). across populations, suggestive of possible pleiotropic relationships. Nevertheless, NDF concentration is a heritable trait in Conversely, divergent selection for PSRI resulted in weak and/or several forage species, with genetic progress demoninconsistent changes in NDF, suggesting that there are multiple mechstrated in smooth bromegrass (Casler, 1999a), reed caanisms by which PSRI is regulated in smooth bromegrass leaves. narygrass, Phalaris arundinacea L. (Surprenant et al., Decreased NDF concentration uniformly results in increased PSRI, 1988), and maize, Zea mays L. (Wolf et al., 1993). Rates but PSRI can probably be increased by alterations to the cell wall or to changes in cell types without changes to NDF concentration of gain from selection for reduced NDF concentration of herbage. of herbage are reported to be as high as 12 g NDF kg 1 DM year 1 (Casler and Vogel, 1999). Voluntary intake is also limited by the rate at which rumen digesta can be degraded and cleared from the P breeding efforts to develop forage crops with improved intake potential are increasing (Casler rumen (Weston, 1996; Wilson and Kennedy, 1996). Most and Vogel, 1999). While there has been considerable feed particles do not pass from the rumen until they effort at making genetic improvements in digestibility have been reduced to a particle size sufficient to pass (Casler and Vogel, 1999), most ruminant nutritionists a 1-mm sieve for sheep (Reid et al., 1977) or a 1to consider voluntary intake to be more important than 2-mm sieve for cattle (Poppi et al., 1985; Waghorn et digestibility in limiting animal performance (Fahey and al., 1989). Chewing and rumination are the major mechHussein, 1999). Up to 70% of the variation in animal anisms of particle size reduction in ruminants (McLeod production can be attributed to variation in intake, while and Minson, 1988; Wilson et al., 1989). Indeed, volunonly 20% can be attributed to variation in digestibility tary intake of ruminants can be predicted by laboratory (Crampton et al., 1960). measures of breakdown resistance of forages. Examples Physical distension of the rumen is the major factor include particle size distribution following artificial maslimiting voluntary intake of high producing ruminants tication (Troelson and Bigsby, 1964), energy required on high-forage diets (Mertens, 1994). For most highto grind forages through a given sieve size (Weston, forage diets, intake of fibrous bulk generally causes ru1985), energy required to shear or compress forage tismen fill and satiation before the ruminant has maxsue (Baker et al., 1993), and particle size reduction index imized its caloric intake, resulting in a reduced plane of (Culvenor and Casler, 1999). Particle size reduction innutrition (Van Soest, 1994). Voluntary intake can be dex (PSRI) is the percentage of ball-milled leaf particles increased by reducing bulk volume of the feed, which that pass through a 1-mm screen in a dry sieve (Casler increases intake prior to satiation, or by increasing fiber et al., 1996). clearance from the rumen, which reduces the time reHeritability of PSRI in smooth bromegrass leaves is quired to stimulate appetite. moderate and rate of genetic gain has averaged 2 to Forage-plant breeders face two challenges: to identify 6% per cycle of selection (Culvenor and Casler, 1999). selection criteria that relate to bulk volume of feeds or Divergent selection for PSRI in smooth bromegrass has to their rate of clearance from the rumen, and to deterresulted in fairly consistent changes in NDF concentration (Casler et al., 1996; Culvenor and Casler, 1999). Dep. of Agronomy, Univ. of Wisconsin-Madison, Madison, WI 53706For each percentage unit change in PSRI, NDF concen1597. Received 5 Sept. 2001. *Corresponding author (mdcasler@facstaff. tration changed by 2.1 to 4.1 g kg 1 (Culvenor and wisc.edu). Casler, 1999). This relationship, and its consistency among different populations of smooth bromegrass, inPublished in Crop Sci. 42:1427–1433 (2002).