Forage Quality Variation among Maize Inbreds: In Vitro Fiber Digestion Kinetics and Prediction with NIRS

Because grain is highly digestible, maize hybrids for silage production are generally chosen based on their The nutritive value of forage maize (Zea mays L.) may be improved grain yield. However, the stalk portion of the maize through genetic selection for increased rate of fiber digestion or decreased indigestible fiber concentration. To identify sources of genetic plant contains 50% or more of the whole plant biomass variation, 45 maize inbreds were evaluated for in vitro neutral deterand most of the fiber, which is much less digestible than gent fiber (NDF) digestion kinetic parameters using stem internode fiber in grain (Hunt et al., 1992). For these reasons the tissue harvested at silking during 2 yr. Near infrared reflectance specstalk is the plant structure most commonly identified troscopy (NIRS) was also used to estimate NDF digestion kinetic as a potential target for genetic improvement. Genetic parameters. Maize inbreds varied significantly in NDF concentration variation for fiber concentration and dry matter digestand digestion kinetic parameters using either conventional in vitro ibility of maize stalks was observed by Albrecht et al. analysis or NIRS predictions. Using NIRS predictions, inbreds varied (1986), Dhillon et al. (1990), and Hunt et al. (1992), and in NDF concentration from 497 to 662 g kg2 dry matter (DM), rate decreases in fiber concentration are commonly correof NDF digestion ranged from 0.037 to 0.077 h21, and extent of NDF lated with increased digestibility (Dhillon et al., 1990; digestion was 525 to 735 g kg21 NDF. The ranges for NIRS predicted parameters were less than those observed for the calibration data Hunt et al., 1992). It is not known, however, if rate of set by conventional analysis. Correspondence between conventional fiber digestion can be altered through genetic selection analysis data and NIRS predictions were good, except for lag time. or how selection for decreased fiber concentration or Digestion kinetics calculated from NIRS predicted residues provided increased extent of digestion will affect fiber digestion more precise predictions of lag time and fractional rate of digestion rate. when compared with observations derived from conventional analyBrown-midrib mutants of maize are characterized by ses, than did direct prediction of these kinetic parameters. Correlations low acid detergent lignin concentration and produce between rate of NDF digestion and 18-h NDF digestibility (r 5 0.79) forage higher in fiber digestibility than normal maize or between potential extent of NDF digestion and 96-h NDF digestibilgenotypes (Lechtenberg et al., 1972). Recent work sugity (r 5 0.95) were large enough that these two fermentation intervals gests that brown-midrib lignin has a lower degree of might substitute for conducting complete digestion kinetic studies with eight to 10 fermentation times. The substantial genetic variation polymerization than normal lignin (Lam et al., 1996). among these maize inbreds shows good potential for development of Brown-midrib mutants have expressed faster fractional silage hybrids with improved fiber digestion parameters. Year and rates of in vitro fiber digestion than normal maize in year 3 genotype interactions were significant suggesting that identifisome experiments (Muller et al., 1972) and had inconsiscation of superior inbred lines will require evaluations in multiple envitent effects on digestion rates in other studies (Thorsronments. tensson et al., 1992). Poor agronomic performance of maize hybrids possessing the brown-midrib trait has limited its use in the production of commercially available U of maize silage by ruminant livestock may be hybrids (Miller et al., 1983). improved through genetic selection for decreased A key factor limiting the development of maize genofiber concentration or increased rate or extent of fiber types with improved digestion kinetics is the high redigestion (Jung and Allen, 1995). Decreasing fiber consource requirement for analysis of digestion kinetics in centrations of forages can increase dry matter intake vitro. Near infrared reflectance spectroscopy (NIRS) is (Mertens, 1973; Waldo, 1985) and increasing fiber dia rapid method for predicting forage quality and has gestibility of maize can increase dry matter intake and been used successfully to estimate fiber concentration rate of gain for steers (Roth and Klopfenstein, 1987). and in vitro dry matter digestibility (Villalobos et al., Theoretical models have also shown that increasing fi1991; Gabrielsen et al., 1988; Marten et al., 1988) of ber digestion rate may improve fiber digestibility by forage grasses. If NIRS equations could be developed permitting a greater extent of digestion before particles that predict fiber digestion kinetics accurately, plant pass from the rumen (Allen and Mertens, 1987). breeders could evaluate much larger populations than is possible with standard in vitro techniques. Additionally, determination of fiber digestion kinetics is extremely H.G. Jung, USDA-ARS-PSR, U.S. Dairy Forage Res. Ctr. Cluster, laborious because of the requirement that many time 411 Borlaug Hall, Univ. of Minnesota, St. Paul, MN 55108; D.R. Mertens, USDA-ARS, U.S. Dairy Forage Res. Ctr., 1925 Linden points be measured during the course of digestion. If Drive West, Univ. of Wisconsin, Madison, WI 53706; D.R. Buxton, selected times could substitute for conducting complete USDA-ARS-FCR, and U.S. Dairy Forage Res. Ctr. Cluster, 1577 digestion studies with eight to 10 fermentation times, Agronomy Hall, Iowa State Univ., Ames, IA 50011. Mention of a estimates of digestion kinetic parameters could be proprietary product does not constitute a recommendation or warranty of the product by the University of Minnesota, Iowa State streamlined. University, or the United States Department of Agriculture, and does The objectives of this study were to: (i) evaluate a not imply approval to the exclusion of other suitable products. Received 13 Jan. 1997. *Corresponding author (jungx002@maroon. Abbreviations: NDF, neutral detergent fiber; INDF, indigestible NDF tc.umn.edu). concentration; NIRS, near infrared reflectance spectroscopy; DM, dry matter; LSD, least significant difference. Published in Crop Sci. 38:205–210 (1998).