Nonstructural Carbohydrate and Digestibility Patterns in Orchardgrass Swards during Daily Defoliation Sequences Initiated in Evening and Morning

est in morning (Holt and Hilst, 1969; Orr et al., 1997; Barrett et al., 2001). Herbage SC and digestibility conHerbage soluble carbohydrate (SC) levels vary diurnally and livecentrations also vary seasonally, with generally higher stock intake can be higher for herbage harvested or allocated to animals in the evening than in the morning. Few assessments of SC SC levels in spring and lower levels in summer or fall and digestibility patterns have been made during sward depletion in (Dent and Aldrich, 1963; Deinum et al., 1968; Delagarde rotationally stocked orchardgrass (Dactylis glomerata L.). We tested et al., 2000). In contrast, high fall or winter SC levels the hypothesis that simulated evening daily pasture allocation inare reported by Jung et al. (1974) and Mayland et al. creases 24-h mean herbage SC and digestibility levels relative to morn(2000), while Radojevic et al. (1994) reported highest ing allocation. Total nonstructural carbohydrate (TNC) and in vitro levels in late summer and lowest levels in winter. Herbtrue dry matter digestibility (IVTDMD) levels were compared during age digestibility and SC levels also vary among geno24-h clipping sequences initiated at 1900 h (PM) and 0700 h (AM). types (Dent and Aldrich, 1963; Wilson and Ford, 1973; Sward height was progressively reduced from 40 to 8 cm at 6-h intervals Jung et al., 1976), growth stages (Jung et al., 1976; Fulkin October, June, and August. Successively lower horizons from defolierson et al., 1998; Delagarde et al., 2000), plant parts ation sequences and also from control areas that were not under progressive defoliation were analyzed. Digestibility and TNC levels (Terry and Tilley, 1964; Lechtenberg et al., 1971), and varied diurnally and seasonally, and were often higher for PM severtical horizons (Buxton and Marten, 1989; Delagarde quences, but differences among 24-h means were small. Daily mean et al., 2000) within cool-season grass and legume swards. TNC levels for defoliation sequences initiated in PM and AM were Experimental shading treatments and sampling across 138 vs. 132, 93 vs. 88, and 72 vs. 60 g kg 1 in October, June, and August, times of day, genotypes, stages of growth, and environrespectively. In all periods, digestibility decreased from approximately mental conditions have shown positive associations be920 to 800 to 890 g kg 1 during sward depletion and displayed similar tween levels of herbage SC and feed preference (Fisher patterns between defoliation sequences. Patterns of TNC and digestet al., 1999, 2002; Ciavarella et al., 2000; Mayland et ibility during sward depletion may not be represented by those in al., 2000) and energy intake and livestock performance intact swards, and PM allocation of daily herbage may not increase (Michell, 1973; Lee et al., 2000; Miller et al., 2001). 24-h mean dietary TNC density relative to AM allocation. Daily quantities of ingested TNC could be higher for PM herbage allocation if Increased SC levels may also increase efficiency of rulivestock consume proportionately more herbage in the PM than men microbial protein synthesis and livestock N retenwe simulated. tion through improved balance or synchronization of energy and protein levels (Fulkerson et al., 1998; Lee et al., 2000; Miller et al., 2001). Timing of daily herbage M of grazing systems for high animal allocation in rotationally stocked pastures affects diurperformance includes facilitation of high intakes nal patterns of leaf area reduction and may therefore of pasture dry matter (DM) by livestock. Constraints affect the daily balance of sward photosynthetic gain to grazing animal performance include sward structural and respiratory loss and energy intake by livestock. Inand herbage compositional factors that determine rates terpretation of temporal SC patterns has led to suggesand concentrations of dietary energy intake (Orr et al., tions that animal performance may be higher under 1997; Vazquez and Smith, 2000; Barrett et al., 2001). afternoon or evening allocation of daily pasture area, Herbage SC and digestibility concentrations, two indices relative to morning allocation (Lechtenberg et al., 1971; of dietary energy density, usually vary diurnally according Delagarde et al., 2000; Orr et al., 2001). This increase to daily patterns of photosynthesis, respiration, and would be a function of matching higher evening herbage translocation of SC. In temperate climates, herbage SC DM (Orr et al., 1997, 2001; Gibb et al., 1998; Delagarde concentrations are generally highest in evening and lowet al., 2000), SC, and digestibility levels with possibly higher evening intake rate or meal size (Orr et al. (1997) T.C. Griggs and J.W. MacAdam, Dep. of Plants, Soils, and Biometeorand 2001; Gibb et al., 1998; Barrett et al., 2001). In the ology, Utah State Univ., 4820 Old Main Hill, Logan, UT 84322-4820; last 4 wk of a 10-wk grazing trial comparing PM and H.F. Mayland, USDA-ARS Northwest Irrigation and Soils Research AM allocation of daily herbage, milk production was Lab., Kimberly, ID 83341-5076; and J.C Burns, USDA-ARS Plant 6% higher for dairy cattle in the PM treatment (Orr et Science Research Unit, and Depts. of Crop Science and Animal Scial., 2001). ence, Room 1119, Box 7620, North Carolina State Univ., Raleigh, NC 27695-7620. This research was supported by the Utah Agricultural Dietary preferences and intake and performance imExperiment Station and USDA-ARS, Kimberly, ID and Raleigh, provements have been shown for relatively small inNC. Approved as journal paper no. 7583. Received 24 Nov. 2003. creases in herbage SC levels associated with genotype, *Corresponding author ([email protected]). environment, or management (Fisher et al., 1999, 2002; Published in Crop Sci. 45:1295–1304 (2005). Forage & Grazing Lands doi:10.2135/cropsci2003.0613 Abbreviations: DM, dry matter; IVTDMD, in vitro true dry matter digestibility; NIRS, near-infrared reflectance spectroscopy; SC, solu© Crop Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA ble carbohydrates; TNC, total nonstructural carbohydrates. 1295 Published online May 27, 2005