Orchardgrass Soluble Carbohydrate and Digestibility Levels in Sward Horizons under Sequences of Successive Defoliation Initiated in Morning and Evening
T. C. Griggs, J. W. MacAdam, and S. Buffler,
Utah State University, Logan, UT
H. F. Mayland, and J.C. Burns,
USDA-ARS, Kimberly, ID, Raleigh, NC
Diurnal cycles of total nonstructural carbohydrate (TNC) levels in forage canopies, and higher TNC levels for hay cut in evening than in morning, have been documented. Assessment of temporal patterns of TNC levels has been limited in rotationally-stocked swards with daily herbage allocation. Our objective was to compare TNC levels in horizons of an orchardgrass (Dactylis glomerata L.) sward under sequential clipping during 24-hr periods initiated in evening (PM treatments) and morning (AM treatments). Vegetative orchardgrass was clipped to remove 0.33 of current sward height every 6 hr from an initial height of 40 cm to a final height of 8 cm. Sward horizons were 40-27, 27-18, 18-12, and 12-8 cm above soil surface. Clipping sequences were initiated at 7 PM and 7 AM in October, 2000 and June and August, 2001. Data from the June period are reported here. Only the uppermost horizon in sequentially-clipped patches was analyzed at each time point. Whole-canopy control samples were also collected at each time point and sectioned into horizons. All samples were analyzed for levels of TNC via near-infrared reflectance spectroscopy (NIRS). Patterns of TNC levels in horizons removed in the defoliation sequences and corresponding horizons of control samples were dissimilar, suggesting different patterns of TNC metabolism under control vs. treatment conditions. Under defoliation, TNC levels in successive horizons decreased steadily over 24 hours in the PM sequence, but increased and decreased diurnally in the AM sequence. Although initial TNC levels were higher in the PM sequence than in the AM sequence, 24-hr mean TNC levels for PM and AM treatments (9.1 and 8.8% of DM, respectively) were not different. Results suggest that TNC patterns in swards under simulated rotational stocking initiated in PM vs. AM may vary from those observed in canopies managed for mechanical harvest.
A primary objective in the management of grazing systems is to maintain sward conditions favorable for high dry matter intake by livestock with high performance potential. Constraints to herbage intake, and corresponding animal performance on pasture, include sward structural and herbage compositional factors that determine concentrations of dietary energy. Diurnal cycling of herbage TNC concentrations has been documented in grass and legume canopies managed for mechanical harvest, but fewer investigations have been conducted under the more dynamic conditions of sward depletion during rotational stocking. Timing of herbage allocation in pastures may impact the daily balance of sward photosynthetic gain and respiratory loss and therefore energy intake by livestock. Previous findings indicate livestock preference for grass and legume hays harvested in evening vs. morning, and improved individual animal performance for grazing dairy cattle with daily herbage allocation in the evening vs. morning. This performance boost may result from higher diet quality, higher rate of intake, or both, for evening shifts to new paddock areas.
We are investigating impacts of timing (evening vs. morning) of herbage allocation on herbage composition that would result in higher levels of diet quality and animal performance. Higher diet quality could stem from higher concentrations of soluble carbohydrates in the herbage DM. Our objective was to test the hypothesis that afternoon allocation of daily grazing area in a rotationally-stocked orchardgrass pasture results in higher herbage soluble carbohydrate levels during a 24-hour defoliation period than morning allocation.
Clipping techniques were used to simulate progressive defoliation of daily paddock areas in an irrigated orchardgrass pasture on Millville silt loam at Logan, Utah. Soil water and fertility levels were appropriate for high pasture production potential. The experimental area had regrown for approximately 3 wk since previous uniform grazing by beef cattle and had a relaxed sward height of approximately 40 cm. Sampling areas were randomly allocated to each of three blocks in a randomized complete block design. Treatments were two schedules of initiation of progressive sward defoliation by clipping at 6-hr intervals for 24 hr. An evening (PM) treatment sequence was initiated at 1900 hours (7 PM) Mountain Time June 20, 2001 and a morning (AM) sequence was initiated at 700 hours (7 AM) June 21. Diurnal environmental conditions were similar for each defoliation sequence. In each treatment, approximately 0.33 of current relaxed sward height was removed every 6 hr by clipping from a 0.5- by 0.7-m sampling area. Portable adjustable guide rails were used to define sampling area dimensions and clipping heights above soil surface. Sward horizons successively removed by clipping during 24 hr were therefore 40 to 27, 27 to 18, 18 to 12, and 12 to 8 cm above soil surface. Each horizon was removed by clipping paired sampling areas at the beginning and end of each 6-hr interval of sward depletion, as shown in Fig. 1. Sampling of adjacent paired patches allowed subsequent estimation of herbage composition at the midpoint of each time interval. Since PM and AM defoliation sequences were initiated 12 hr apart, each sampling period took 36 hr to complete.
Clipped herbage samples were stored in a cooler with dry ice for up to 2 hr, then moved to a freezer (-9 C). Samples were subsequently lyophilized, ground through an impact mill to pass a 1-mm screen, and scanned in powder cells on a NIR spectrophotometer to obtain spectra for prediction of herbage chemical composition (Mod. 5000 scanning monochromator, NIRSystems, Foss North America, Eden Prairie, MN). A TNC prediction equation was developed from approximately 120 calibration samples that were selected to represent the range and spectral distribution of the sample population and analyzed for TNC according to Smith (1969).
Clear diurnal patterns of herbage TNC concentration were present in all horizons of whole-canopy control samples from undefoliated patches (Fig. 2). Control sample TNC levels (Fig. 3) were higher in evening and lower in morning, and amplitudes fluctuated most for the top (40-27 cm) horizon and least for the horizon at 12-18 cm sward height. Highest mean TNC level across sampling times was in the bottom (12-8 cm) horizon. In the defoliation treatments, TNC levels in successively-lower horizons decreased steadily with time in the PM sequence, but increased and decreased diurnally in the AM sequence. Patterns in corresponding control horizons at the same sampling times were not representative of those in the defoliation treatments. While initial TNC level was higher for the PM than for the AM sequence (13.1 vs. 9.3%), 24-hr mean TNC levels in PM and AM treatments (9.1 vs. 8.8%, respectively) were not different (Table 1). The similarity of TNC means over 24-hr defoliation periods does not support our hypothesis that evening herbage allocation may improve grazing livestock energy intake relative to morning allocation in response to higher TNC density. If defoliation sequences had been terminated at taller residual sward heights, 24-hr mean TNC levels could have differed among treatments under our conditions. Possible differences in ingestive behavior and intake during daytime and nighttime grazing sessions might offer an alternative explanation for improved livestock performance response to evening vs. morning herbage allocation.
While TNC levels in whole-canopy control samples from undefoliated patches were consistent with diurnal patterns documented in other studies involving mechanical harvest management, they were not representative of patterns under progressive sward depletion during 24-hr defoliation periods. Additional work with pasture species will be required for clearer definition of sward TNC dynamics under varying climatic and defoliation regimes. Predictions of diurnal TNC relationships in rotationally-stocked swards based on the behavior of whole canopies under mechanical harvest management may not be valid. The similarity of 24-hr mean herbage TNC levels among defoliation sequences under our conditions suggests that differences in performance of grazing livestock receiving evening vs. morning herbage allocations may be at least partly due to intake factors.
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