Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/20/2004
Publication Date: 7/26/2005
Citation: Griggs, T., Macadams, J.W., Mayland, H.F., Burns, J.C. 2005. Nonstructural carbohydrate and digestibility patterns in orchardgrass swards during daily defoliation sequences initiated in evening and morning. Crop Science. 45:1295-1304 Interpretive Summary: Forage plants increase total nonstructural carbohydrates (TNC) concentration during the day light hours peaking in the late afternoon with a reduction during the night nearing least concentration at sunrise. Harvested tall fescue and alfalfa cut in the PM resulted in forage with greater TNC concentration compared with an AM cut. This difference in favor of the PM harvested forage has been detected in animal preference, intake and digestion trials. This study evaluates the potential changes in TNC concentration between PM and AM as strata are removed from the canopy as would occur in a rotationally grazed pasture. Generally, orchardgrass canopies showed higher TNC in the PM series of strata removed vs. the AM series. As sword depth increased (removal of top most increments of the canopy) samples from PM and AM strata retained differences but these differences were reduced compared with the initial canopy strata. In contrast, digestibility showed minor diurnal variation and decreased readily at the lower strata within the PM and AM sequences. This is consisted with reduced digestibility in lower strata of the canopy. These data do not reflect higher daily energy intake for PM vs AM herbage in grazing systems. However, this simulation does not take into account intake rate and meal size at a particular grazing strata. Increases in these animal responses would be a strategy that would support an advantage to animal performance. If animals were restricted such that subsequent strata were consumed to meet the animal daily demand than livestock performance maybe reduced compared with less ridged rotional stocking.
Technical Abstract: Herbage soluble carbohydrate (SC) levels vary diurnally and livestock intake can be higher for herbage harvested or allocated to animals in the evening than in the morning. Few assessments of SC and digestibility patterns have been made during sward depletion in rotationally-stocked orchardgrass (Dactylis glomerata L.). We tested the hypothesis that simulated evening daily pasture allocation increases 24-h mean dietary SC and digestibility levels relative to morning allocation. Total nonstructural carbohydrate (TNC) and in vitro true dry matter digestibility (IVTDMD) levels were compared during 24-h clipping sequences initiated at 1900 h (PM) and 0700 h (AM). Sward height was reduced from 40 to 8 cm at 6- h intervals in October, June, and August. Successive-lower horizons from defoliation sequences and also from control areas that were not under progressive defoliation were analyzed. Digestibility and TNC levels varied diurnally and seasonally and were often higher for PM sequences, but differences among 24-h means were small. Daily mean TNC levels for defoliation sequences initiated in PM and AM treatments were 138 vs. 132, 91 vs. 88, and 72 vs. 60 g kg-1 in October, June, and August, respectively. In all periods, dry matter digestibility decreased from approximately 920 to 800-850 g kg-1 with sward depletion, and displayed similar patterns between defoliation sequences. Patterns of TNC and digestibility during sward depletion may not be represented by those in intact swards, and PM allocation of daily herbage may not increase 24-h mean dietary TNC density relative to AM allocation. Daily quantities of ingested TNC could be higher for PM herbage allocation if livestock consume proportionately more herbage in the PM than we simulated.