Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 23, 2007
Publication Date: March 19, 2007
Citation: Coblentz, W.K., Brink, G.E., Martin, N.P., Undersander, D.J. 2007. Effects of harvest timing on estimates of rumen degradable protein from alfalfa forages. Crop Science. 48:778-788. Interpretive Summary: Generally, aging reduces concentrations of rumen degradable protein within alfalfa forages, but the effects of harvest season (late spring, early summer, late summer, or fall) remain unclear. Within harvest season, rumen degradable protein declined over time, regardless of whether it was expressed on a whole-plant DM or CP basis. In contrast, concentrations of CP insoluble in neutral detergent, and presumably associated with the cell wall, remained relatively stable across harvest periods, as well as across days within each harvest period. Because this CP fraction is generally resistant to ruminal degradation, and comprises a substantial proportion of the total undegradable protein pool, concentrations of rumen undegradable protein, expressed as a proportion of whole-plant DM, also remained relatively stable across all treatment factors. Given these relatively stable concentrations of proteins associated with the cell wall, any declines in rumen degradable protein are most likely related to reductions of highly-degradable, cell-soluble protein that also were observed as a function of the aging.
Technical Abstract: Alfalfa (Medicago sativa L.) proteins ingested by dairy cows typically degrade at rapid rates, and a low percentage escapes the rumen intact. Our objective was to quantify rumen degradable intake protein (DIP) for 'Affinity ' alfalfa harvested over a range of ages (0, 5, 10, 15, and 20 d from Stage 2) within each of four harvest periods (spring, early and late summer, and fall). For 2004, there were no interactions (P 0.372) between harvest period and days within period for any protein component. Crude protein (CP), neutral-detergent-soluble CP (g kg-1 DM), and DIP (g kg-1 DM) declined in a quadratic (P 0.026) relationship with sampling dates. A quadratic (P = 0.002) pattern also was observed for undegraded intake protein, but the overall range was small (60.4 to 66.5 g kg-1 DM); similarly, neutral-detergent-insoluble CP exhibited only a quadratic trend (P = 0.077), and a small overall range (26.3 to 29.0 g kg-1 DM). On a CP basis, DIP declined linearly (P < 0.001) from 720 to 659 g kg-1 CP during 2004. For 2005, there were interactions (P 0.020) of harvest period and days within period for all protein-related response variables. Although more erratic than 2004, individual CP pools exhibited trends over time within each harvest period that generally were similar to those observed the previous year. Overall, DIP declined as alfalfa plants aged, but these responses were due primarily to reduced concentrations of CP within the highly degradable cell-soluble fraction.