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Harvest Timing Effects on Estimates of Rumen Degradable Protein from Alfalfa Forages

^a USDA-ARS, U.S. Dairy Forage Research Center, 8396 Yellowstone Dr., Marshfield, WI 54449
^b USDA-ARS, U.S. Dairy Forage Research Center, Madison, WI 53706
^c Dep. of Agronomy, University of Wisconsin, Madison, WI 53706. Mention of trade names or commercial products is solely for the purpose of providing specific information about scientific procedures and/or subsequent evaluation of results, and does not imply any recommendation or endorsement by the USDA

^* Corresponding author (wayne.coblentz{at}ars.usda.gov).

Alfalfa (Medicago sativa L.) proteins ingested by dairy cows typically degrade at rapid rates and exhibit extensive ruminal degradability. Although the effects of conservation method (hay or silage) on these characteristics have been evaluated extensively, agronomic factors, such as harvest timing, have not. Our objective was to quantify rumen degradable protein (RDP) for ‘Affinity’ alfalfa harvested over a range of ages (0, 5, 10, 15, and 20 d following 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 harvest period for any protein component. Crude protein (CP), neutral-detergent soluble CP (NDSCP; g kg^–1 dry matter [DM]), and RDP (g kg^–1 DM) declined in a quadratic (P 0.026) relationship with days following Stage 2. A quadratic (P = 0.002) pattern also was observed for rumen undegradable protein (RUP), but the overall range was small (60.4–66.5 g kg^–1 DM). On a CP basis, RDP 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, but trends over time within each harvest period generally were similar to those observed in 2004. Overall, RDP declined as alfalfa plants aged within harvest period, but these responses were due pri marily to reduced concentrations of CP within the cell-soluble fraction.

Abbreviations: CP, crude protein • DM, dry matter • ES, early-summer harvest period • FA, fall harvest period • GDD, growing degree days • LS, late-summer harvest period • NDF, neutral-detergent fiber • NDICP, neutral-detergent insoluble CP • NDSCP, neutral-detergent soluble CP • RDP, rumen degradable protein • RUP, rumen undegradable protein • SP, spring harvest period

Received for publication June 1, 2007.