ARS | Publication request: Effects of Harvest Timing on Estimates of Rumen Degradable Protein from Alfalfa Forages

Submitted to: Joint Meeting of the ADSA, AMSA, ASAS and PSA
Publication Type: Abstract
Publication Acceptance Date: February 12, 2007
Publication Date: N/A
Reprint URL: http:////jds.fass.org/

Technical Abstract: Alfalfa (Medicago sativa L.) proteins ingested by dairy cows typically degrade at rapid rates, resulting in low percentages of dietary crude protein (CP) that escape the rumen intact. Our objectives were to determine rumen degradable protein (RDP) for alfalfa managed in a four-harvest system that was clipped on five dates within each harvest. During 2004 and 2005, 'Affinity ' alfalfa was harvested four times (spring, early and late summer, and fall) at Prairie du Sac, WI; within each harvest, plots were clipped initially (d 0) when plants were at least 30 cm tall, but did not exhibit any evidence of buds. Additional sampling dates were scheduled at 5-d intervals for the next 20 d, resulting in a total of five clipping dates (0, 5, 10, 15, and 20 d) within each individual harvest. Forages were evaluated for CP, neutral-detergent soluble CP (NDSCP), and neutral-detergent insoluble CP (NDICP); in addition, RDP and rumen undegradable protein (RUP) were determined by the Streptomyces griseus protease method. For 2004, there were no interactions (P = 0.372) between harvest number and clipping date for any protein component. Crude protein, NDSCP, and RDP declined in a quadratic (P = 0.026) relationship with clipping dates. A quadratic (P = 0.002) pattern also was observed for RUP, but the overall range was small (6.04 to 6.65% of DM); similarly, NDICP exhibited only a quadratic trend (P = 0.077) and a small overall range (2.63 to 2.90% of DM). On a percentage of CP basis, RDP declined linearly (P < 0.001) from 72.0 to 65.9% of CP during 2004. For 2005, there were interactions (P = 0.020) of harvest number and clipping date within harvest for all response variables. Although less consistent than in 2004, trends for individual CP pools generally were similar over clipping dates within harvest. Concentrations of RDP (% of CP) declined (P = 0.042) with clipping date during three of four harvests, but specific polynomial effects varied with harvest number. Overall, RDP (% of CP) declined as alfalfa plants aged, but these responses were due primarily to reduced concentrations of CP within the highly degradable cell-soluble fraction.