Effects of summer management and fall harvest date on ruminal in situ degradation of crude protein in stockpiled bermudagrass

Limited information is available describing digestive kinetics of crude protein (CP) in stockpiled bermudagrass (Cynodon dactylon (L.) Pers.) during late fall and early winter. ‘Greenfield’ bermudagrass was stockpiled at two sites following summer hay or pasture management, and samples were taken at 4-week intervals under caged exclosures (UNGRAZED) and outside exclosures (GRAZED) between 17 October 1997 and 9 January 1998. Forage samples were subsequently incubated in situ in five ruminally cannulated, crossbred steers to determine effects of fall harvest date and previous summer management on in situ degradation kinetics of forage CP. At the hay site, concentrations of total plant CP declined (P<0.05) for UNGRAZED forages between 17 October (134.4 g kg−1) and 12 December (119.4 g kg−1), but concentrations of CP did not differ (P>0.05) on the 17 October and 9 January harvest dates. From the pasture site, concentrations of total CP for UNGRAZED forages did not change (P>0.05) throughout the trial. In situ degradation rates of CP from the hay site were not different (P>0.05) for UNGRAZED forages on 17 October and 9 January (overall mean=0.056 h−1). The ruminal degradation of CP decreased (P<0.05) by 63 g kg−1 CP between 14 November and 12 December, but did not change during the first and last sampling intervals. At the pasture site, rates of CP degradation increased (P<0.05) for UNGRAZED forages between 17 October (0.044 h−1) and 9 January (0.080 h−1) due to the presence of immature, winter annual weeds. Similarly, the ruminal degradation of CP increased (P<0.05) between 12 December (580 g kg−1) and 9 January (624 g kg−1) in response to winter annual weed presence. Ruminal availability of CP in stockpiled bermudagrass decreases as the dormant forage ages, but may be adequate to meet the minimum CP requirements of dry, pregnant beef cows.