PS6-18. Effects of DMI and ADG Following an Abrupt Dietary Shift to High- and Low-quality Forage in Beef Cattle.

Abstract Dietary interventions have long been recognized as a strategy to influence the gastrointestinal environment of cattle. In terms of preharvest food safety, abrupt dietary shifts have been suggested to reduce gastrointestinal microbial food safety risk; however, understanding its effect on performance metrics with different forage qualities remains unclear. An objective of the study was to measure the effects of an abrupt dietary shift to differing forage qualities on dry matter intake (DMI) and average daily gain (ADG) in beef cattle. Twenty-seven (N = 27) crossbred Angus steers and heifers (236±32.6 kg) were enrolled into a 43 day randomized block design, blocking for weight and sex at the University of Georgia Double Bridges Farm. Animals (n = 9; 6 steers, 3 heifers) were assigned to one of three treatments: 1) CTRL: high grain diet, 2) LQF: Bermuda Grass hay, 3) HQF: Endophyte Free Alfalfa and Tall Fescue hay. Cattle were housed in separate pens based on treatment; each pen equip with SmartFeed Pro™ (C-Lock Inc., Rapid City, SD) to monitor feed intake. Cattle were initially fed a high-grain diet for 21 days and were abruptly shifted to their respective treatment for an additional 22 days. Body weights were recorded on days 0, 21, 23, 25, 29, and 43 and feed intake recorded daily. For DMI and ADG, no differences were observed (P > 0.30) on d 0 to 21. Immediately following the abrupt shift from d 21 to 23, ADG was greater in HQF compared to CTRL and LQF (P < 0.01) while DMI was greatest in CTRL and HQF (P < 0.01). From d 23 to 25 and d 25 to 29 only DMI differed, with CTRL being greater than LQF on d 23 to 25 (P < 0.02) and HQF being greater then LQF on d 25 to 29 (P < 0.01). From d 29 to 43 both DMI and ADG differed with CTRL and HQF being greater then LQF (P < 0.01). When looking at the entire post shift period (d 21 to 43), DMI was greater in the HQF and CTRL groups compared to the LQF group (P < 0.01) and ADG being greater in HQF followed by CTRL then LQF(P < 0.01); however, during the entire study period (d 0 to 43), DMI of HQF was greater than LQF (P < 0.01) while ADG showed HQF being the greatest compared to CTRL and LQF (P < 0.01). Difference between the post-shift period and the entire study period may be attributed to gut fill in HQF group. Results suggest that an abrupt dietary shift to solely forage alters cattle performance metrics and further supports that nutrient composition is an important determinant of performance metrics. Furthermore, nutrient composition may be central to unraveling the mechanisms and timeline associated with effective preharvest reduction strategies.