Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/12/2006
Publication Date: 4/1/2007
Citation: Huntington, G., Burns, J.C. 2007. Afternoon harvest increases readily fermentable carbohydrate concentration and voluntary intake of gamagrass and switchgrass fed to beef steers. Journal of Animal Science. 85:276-284. Interpretive Summary: In previous trials the harvest of forage in the PM, vs. the AM, has shown it to contain greater concentrations of soluble carbohydrates and was detectible by steers, sheep and goats. Animals preferred the PM harvest and generally showed greater dry matter intakes and the dry matter consumed was frequently greater in digestibility. This simple management strategy has resulted in the harvesting of forage with greater nutritive value. This study examined the nitrogen fractions in the silages relative to the utilization of the additional soluble carbohydrates that accumulate in the PM silage. The preservation of grasses as baleage increases the non-protein nitrogen delivered to the rumen and an increase of soluble carbohydrate in the rumen aids the conversion of that nitrogen into microbial mass which would be instrumental in increasing dry matter digestion and subsequent dry matter intake. Harvesting forage in the PM is a management strategy to improve animal daily response without any additional economic input, but results in an economic benefit.
Technical Abstract: Our objective was to determine if AM (0600) vs. PM (1800) harvest affects composition and voluntary intake of gamagrass (GG) or switchgrass (SG) stored as baleage. Iuka GG (Tripsacum dactyloides L.) and Alamo SG (Panicum virgatum L.) were direct-cut and stored as baleage in round bales wrapped in plastic. Beef steers (255 ± 7 kg BW) were assigned (5 steers per treatment) to GG/AM harvest, GG/PM harvest, SG/AM harvest, or SG/PM harvest. Ad libitum intake was measured for 21 d (7 d adjustment and 14-d intake estimate) followed by 7-d adjustment and 5-d balance trial in indoor digestion crates. Chewing behavior was recorded during the balance trial. Silage concentrations are g/kg DM. Compared to AM harvest, PM harvest had more (P < 0.01) starch (9.3 vs. 4.7), total nonstructural nonstructural carbohydrate (30.4 vs. 19.0), and monosaccharides (17.1 vs. 11.2). Compared to AM harvest, PM harvest had less (P = 0.05) acetate (13.0 vs. 18.6) and propionate (0.29 vs. 0.82), and tended (P < 0.13) to have less lactate (2.9 vs. 3.5) and butyrate (3.9 vs. 5.1). Compared to SG, GG had more (P = 0.01) starch (9.4 vs. 4.7), total nonstructural carbohydrate (34.2 vs. 15.2), and monosaccharides (20.8 vs. 7.4). Compared to GG, SG had higher (P = 0.01) pH (5.79 vs. 5.32) and more (P < 0.01) ethanol (27.3 vs. 18.7), acetate (19.2 vs. 12.3), propionate (1.11 vs. 0.00), and butyrate (8.4 vs. 0.6), but less (P = 0.01) DM (242 vs. 324 g/kg silage), and DM concentrations of CP (97 vs. 114) and lactate (1.6 vs. 4.8). Daily DMI (2.16 vs. 1.83 % BW) and digestible DMI (1.15 vs. 0.95 % BW) were greater (P = 0.03) for PM vs. AM harvest. Serum urea N concentrations at the end of the ad libitum intake phase were greater (P = 0.01) for AM (3.91 mM) than for PM harvest (2.31 mM) and greater (P = 0.07) for GG (3.51 mM) than for SG (2.71 mM). Steers fed PM harvest spent more time eating (P = 0.04) and less time resting (P = 0.01) during meals than steers fed AM harvest . Apparent digestibility of DM or fiber components were not affected ( 0.73 < P < 0.18) by treatment. Apparent digestibility and retention of N decreased from PM to AM harvest of SG, but increased for GG (interaction, P = 0.05). Retention of N, as a percentage of N intake or N digested, was greater (P < 0.01) for PM vs. AM harvest. We conclude that increased soluble carbohydrate content of the PM harvest of these grasses stored as baleage caused increased voluntary intake and improved use of dietary N by beef steers.