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Title: The interaction of harvesting time of day of switchgrass hay and ruminal degradability of supplemental protein to beef steers

item Burns, Joseph

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/17/2007
Publication Date: 1/15/2008
Citation: Huntington, G.B., Burns, J.C. 2008. The interaction of harvesting time of day of switchgrass hay and ruminal degradability of supplemental protein to beef steers. Journal of Animal Science. 86:159-166.

Interpretive Summary: Forage cut in the PM has greater concentrations of total nonstructural carbohydrates (TNC) than forage cut in the AM. This difference has been reported for cool season grasses with animals selecting the PM forage when given a choice between AM and PM cut forage. This difference was recently reported for switchgrass, a warm season grass, but the magnitude of change was rather small ie., 0.7 to 1.2 percentage units. This study examines the long-time effect of increased TNC in switchgrass on animal response when fed two levels of a nitrogen supplement differing in degradability. Forage from the PM cut (high TNC) showed greater fiber digestion and greater response from high vs. low ruminal degradable protein. Forage from the AM cut showed increased fiber digestion from the low vs. the high degradable protein. This indicates the importance of having both energy and nitrogen in the proper proportions to maximize nutrient utilization. Time of cut during the day altered forage composition sufficiently to alter both the composition and concentration of the fiber fraction. Forage cut in the PM had greater nutritive value and subsequent greater intake and fiber digestibility.

Technical Abstract: The objective of this study was to interact AM ( 0600 ) vs PM ( 1800 ) harvest with ruminal degradability of a protein supplement (HI or LO ) to change voluntary intake, apparent digestibility or N retention by steers fed Alamo switchgrass (Panicum virgatum L.) hay. Black steers (255 ± 14kg BW) were blocked by BW, then randomly assigned (5 steers each) to AM /HI, PM /HI, AM /LO, or PM /LO. Steers were group-housed in covered, outdoor pens with individual feeding gates. After adaptation to the facility and 14 d standardization, ad libitum hay intake was measured for 21d (7 d adjustment and 14 d intake estimate) followed by a digestion trial (7 d of adjustment and 5 d of total collection). Steers were fed 767 (LO) or 825 (HI) g/d supplement to provide 268 g CP/d. PM, compared to AM, had greater (P = 0.01) DM concentrations of total nonstructural carbohydrate (TNC, 71 vs 56 g/kg DM), and less concentrations of NDF (760 vs 770 g/kg DM, (P = 0.02), ADF (417 vs 427 g/kg DM, (P = 0.02), and CP (55.9 vs. 58.6 g/kg DM, (P = 0.07). Protein fractions A, B2, and B3 were similar for AM and PM, but HI contained more (P < 0.02) g/kg protein of A (694 vs 296) and less B2 fraction (174 vs 554) than LO. Harvest interacted with supplement to increase (P = 0.07) ad libitum digestible DM intake (g/kg BW) for steers fed PM/HI (11.4) than for steers fed PM/LO (10.2), but no difference for steers fed AM/LO or AM/HI (10.7). Apparent digestibility (g digested/ kg of intake) of DM (594 vs 571), NDF (591 vs 562), ADF (585 vs 566), and N (651 vs 632) were greater (P < 0.04) for PM than AM. Apparent digestibility of N was greater (P = 0.02) for HI (652) vs LO (631). Interactions between harvest and supplement for apparent digestibilities of NDF (P = 0.09) and ADF (P = 0.03) were due to no change or an increase in digestibility in response to increased ruminal degradability of supplement in steers fed PM harvest while decreased ruminal degradability of supplement increased digestibility of NDF and ADF in steers fed AM harvest. Treatments did not affect hay intake (3.93 kg/d), N retained (15.8 g/d) or serum urea-N (5.25 mM) during ad libitum intake. Increased TNC was not sufficient by itself to increase voluntary intake, but increased protein degradability interacted with PM harvest to increase ruminal fiber digestion and voluntary intake of beef steers.