Alfalfa cut at sundown and harvested as baleage increases bacterial protein synthesis in late-lactation dairy cows

• Published in: Journal of dairy science 2009-03, Vol.92 (3), p.1092-1107
• Main Authors: Brito, A.F, Tremblay, G.F, Lapierre, H, Bertrand, A, Castonguay, Y, Bélanger, G, Michaud, R, Benchaar, C, Ouellet, D.R, Berthiaume, R
• Format: Article
• Language: English
• Subjects: alfalfa; Amino Acids - metabolism; Animal productions; Animals; Bacteria - growth & development; Bacteria - metabolism; bacterial proteins; baleage; Biological and medical sciences; carbohydrate content; Carbohydrates - analysis; Cattle - metabolism; Cattle - microbiology; Cattle - physiology; Cross-Over Studies; dairy cows; Dairying; Digestion; Eating - physiology; Female; Food industries; forage evaluation; Fundamental and applied biological sciences. Psychology; harvesting; Lactation - physiology; late lactation; Medicago sativa - chemistry; Medicago sativa - physiology; Milk - chemistry; Milk - secretion; Milk and cheese industries. Ice creams; nutrient use efficiency; omasum; Omasum - metabolism; organic matter; photoperiod; protein synthesis; Rumen - microbiology; ruminant nutrition; Terrestrial animal productions; Time Factors; total nonstructural carbohydrates; Vertebrates
• ISSN: 0022-0302; 1525-3198
• DOI: 10.3168/jds.2008-1469

Alfalfa (Medicago sativa L.) cut at sundown (p.m.) has been shown to have a greater concentration of total nonstructural carbohydrates (TNC) than when cut at sunup (a.m.). Eight ruminally cannulated Holstein cows that were part of a larger lactation trial were used in a crossover design (24-d periods) to investigate the effects of alfalfa cutting time on digestibility and omasal flow of nutrients. Alfalfa was cut at sundown or sunup, field-wilted, and harvested as baleage (530 ± 15.0 g of dry matter/kg of fresh matter). The difference in TNC concentration between p.m. and a.m. alfalfa within each pair of bales fed daily during the 10 d of data and sample collection varied from -10 to 50 g/ kg of dry matter. Each pair of bales was fed for ad libitum intake to cows once daily with no concentrate. During the 3 d of omasal sampling, intake (+0.8 kg/d) and omasal flow of organic matter (OM; +0.42 kg/d) tended to be greater when cows were fed p.m. vs. a.m. alfalfa, but no differences were found for ruminal and postruminal digestion of this nutrient. Similarly, N apparently digested ruminally and postruminally did not differ when feeding p.m. vs. a.m. alfalfa. However, N truly digested in the rumen, as a proportion of N intake, was significantly greater in cows fed p.m. (79%) vs. a.m. alfalfa (74%), thus suggesting that longer wilting time of alfalfa cut at sundown increased forage proteolysis. Supply of rumen-degradable protein did not change (2,716 g/d) when averaged across treatments, whereas omasal flow of non-NH₃ nonbacterial N was significantly decreased (-29 g/d) when feeding p.m. vs. a.m. alfalfa. Omasal flow of total bacterial non-NH₃-N (NAN) increased (+21 g/d) significantly when cows were fed p.m. vs. a.m. alfalfa possibly because bacteria from cows fed p.m. alfalfa captured significantly more NH₃ than those from cows fed a.m. alfalfa. Therefore, greater availability of fermentable energy as TNC appears to increase the capacity of microbes to uptake NH₃-N and convert it to microbial protein. Enhanced OM intake can also explain the observed increase in bacterial protein synthesis with p.m. alfalfa. Efficiency of bacterial protein synthesis, expressed on a fermented OM basis or as grams of bacterial NAN per gram of rumen-degradable N, did not differ between p.m. and a.m. alfalfa. Conversely, bacterial efficiency, as grams of bacterial NAN per gram of N intake, was significantly increased when cows were fed p.m. baleage. No significant difference between