|Hall, Mary Beth|
Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 11, 2007
Publication Date: January 1, 2008
Citation: Hall, M., Mertens, D.R. 2008. In vitro fermentation vessel type and method alter fiber digestibility estimates. Journal of Dairy Science. 91(1):301-307. Interpretive Summary: Neutral detergent fiber (NDF) in forages and feeds is an important source of digestible nutrients for dairy cattle. Digestibility of NDF varies greatly among feeds and this can have great impact on the nutritional value of the feed to cattle: the more digestible the NDF, the more nutrients an animal will receive to meet their daily requirements. The most common way of evaluating NDF digestibility in the laboratory has been done by using rumen microbes to ferment the feeds. But, a variety of different methods are in use -- do the methods all give similar results? Most of the common methods we tested -- using Erlenmeyer flasks, serum vials, and centrifuge tubes as vessels, with continuous gassing with carbon dioxide or sealing the vessels with or without manual release of fermentation gas -- gave similar values for NDF digestibility. However, centrifuge tubes with gas release valves (which release fermentation gasses automatically) gave the lowest NDF digestibility, and underpredicted the digestibility of fiber relative to the other methods. Because NDF digestibility numbers are used for predicting nutrient value of feedstuffs to formulate diets, care in selecting a method is important. Use of tubes with gas release valves does not appear to be the best option for quantitative evaluation of NDF digestibility.
Technical Abstract: Vessel types commonly used for in vitro fermentation were compared to evaluate their effects on determinations of neutral detergent fiber (NDF) digestibility. Vessel treatments (trt) included 125 mL Erlenmeyer flasks with continuous gassing with CO2 (trt 1), 125 mL serum vials sealed with stoppers and crimp seals without (trt 2) or with (trt 3) manual gas release, and 50 mL polyethylene centrifuge tubes with continuous gassing with CO2 (trt 4), with Bunsen valves (trt 5), or with septum stoppers, continuous shaking and manual gas release (trt 6). Goering and Van Soest medium and blended ruminal inoculum from four lactating cows were used. Alfalfa hay, corn silage, ryegrass hay, and soyhulls were used as substrates. Gas was released and measured at 3, 6, 9, 11.5, 23.5, 29.5, and 47.5 h using a syringe with hypodermic needle to pierce the septa. All trt were run in each of two fermentations, with vessels harvested at 0, 6, 12, 24, 30 and 48 h and subsequently analyzed for NDF. Medium pH did not decrease below 6.4 during fermentation. Vessels did not give similar NDF disappearance (trt, P=0.024; trt x hour of fermentation, P=<0.01). Least squares means by trt of NDF digestibility for the entire fermentation run were 0.433, 0.431, 0.434, 0.408, 0.371 and 0.425, and for trt 1 through 6 respectively (standard error of the difference = 0.013). The Bunsen valve + tube trt had the greatest residual NDF and differed from all other trt (P < 0.06) except continuous gassing + tube (P=0.20). No other differences were detected among trt. The differences between Bunsen valves+tubes and trt 1, 2, 3, and 6 were noted at 24 h (P<0.06), 30 h (P<0.05) and 48 h (P<0.01). Results from continuous gassing + tube also differed from the Bunsen valve + tube at 48 h (P<0.01). The lack of difference between serum vials with or without gas release suggests that increase in gas content of a vial does not affect NDF fermentation unless it acidifies the medium below acceptable levels. Gas production per gram of substrate dry matter in the entire fermentation tended to differ between serum vials (trt 3; 198.0 mL) and tubes (trt 6; 0.157.8 mL) (P=0.08), but slopes of the gas production by time curves for trt did not differ (trt x fermentation hour, P = 0.23). In conclusion, fermentation vessel treatment did alter NDF disappearance, with polyethylene tubes + Bunsen valves giving the lowest digestion. Method of sealing or gassing vessels may have greater impact than the vessel type.