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Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 7/24/2013 Publication Date: 11/1/2013 Citation: Hall, M. 2013. Dietary starch source and protein degradability in diets containing sucrose: effects on ruminal measures and proposed mechanism for degradable protein effects. Journal of Dairy Science. 96:7093-7109. Interpretive Summary: Microbes in the rumen digest feed for the cow and supply her with high-quality nutrients, such as organic acids which provide cows with energy. Do rumen microbes digest feed differently when rations differ in the rate of carbohydrate fermentation or in the amount of protein that microbes can degrade? In this study, the answer was ‘yes.’ In cows fed diets with more degradable protein, microbes made more organic acids and made them more quickly, particularly with a rapidly fermented starch (carbohydrate) source. The change in organic acids could be helpful by providing more energy and perhaps more microbes to the cow, but could also make the rumen more acidic, which can create problems if it goes too far. Learning more about how to manipulate microbial digestion will let us formulate diets to maintain a healthy rumen and cow and most efficiently meet nutrient requirements for the cow by optimizing ruminal and post-ruminal digestion. Technical Abstract: A feeding study was conducted to evaluate ruminal effects of starch source (STA) and ruminally degraded dietary protein (RDP) in diets with added sucrose. The experimental design was an incomplete Latin square with three 21-d periods, 8 ruminally cannulated lactating cows, and a 2 x 2 factorial arrangement of treatments. Treatments were STA (DG: dry ground corn, HM: high-moisture corn) as more slowly and more rapidly fermenting starch sources, respectively, and relative amount of RDP (+RDP: added protein from soybean meal; -RDP: heat-treated expeller soybean product partially substituted for soybean meal). Diets were formulated to be isonitrogenous and similar in starch and NDF concentrations. DMI was 1 kg greater on +RDP as compared to -RDP diets. For ruminal digesta data collected 2 h post-feeding, digesta DM kg was unaffected by treatment; total wet digesta kg and liquid kg tended to be greater with +RDP than with -RDP, and there was no effect of STA x RDP. Digesta DM% was greater with -RDP than with +RDP. At 2 h post-feeding, ruminal pool sizes (moles) of lactate and total amino acids were larger and those of total organic acids (OA) and ammonia tended to be larger with +RDP than with -RDP; no effects of STA or STA x RDP were detected. RDP effects on lactate and OA pool sizes may be due to a protein-mediated increase in fermentation rate of carbohydrate. OA concentrations at 2 h post-feeding did not show the same response pattern or significance as the pool size data. HM tended to be greater than DG, and there was no effect of RDP or of STA x RDP. Concentration and pool size for OA were more weakly correlated (R2=0.66) than was the case for other ruminal analyses (R2>0.80). OA pool size and digesta liquid kg were strongly correlated (R2=0.79), whereas concentration and liquid kg were much less so (R2=0.21). The correlation of OA moles with liquid kg likely relates to the homeostatic mechanism of water flux across the rumen wall to reduce the osmotic gradient with blood as intraruminal moles of solute change. This action compresses the range of ruminal OA concentrations. With ruminal liquid kg differing across individual measurements, the ruminal OA concentration data are not on an equivalent basis required to be reliably useful for assessing impact of treatments. Further evaluation of protein effects on carbohydrate fermentation and of methods that allow accurate comparison of treatments for their impact on ruminal OA production are warranted. |
