Dietary starch source and protein degradability in diets containing sucrose: effects on ruminal measures and proposed mechanism for degradable protein effects.

A feeding study was conducted to evaluate ruminal effects of starch source (STA) and rumen-degradable 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 × 2 factorial arrangement of treatments. Treatments were STA (dry ground corn or 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 neutral detergent fiber concentrations. Dry matter (DM) intake was 1 kg greater with +RDP compared with -RDP diets. For ruminal digesta measures made 2 h postfeeding, weight of digesta DM was unaffected by treatment; total kilograms of wet digesta and kilograms of liquid tended to be greater with +RDP than with -RDP, and no effect was observed of STA × RDP. Digesta DM percentage was greater with -RDP than with +RDP. At 2 h postfeeding, ruminal pool sizes (mol) of lactate and total AA 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 × RDP were detected. Rumen-degradable protein effects on lactate and OA pool sizes may be due to a protein-mediated increase in fermentation rate of carbohydrate. Organic acid concentrations at 2 h postfeeding did not show the same response pattern or significance as the pool size data; high-moisture corn tended to be greater than dry ground corn and no effect was observed for RDP or STA × RDP. Concentration and pool size for OA were more weakly correlated [coefficient of determination (R(2)) = 0.66] than was the case for other ruminal analytes (R(2) >0.80). Organic acid pool size and kilograms of digesta liquid were strongly correlated (R(2) = 0.79), whereas concentration and kilograms of liquid were much less so (R(2) = 0.21). The correlation of OA moles with kilograms of liquid 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 kilograms of ruminal liquid differing across individual measurements, the ruminal OA concentration data are not on the equivalent basis required to be reliably useful for assessing the effect of treatments. Further evaluation of protein effects on carbohydrate fermentation and of methods that allow accurate comparison of treatments for their effect on ruminal OA production are warranted.