Feeding diets rich in rapidly fermentable non-structural carbohydrates can lead to the development of ruminal acidosis. This study was conducted to determine if the duration of time that cattle are fed a high-grain diet affects their absorption of short-chain fatty acids (SCFA) and susceptibility to, and recovery from, ruminal acidosis. Sixteen Angus heifers (BW ± SEM, 261 ± 6.1 kg) were assigned to 1 of 4 blocks, and fed a backgrounding diet consisting of 60% barley silage, 30% barley grain, and 10% supplement (DM basis). Within block, cattle were randomly assigned to 1 of 2 treatments differing in the number of days they were fed the high-grain diet prior to an acidosis challenge: 34 d for long-adapted (LA) and 8 d for short-adapted (SA). All cattle were exposed to the same 20-d dietary transition using 5 dietary steps until achieving the final diet that contained 9% barley silage, 81% barley grain, and 10% supplement (DM basis). Data were collected during an 8-d baseline period (BASE), on the d of the acidosis challenge (CHAL), and during two consecutive 8 d recovery periods (REC1 and REC2). Ruminal acidosis was induced by restricting feed to 50% of DMI:BW for 24 h followed by an intraruminal infusion of ground barley at 10% DMI:BW. Cows were then given their regular diet allocation 1 h after the intraruminal infusion. The duration of time fed the high-grain diet did not affect ruminal pH, lactate, or SCFA concentrations (P > 0.050). However, during BASE and on the day of CHAL the SA heifers experienced greater linear (P = 0.031), quadratic (P = 0.016), and cubic (P = 0.008) between day change in the duration of time that pH was < 5.5 than LA heifers. Relative to BASE, inducing acidosis increased daily duration (531 to 1020 min/d; P < 0.001) and area (176 to 595 (min × pH)/d; P < 0.001) that pH was < 5.5. Inducing ruminal acidosis also increased the daily mean (0.3 to 11.4 mM; P = 0.013) and maximum (1.3 to 29.3 mM; P = 0.008) rumen fluid lactate concentrations relative to BASE, suggesting that an acute bout of ruminal acidosis was induced. In addition, a treatment × day interaction for the duration that pH was < 5.5 during REC1 suggests that LA cattle tended to recover from the CHAL more rapidly than SA cattle (P = 0.085). Indeed, analysis of covariance confirmed that the LA heifers experienced a quicker linear (P = 0.019) recovery over time from CHAL. The greater rate of recovery possibly resulted from the LA heifers having greater rates of both fractional butyrate (45 vs. 36 %/h; P = 0.019) and propionate absorption (42 vs. 34 %/h; P = 0.045), and tending to have greater rates, on an absolute basis, of butyrate absorption (94 vs. 79 mmol/h; P = 0.087) iii and, on a fractional basis, of total SCFA absorption (37 vs. 32 %/h; P = 0.100). Treatment × period interactions revealed that LA heifers had greater serum D-lactate concentrations (P = 0.003), and fractional rates of lactate absorption (P = 0.024) than SA heifers, during CHAL and REC1, respectively. When treatments were pooled, the absorption (%/h and mmol/h) of acetate, propionate, butyrate, and total SCFA increased between REC1 and REC2, with intermediate values for BASE (P ≤ 0.05). Corresponding to a reduction in absorption during REC1 (2 d post CHAL), saliva production (kg/h; P = 0.018) increased between BASE and REC1, with intermediate values for REC2. These results indicate that the duration of time cattle are fed a high-grain diet may stabilize rumen pH, both prior to and after an induced bout of acute ruminal acidosis, likely through increased ruminal absorptive capacity for SCFA and lactate. In addition, this study found evidence to suggest that beef cattle possess the ability to increase saliva secretion in order to compensate for decreased absorptive capacity.

A series of experiments were conducted to determine nutritional strategies to improve the efficiency of N utilization in dairy cows when feeding co-products including wheat-based (W-DDGS) and corn-wheat blend distillers grains with solubles (B-DDGS), and dried whey permeate (DWP). In Experiment 1, the objective was to determine the effects of replacing canola meal (CM) as the major protein source with W-DDGS on ruminal fermentation, microbial protein production, omasal nutrient flow, and animal performance. Cows were fed either a standard barley silage-based total mixed ration containing CM as the major protein supplement (0% W-DDGS, control) or diets formulated to contain 10, 15 and 20% W-DDGS (dry matter [DM] basis), with W-DDGS replacing primarily CM. Diets were isonitrogenous (18.9% crude protein [CP]). Inclusion of W-DDGS to the diet did not negatively affect ruminal fermentation, microbial protein production, and omasal nutrient flow. However, there was a 0.7- to 2.4-kg increase in DM intake, and a 1.2- to 1.8-kg increase in milk yield after the addition of W-DDGS in place of CM. In Experiment 2, the objective was to delineate the effects of including either W-DDGS or B-DDGS dried distillers grains with solubles as the major protein source in low or high CP diets fed to dairy cows on ruminal function, microbial protein synthesis, omasal nutrient flows, urea-N recycling, and milk production. The treatment factors were type of distillers co-product (W-DDGS vs. B-DDGS) and dietary CP content (15.2 vs. 17.3%; DM basis). The B-DDGS was produced from a mixture of 15% wheat and 85% corn grain. All diets were formulated to contain 10% W-DDGS or B-DDGS on a DM basis. Feeding up to 10% of dietary DM as B-DDGS or W-DDGS as the major source of protein did not have negative effects on metabolizable protein (MP) supply and milk production in dairy cows. However, reducing dietary CP content from 17.3 to 15.2% decreased milk production. This response was attributed to an insufficient supply of ruminally degradable protein (RDP) that suppressed microbial nonammonia N (NAN) synthesis in the rumen, thus decreasing intestinal MP supply. In Experiment 3, the objective was to determine the effects of replacing barley or corn starch with lactose (as DWP) in diets containing 10% W-DDGS on ruminal function, omasal nutrient flow, and lactation performance. The treatment factors were source of starch (barley vs. corn) and dietary inclusion level of DWP (0 vs. 6%; DM basis) as a partial replacement for starch. Diets were isonitrogenous (18% CP) and contained 3 or 8% total sugar. The starch content of the low sugar diet was 24% compared to 20% for the high sugar diet. Dry matter intake, and milk and milk component yields did not differ with diet. However, partially replacing dietary corn or barley starch with sugar up-regulated ruminal acetate and propionate absorption, and reduced ruminal NH3-N concentration, but had no effect on ruminal pH, microbial protein synthesis, omasal nutrient flow and production in dairy cows. In summary, data presented in this thesis indicate that W-DDGS and B-DDGS can be included as the major source of protein in dairy cow diets without compromising ruminal function, nutrient supply and milk production in dairy cows. Feeding medium to low CP diets, and partial replacement of starch with sugar in diets containing W-DDGS and B-DDGS can improve N utilization efficiency in dairy cows. Additionally, an upregulation of facilitated transport of acetate and propionate across epithelial cells possibly prevents the occurrence of ruminal acidosis when lactose partially replaces starch in cow diets.