Effects of dietary cation-anion difference (DCAD) and Na:K on dairy cows in early lactation, and the interaction of particle size reduction via mastication and rumination with digestion and passage in cattle

Cattle spend many hours per day chewing, either eating or ruminating. Comminution of feed and digesta particles affects the kinetics of digestion and passage, and can also affect voluntary feed intake. These, in turn, determine nutrient availability and productive efficiency. Our objective was to incorporate relevant data into a framework leading towards a dynamic mathematical model for comminution from feed through feces in cattle. Although large particles (i.e., those retained on a screen with 1.18-mm pores) often comprise 80 to 90% of swallowed forage dry matter, they account for about 35% of fecal dry matter. Large particles can be a minority of those in the reticulorumen at any given time; therefore, size is not the only criterion determining passage to the lower gut. Current data support the conclusion that synergism exists between animal and microbial effects; i.e., mastication during eating enhances microbial fermentation, which increases the effectiveness of comminution during rumination. Significant amounts of variation in the particle size distributions of boluses entering the reticulorumen can now be explained from knowledge of feed characteristics. Our understanding of mastication and rumination effects on digestion and passage in cattle is limited because no information is available for mixed diets and few data exist for many common types of forage (none for silages or which address the effects of plant maturity). Data amenable to studying the dynamics of particle size distributions are few and relate to near steady-state conditions; therefore, synergies between mastication, digestion, and rumination under practical conditions remain to be examined. Six multiparous Holstein cows, fitted with rumen cannulas, averaging 122 ± 31 days in milk were randomly assigned to six treatments allocated in an equiradial (pentagonal) second-order response surface design with a center point to examine the effects of dietary cation-anion difference (DCAD) and Na:K on lactating dairy cows. Replication of treatments within a 6 x 6 Latin square minimized the potential effects of outliers and allowed a surface covering a 3 × 3 matrix of DCAD and Na:K combinations to be examined. Ranges in DCAD and Na:K were chosen to be equally spaced on logarithmic scales; tripling each time from 0.25 for the former, and 1.5-fold each time from 25 meq/100 g of DM for the latter. The response surface was centered on a molar Na:K of 0.75 (0.60% Na and 1.37% K in DM) and a DCAD of 37.5 meq/100 g of DM. The other 5 treatments were: 1.63, 50.0 (Na:K, DCAD); 0.46, 53.8; 0.25, 35.2; 0.63, 25.1; and 2.00, 31.2. Percentages of Na and K in DM of the TMR for vertices of the pentagon were calculated as 1.05, 1.10; 0.56, 2.08; 0.27, 1.84; 0.44, 1.17; and 0.84, 0.72. Diets were based on corn silage and a corn-based grain mix. The Na:K ratios were varied with NaHCO3 and K2CO3. Periods were 14 d. Daily feed intake of each cow was recorded during each period; samples of feed and orts were collected daily. Milk production was measured daily; samples were collected weekly and analyzed for components. Rumen and urine samples were collected and analyzed for pH on the last 3 d of each period. The MIXED procedure of SAS was used for ANOVA. There were no response surface effects of treatment on milk production and components, or DMI (P<0.05). Acetate, propionate, and butyrate concentrations in the rumen were all affected by treatment (P <0.05). There were multiple significant effects on acetate, including an interaction of DCAD and ratio. There were both linear and quadratic effects of ratio on propionate and butyrate. Linear (P <0.05) and quadratic effects (P <0.05) of DCAD on rumen pH were also indicated. A quadratic effect of Na:K (P < 0.01) and interaction of DCAD (P < 0.003) indicated that urine pH was maximal (8.24 or above) at high DCAD and low Na:K. Milk production and components were similar across treatments, but rumen fermentation was affected. Rumen and urine samples were collected and analyzed for pH on the last 3 d of each period. There was a relationship between pH6 -h and ruminal pH (r2= 0.64, P < 0.001). The relationship between mean ruminal pH and mean urinary pH explained 15% of the variation (P < 0.022), but few data were below pH 6. The relationship between mean urinary pH and mean ruminal pH6-h explained 28% of the variation (P < 0.001). Few published data compare ruminal and urinary pH. A relationship between ruminal and urinary pH was measured. More data are necessary to further elucidate this relationship before making determinations of the presence of SARA