Two experiments were conducted 1) to validate a field protocol for the determination of ruminal pH and 2) to develop a strategy to interpret ruminal pH data from groups of cows. In the first experiment, ruminal fluid was collected from 30 lactating dairy cows. Ruminal fluid pH was 0.28 pH units lower for fluid collected by rumenocentesis than for fluid collected through a ruminal cannula. Concentrations of volatile fatty acids were correspondingly higher in samples collected by rumenocentesis. A portable pH meter capable of measuring pH of a very small volume of ruminal fluid yielded very similar pH readings as did a standard meter with a pH probe. Filtration or aspiration of ruminal fluid had no effect on pH. In the second experiment, a strategy was developed to use ruminal pH values from a subsample of cows to distinguish between groups fed either a low or higher forage diet. Groups could be distinguished using a cut point of 5.5 ruminal pH, a sample size of 12 cows, and a critical value of 3 or more cows below the cut point. This strategy had the lowest theoretical error rate for herds with either a high or low prevalence of cows with a low ruminal pH.

The objective of this experiment was to determine the relationship between corn kernel vitreousness and ruminal in situ starch degradation. Fourteen U.S. and five Brazilian corn hybrids cultivated in their respective countries were evaluated. The U.S. dent hybrids were harvested at one-half milk line, black layer, and 21 d after black layer stages of maturity. Brazilian flint hybrids were harvested only at the latest stage of maturity. Vitreousness was determined by manual dissection of the kernels. Ruminal in situ degradation of starch was determined in three lactating Holstein cows fitted with ruminal cannulae. Vitreousness of the five mature Brazilian hybrids averaged 73.1% (range of 64.2% to 80.0%), while vitreousness of the 14 mature U.S. hybrids averaged 48.2% (range of 34.9% to 62.3%). Within the 14 U.S. hybrids, average vitreousness increased from 42.8% to 48.2% as stage of maturity progressed from one-half milk line to 21 d after black layer. The correlation between kernel density and vitreousness was 0.87. The correlations between kernel vitreousness or density and ruminal starch availability were -0.93 and -0.87, respectively. With advancing maturity, kernel vitreousness and density increased while ruminal starch availability decreased. Kernel vitreousness and density may be useful parameters for which to select corn hybrids for high ruminal starch availability. Density may be a more practical measurement than vitreousness for screening large corn data sets.

Chemical analysis of neutral detergent fiber (NDF) provides a useful description of forages and other feeds. However, use of NDF as the sole measure of the fiber contribution of a feed has proved problematic for two classes of feeds: forages processed into differing physical forms and high fiber by-products. By-products and physically fine forages contribute to the fiber value of dairy rations, but contribute less than long forages do. Therefore, some discount factors must be assigned to these feeds if fiber requirements are to be used in balancing the carbohydrate portion of dairy diets. The effectiveness factors applied to NDF from these feeds provide an improved measure of fiber value. The assumptions and trial designs used to measure the effectiveness of NDF based on any single animal response variable are discussed, and improved approaches are suggested. The use of different response variables to measure physical and overall fiber effectiveness is discussed. Measured effectiveness of a high fiber feed differs when estimated by chewing, by the ratio of acetate to propionate, or by milk fat concentration. In all cases, inclusion of negative control treatments is necessary to measure the effect of removing fiber without introducing a substitute fiber source.

The effects of rumen digesta volume and pH on VFA absorption and its relation to rumen wall morphology were evaluated. Nine rumen cannulated cows formed 3 groups based on desired variation in rumen morphology: The High group was formed by Holsteins yielding 25.9 kg milk/d and fed on a high-grain total mixed ration (TMR); the Medium group by Holstein-Zebu crossbreds yielding 12.3 kg milk/d and fed on corn silage, tropical pasture, and a commercial concentrate; and the Dry group by nonlactating grazing Jerseys fed exclusively on tropical pasture. Within each group, a sequence of 3 ruminal conditions was induced on each cow in 3 × 3 Latin Squares, with 7-d periods: high digesta volume and high pH (HVHP), low volume and high pH (LVHP), and low volume and low pH (LVLP). Rumen mucosa was biopsied on the first day of Period 1. Ruminal morphometric variables evaluated were mitotic index, absorptive surface and papillae number per square centimeter of wall, area per papillae, papillae area as a percentage of absorptive surface, and epithelium, keratinized layer, and nonkeratinized layer thickness. There was marked variation in rumen morphology among the groups of cows. Grazing Jerseys had decreased rumen wall absorptive surface area and basal cells mitotic index, and increased thickness of the epithelium and of the keratin layer compared with cows receiving concentrates. Mean rumen pH throughout the 4 h sampling period was: 6.78 for HVHP, 7.08 for LVHP, and 5.90 for LVLP (P < 0.01). The capacity of the rumen wall to absorb VFA was estimated by the Valerate/CrEDTA technique. The fractional exponential decay rate for the ratio of valeric acid to Cr (k Val/Cr) was determined by rumen digesta sampling at 20-min intervals during 4 h, after the mixing of markers and the return of the evacuated ruminal content. The k Val/Cr values for treatments HVHP, LVHP, and LVLP were, respectively: 19.6, 23.9, and 35.0 %/h (SEM = 2.01; P = 0.21 for contrast HVHP vs. LVHP and P < 0.01 for contrast LVHP vs. LVLP). The k Val/Cr was faster under low pH, but decreasing digesta volume under high pH did not elicit such a response. The correlation between the absorptive surface area per square centimeter of rumen wall and the mean of the 3 k Val/Cr values of each cow was 0.90 (P < 0.01). Cows capable of maintaining a less-acidic rumen environment had greater inflow of water into the digestive cavity, had a more developed rumen mucosa, and were more efficient VFA absorbers. Copyright © 2013. American Society of Animal Science .

Dietary yeast supplementation may improve the digestive efficiency of ruminants, but responses depend on the yeast strain and the diet composition. Corn silage and citrus pulp are usual carbohydrate sources for dairy cows in southeast Brazil. This study evaluated the supplementation of dairy cows fedding on corn silage-citrus pulp-based diets with Saccharomyces cerevisiae CNCM I-1077 (Lallemand SAS, Toulouse, France). Twenty multiparous, midlactation Holstein cows were assigned to two treatments in crossover design. Treatments were: live yeast on oyster meal capable of supplying a daily minimum of 1 × 10(10) CFU per cow or oyster meal top-dressed at 10 g to the morning meal. Diet contained (% of dry matter): 16.8% crude protein, 30.9% neutral detergent fiber, 43.9% corn silage, 2% tifton hay, 14.4% steam flaked corn, 16.9% citrus pulp and 21.7% soybean meal. Yeast supplementation increased daily yields of milk (29.4 vs. 28.5 kg, p = 0.11), protein (0.939 vs. 0.908 kg, p = 0.05), and lactose (1.294 vs. 1.241 kg, p = 0.06), but did not affect milk fat contents (p = 0.59). Daily dry matter intake was 21.4 with yeast and 20.7 kg for the control (p = 0.11). Total tract apparent digestibility of the neutral detergent fiber was 48.1% with yeast and 43.2% for the control (p = 0.08). There was a trend for increased intake of digestible organic matter with yeast supplementation (p = 0.07). The positive milk protein yield response to yeast supplementation may have resulted from the increased fiber digestibility, but the response mechanism could not be elucidated.