Rumen morphometrics and the effect of digesta pH and volume on volatile fatty acid absorption1

Abstract

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.

Morphological Response of the Ruminal and Omasal Mucosae to the Variation in Diet Energy

“…As reported by Resende Júnior, et al (2006b), the inaccuracies in the macroscopic measurement technique may decrease accurate detection of behavior differences in the extension of the rumen's absorptive surface. However, other authors have reported accurate measurements of the extent of the absorptive surface using the same technique described in the current experiment (Melo et al, 2013).…”

“…This finding is in contrast to reports of animals in early lactation, likely because the animals in this study had a lower metabolic demand, as they were neither pregnant nor lactating. While the ruminal epithelium is known to respond positively to introduction of high-energy diets (Dirksen et al, 1984;Melo et al, 2013), the response of the omasal epithelium to high-energy diets has not been evaluated.…”

“…Eighteen days after introduction of the highenergy diet, the lower concentration of VFA in rumen fluid likely indicated that the rumen had adapted to the new conditions (Melo et al, 2013;Resende Júnior et al, 2006b), probably due to higher fractional absorption rate. The absence of a fermentable substrate during the fasting period explains decreased fluid VFA concentrations during this time.…”

Omasal Morphology of Dairy Cows Fed With High or Low Grain Content Diet Prior Parturition

“…A possible explanation for the effect of the transitional diet on post-partum DMIBW could be the adaptation of the rumen epithelium (Melo et al, 2013;Teófilo et al, 2011;Dirksen et al, 1984). This adaptation could have induced higher synchrony between the production and removal of VFAs in the reticulorumen, which enables greater equilibrium in the rumen environment, with a pH that is more stable and closer to normality.…”

“…Although studies that measured the height and area of the omasum papillae were not found in the literature, it is widely known that the area and height of the rumen papillae respond positively to a high calorie diet, inducing a larger absorptive surface for VFAs (Melo et al, 2013;Teófilo et al, 2011;Dirksen et al, 1984;Beharka, 1998;Shen et al, 2004). One week before parturition and two weeks postpartum, the height of the omasum papillae tended to be higher when the cows were fed the HGC diet (Figure 5), in which less abrasiveness was expected.…”