Research note: a method for studying local differences in ruminal fermentation in dairy cattle

Technical Note: The comparison of pH and redox potential in different locations in the reticulo-rumen of growing beef steers supplemented with different levels of quebracho extract

Rumen acidosis is a common metabolic disorder occurring when organic acid production exceeds clearance capacity, reducing ruminal pH. The occurrence of acidosis has been directly correlated to the ratio of concentrate to forage in the diet. However, rates of substrate fermentation and acid absorption vary at different locations in the reticulo-rumen. The objective of this study was to determine the pH and redox potential (Eh) in different locations of the reticulo-rumen using 16 ruminally cannulated steers (309 ± 43 kg) receiving different supplementation levels of quebracho extract (QT; Schinopsis balansae) within a grower type diet (CP: 13.4%; total digestible nutrients [TDN]: 70.4%; and ME: 2.55 Mcal/kg, dry matter [DM] basis). Animals were randomly assigned to one of four dietary treatments: QT at 0%, 1%, 2%, and 3% of DM (QT0, QT1, QT2, and QT3, respectively), containing about 0%, 0.7%, 1.4%, and 2.1% of condensed tannins (CT), DM basis, respectively. Animals were adapted to the basal diet for 12 d before being introduced to predetermined treatments for 4 weeks (wk), with diets provided twice daily to allow ad libitum intake. Weekly measurements of ruminal fluid pH and Eh were taken 4 h post-feeding using a portable pH meter with two probes (pH and redox) in four locations of the reticulo-rumen (reticulum, cranial sac, dorsal sac, and ventral sac). Data were analyzed using a random coefficients model with the pen as a random effect and wk as repeated measures, with DM intake included as a covariate. There was no interaction among treatments, location, and wk (P ≥ 0.882) on reticulo-ruminal pH. Overall, ruminal pH was lower for QT0 and QT1 compared to QT3 (P < 0.001). The pH in the reticulum was greater than those of the ventral and dorsal sacs (6.05 vs. 5.94, 5.89, respectively; P ≤ 0.001) but similar to cranial sac (6.00). Reticular pH was positively correlated with the ruminal locations (≥0.78; P < 0.001). The linear equation to estimate ruminal mean pH using reticulum pH had an intercept and slope different from zero (P ≤ 0.04), but CT (% DM) was not different from zero (P = 0.15), root mean square error of 0.15, and R2 of 0.778: 0.723 (±0.36) + 0.857 (±0.059) × reticulum pH + 0.033 (±0.023) × CT. The Eh was lower for QT0 in week 1 than all other treatments (P < 0.001). We concluded that reticulo-ruminal pH differs among locations in the rumen regardless of QT supplementation level and days on feed, with reticular pH being the highest.

Invited review: Role of physically effective fiber and estimation of dietary fiber adequacy in high-producing dairy cattle

Highly fermentable diets require the inclusion of adequate amounts of fiber to reduce the risk of subacute rumen acidosis (SARA). To assess the adequacy of dietary fiber in dairy cattle, the concept of physically effective neutral detergent fiber (peNDF) has received increasing attention because it amalgamates information on both chemical fiber content and particle size (PS) of the feedstuffs. The nutritional effects of dietary PS and peNDF are complex and involve feed intake behavior (absolute intake and sorting behavior), ruminal mat formation, rumination and salivation, and ruminal motility. Other effects include fermentation characteristics, digesta passage, and nutrient intake and absorption. Moreover, peNDF requirements depend on the fermentability of the starch source (i.e., starch type and endosperm structure). To date, the incomplete understanding of these complex interactions has prevented the establishment of peNDF as a routine method to determine dietary fiber adequacy so far. Therefore, this review is intended to analyze the quantitative effects of and interactions among forage PS, peNDF, and diet fermentability with regard to rumen metabolism and prevention of SARA, and aims to give an overview of the latest achievements in the estimation of dietary fiber adequacy in high-producing dairy cattle. Recently developed models that synthesize the effects of both peNDF and fermentable starch on rumen metabolism appear to provide an appropriate basis for estimation of dietary fiber adequacy in high-producing dairy cows. Data suggest that a period lasting more than 5 to 6h/d during which ruminal pH is <5.8 should be avoided to minimize health disturbances due to SARA. The knowledge generated from these modeling approaches recommends that average amounts of 31.2% peNDF inclusive particles >1.18mm (i.e., peNDF(>1.18)) or 18.5% peNDF inclusive particles >8mm (i.e., peNDF(>8)) in the diet (DM basis) are required. However, inclusion of a concentration of peNDF(>8) in the diet beyond 14.9% of diet DM may lower DM intake level. As such, more research is warranted to develop efficient feeding strategies that encourage inclusion of energy-dense diets without the need to increase their content in peNDF above the threshold that leads to lower DM intake. The latter would require strategies that modulate the fermentability characteristics of the diet and promote absorption and metabolic capacity of ruminal epithelia of dairy cows.

Digesta characteristics of dorsal, middle and ventral rumen of cows fed with different hay qualities and concentrate levels

The influence of fibre content of hay (H) and concentrate level (C) on local differences in the composition of ruminal digesta (ratio of solid to fluid digesta, DM, NDF, ADF and ADL content), particle size (MPL), specific gravity (SG) and fermentation (pH and concentrations of SCFA and bicarbonate) have been tested on two ruminally cannulated Friesian cows (520 kg BW) which were fed restricted, using individual cows as experimental units. Digesta samples were collected via cannula from three rumen layers: 5 to 10 cm (top) and 25-35 cm beneath the top of the particle mat (middle) and 5-10 cm above the rumen floor (bottom). For a main plot treatment (H x C), repeated samples were collected at four time intervals (1 h before and 2, 5 and 10 h after morning feeding) on each of two days. From top to bottom rumen the share of solid digesta mass (SM), DM and NDF contents of squeezed digesta fluid (SRF) and concentration of SCFA decreased (P < 0.05); pH and bicarbonate concentration increased (P < 0.05), while DM, NDF, ADF and ADL contents in SM, MPL and SG did not differ. Higher NDF content of hay (from 47-62%) increased SM, fibre fractions in SM, MPL, pH and concentration of bicarbonate in ruminal digesta, especially when 50% concentrate was given, while SG decreased. When the concentrate level was enhanced from 20 to 50%, digesta SM, MPL and the content of DM and NDF in SRF increased, while pH, concentrations of SCFA and acetate decreased when low-fibre hay was given. With longer time after feeding the digesta SM was reduced and fibre content in SM increased. The increase of the fibre content of hay reduced the possible negative effect of high concentrate level on the stratification of ruminal digesta. The decrease of the fibre content of hay promised better conditions for fibre digestion in the rumen when concentrate availability is limited.