The main objective of this study was to develop practical models to assess and predict the adequacy of dietary fiber in high-yielding dairy cows. We used quantitative methods to analyze relevant research data and critically evaluate and determine the responses of ruminal pH and production performance to different variables including physical, chemical, and starch-degrading characteristics of the diet. Further, extensive data were used to model the magnitude of ruminal pH fluctuations and determine the threshold for the development of subacute ruminal acidosis (SARA). Results of this study showed that to minimize the risk of SARA, the following events should be avoided: 1) a daily mean ruminal pH lower than 6.16, and 2) a time period in which ruminal pH is <5.8 for more than 5.24 h/d. As the content of physically effective neutral detergent fiber (peNDF) or the ratio between peNDF and rumen-degradable starch from grains in the diet increased up to 31.2 +/- 1.6% [dry matter (DM) basis] or 1.45 +/- 0.22, respectively, so did the daily mean ruminal pH, for which a asymptotic plateau was reached at a pH of 6.20 to 6.27. This study also showed that digestibility of fiber in the total tract depends on ruminal pH and outflow rate of digesta from reticulorumen; thereby both variables explained 62% of the variation of fiber digestibility. Feeding diets with peNDF content up to 31.9 +/- 1.97% (DM basis) slightly decreased DM intake and actual milk yield; however, 3.5% fat-corrected milk and milk fat yield were increased, resulting in greater milk energy efficiency. In conclusion, a level of about 30 to 33% peNDF in the diet may be considered generally optimal for minimizing the risk of SARA without impairing important production responses in high-yielding dairy cows. In terms of improvement of the accuracy to assessing dietary fiber adequacy, it is suggested that the content of peNDF required to stabilize ruminal pH and maintain milk fat content without compromising milk energy efficiency can be arranged based on grain or starch sources included in the diet, on feed intake level, and on days in milk of the cows.
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.
The objective of this study was to evaluate effects of feeding increasing proportions of barley grain on acute phase response in lactating dairy cows. Eight cannulated primiparous (60 to 140 d in milk) Holstein dairy cows were assigned to 4 diets in a 4 x 4 Latin square experimental design. The experimental period lasted for 21 d, with 11 d of adaptation and 10 d of measurements. Cows were fed the following diets: 1) no barley grain in the diet, 2) 15% barley grain, 3) 30% barley grain, and 4) 45% barley grain, as well as barley and alfalfa silage and alfalfa hay at 85, 70, 55, and 40% [dry matter (DM) basis]. All cows were supplemented with a 15% concentrate mix. Blood and rumen fluid samples were collected on d 1, 3, 5, 7, and 10 of the measurement period, and pH and endotoxin content were measured in rumen samples. Concentrations of serum amyloid A, lipopolysaccharide-binding protein, haptoglobin, and C-reactive protein in plasma were measured by ELISA. Feeding high proportions of barley grain at 0, 15, 30, and 45% of DM was associated with lower feed intake (32.6, 32.9, 27.34, and 25.18 kg/d +/- 1.30, respectively), lower ruminal pH (6.8, 6.7, 6.7, and 6.5 +/- 0.03, respectively), and higher DM intake (13.33, 15.28, 14.68, and 16.04 +/- 0.63 kg/d, respectively) and milk production (27.2, 28.2, 29.0, and 31.0 +/- 1.2 kg/d, respectively). Ruminal endotoxin increased in cows receiving 30 and 45% barley grain (5,021, and 8,870 +/- 393 ng/mL, respectively) compared with those fed no grain or 15% barley grain (654 and 790 +/- 393 ng/mL, respectively). Plasma concentrations of serum amyloid A, lipopolysaccharide-binding protein, and C-reactive protein increased in cows given higher (30 and 45%) proportions of grain. Plasma haptoglobin was not affected by treatments. In conclusion, feeding dairy cows high proportions (30 and 45% DM basis) of barley grain was associated with lower feed intake and rumen pH, increased endotoxin in the rumen fluid, and stimulation of an inflammatory response.
The objective of this study was to evaluate the effects of feeding essential oils from garlic (GAR) and juniper berry (JUN), or monensin (MO) on feed intake, ruminal fermentation, the site and extent of digestion, microbial protein synthesis, milk production, and immune status in dairy cows. Four midlactating Holstein cows fitted with ruminal and duodenal cannulas were used in a 4 x 4 Latin square design with 21-d periods and 4 treatments: control (no additive), MO (330 mg/cow per d), GAR (5 g/cow per d), and JUN (2 g/cow per d). Cows were fed ad libitum a TMR consisting of 40% forage and 60% barley-based concentrate. Dry matter intake averaged 20.4 kg/d and was not affected by dietary additives. Total tract digestibilities of dry matter, organic matter, fiber, and starch were not affected by experimental treatments. However, ruminal digestibilities of dry matter and organic matter were higher (+13%) for GAR and JUN than for the control diet, mainly because of increased crude protein digestion in the rumen. Feeding GAR and JUN increased ruminal digestion of dietary protein by 11% as compared with the control. In contrast, ruminal digestion of dietary protein was reduced by 11% with MO as compared with the control. Milk fat content was lower for MO (2.68%) than for the GAR (3.46%), JUN (3.40%), and control (3.14%) diets. No effects of GAR, JUN, or MO were observed on milk production, ruminal microbial protein synthesis, ruminal pH, and ruminal concentrations of volatile fatty acids and ammonia N. The total and differential numbers of white blood cells as well as serum amyloid A and haptoglobin were not affected by the treatments, suggesting that additives had no effect on the immune status of cows. Results of this study indicate that supplementing dairy cows with GAR (5 g/d) and JUN (2 g/d) essential oils improved feed digestibility in the rumen, but possibly at the expense of a reduction in the flow of bypass protein to the small intestine. Feeding monensin could be beneficial in terms of increasing bypass protein from the rumen but did not improve feed digestion or milk production under the current experimental conditions.
This study presents the first application of metabolomics to evaluate changes in rumen metabolites of dairy cows fed increasing proportions of barley grain (i.e., 0, 15, 30, and 45% of diet dry matter). 1 H-NMR spectroscopy was used to analyze rumen fluid samples representing 4 different diets. Results showed that for cows fed 30 and 45% grain, increases were observed in the concentration of rumen methylamine as well as glucose, alanine, maltose, propionate, uracil, valerate, xanthine, ethanol, and phenylacetate. These studies also revealed lower rumen 3-phenylpropionate in cows fed greater amounts of cereal grain. Furthermore, ANOVA tests showed noteworthy increases in rumen concentrations of N-nitrosodimethylamine, dimethylamine, lysine, leucine, phenylacetylglycine, nicotinate, glycerol, fumarate, butyrate, and valine with an enriched grain diet. Using principal component analysis it was also found that each of the 4 diets could be distinguished on the basis of the measured rumen metabolites. The two closest clusters corresponded to the 0 and 15% grain diets, whereas the 45% barley grain diet was significantly separated from the other clusters. Unhealthly levels of a number of potentially toxic metabolites were found in the rumen of cattle fed 30 and 45% grain diets. These results may have a number of implications regarding the influence of grain on the overall health of dairy cows.
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