Mary Beth Hall

Ruminant Nutrition Symposium: Productivity, digestion, and health responses to hindgut acidosis in ruminants.

Microbial fermentation of carbohydrates in the hindgut of dairy cattle is responsible for 5 to 10% of total-tract carbohydrate digestion. When dietary, animal, or environmental factors contribute to abnormal, excessive flow of fermentable carbohydrates from the small intestine, hindgut acidosis can occur. Hindgut acidosis is characterized by increased rates of production of short-chain fatty acids including lactic acid, decreased digesta pH, and damage to gut epithelium as evidenced by the appearance of mucin casts in feces. Hindgut acidosis is more likely to occur in high-producing animals fed diets with relatively greater proportions of grains and lesser proportions of forage. In these animals, ruminal acidosis and poor selective retention of fermentable carbohydrates by the rumen will increase carbohydrate flow to the hindgut. In more severe situations, hindgut acidosis is characterized by an inflammatory response; the resulting breach of the barrier between animal and digesta may contribute to laminitis and other disorders. In a research setting, effects of increased hindgut fermentation have been evaluated using pulse-dose or continuous abomasal infusions of varying amounts of fermentable carbohydrates. Continuous small-dose abomasal infusions of 1 kg/d of pectin or fructans into lactating cows resulted in decreased diet digestibility and decreased milk fat percentage without affecting fecal pH or VFA concentrations. The decreased diet digestibility likely resulted from increased bulk in the digestive tract or from increased digesta passage rate, reducing exposure of the digesta to intestinal enzymes and epithelial absorptive surfaces. The same mechanism is proposed to explain the decreased milk fat percentage because only milk concentrations of long-chain fatty acids were decreased. Pulse-dose abomasal fructan infusions (1 g/kg of BW) into steers resulted in watery feces, decreased fecal pH, and increased fecal VFA concentrations, without causing an inflammatory response. Daily 12-h abomasal infusions of a large dose of starch (~4 kg/d) have also induced hindgut acidosis as indicated by decreased fecal pH and watery feces. On the farm, watery or foamy feces or presence of mucin casts in feces may indicate hindgut acidosis. In summary, hindgut acidosis occurs because of relatively high rates of large intestinal fermentation, likely due to digestive dysfunction in other parts of the gut. A better understanding of the relationship of this disorder to other animal health disorders is needed.

Carbohydrate source and protein degradability alter lactation, ruminal, and blood measures.

Thirty-eight lactating dairy cows including 6 ruminally cannulated cows were used in a feeding study to assess effects of feed sources that differed in dietary nonfiber carbohydrate (NFC) composition and ruminal degradability of dietary protein (RDP) on production, ruminal, and plasma measures. The design was a partially balanced, incomplete Latin square with three 21-d periods and a 3 x 2 factorial arrangement of treatments. Samples and data were collected in the last 7 d of each period. Feed sources that differed in NFC profile were dry ground corn (GC; starch), dried citrus pulp (DCP; sugar and pectins), and sucrose+molasses (SM; sugar). Dietary RDP was altered by providing CP with soybean meal (+RDP) or substituting a heat-treated expeller soybean product for a portion of the soybean meal (-RDP). Diets were formulated to be isonitrogenous and similar in NFC concentration. Cows consuming GC had the greatest milk urea nitrogen and milk protein percentage and yield, tended to have the greatest dry matter intake, but had a lesser milk fat percentage compared with cows consuming DCP and SM. Sucrose+molasses diets supported greater dry matter intake, milk protein yield, and 3.5% fat- and protein-corrected milk yield than did DCP diets. On -RDP diets, milk protein percentage was less and milk urea nitrogen and protein yield tended to be less than for +RDP diets. Dry ground corn diverged from DCP and SM in the effect of NFC x RDP, with cows consuming GC having lesser milk yield, 3.5% fat- and protein-corrected milk yield, and efficiency with -RDP as compared with +RDP, whereas these production measures were greater with -RDP than +RDP for cows consuming DCP and SM. In contrast, in situ NDF digestibility at 30h for GC and SM was greater for -RDP as compared with +RDP, but the reverse was true for DCP. The lowest ruminal pH detected by 6h postfeeding was also influenced by the interaction of NFC x RDP, with cows consuming SM having a lower pH with +RDP than with -RDP and cows consuming DCP having a similar pH on either RDP treatment. Total rumen volatile fatty acid concentrations did not differ among diets, but acetate molar percent was greater for DCP than for SM, and GC had the lowest molar percent for butyrate and valerate and greatest branched-chain volatile fatty acid concentration. Valerate molar percent and NH(3) concentration tended to be greater with +RDP than with -RDP. Plasma glucose and insulin were both greater in cows receiving SM than in those receiving DCP. Protein degradability, NFC source, and their interactions affected lactation, ruminal, and blood measures, suggesting that these dietary factors warrant further consideration in diet formulation.

A ring test of in vitro neutral detergent fiber digestibility: Analytical variability and sample ranking1

In vitro neutral detergent fiber (NDF) digestibility (NDFD) is an empirical measurement of fiber fermentability by rumen microbes. Variation is inherent in all assays and may be increased as multiple steps or differing procedures are used to assess an empirical measure. The main objective of this study was to evaluate variability within and among laboratories of 30-h NDFD values analyzed in repeated runs. Subsamples of alfalfa (n=4), corn forage (n=5), and grass (n=5) ground to pass a 6-mm screen passed a test for homogeneity. The 14 samples were sent to 10 laboratories on 3 occasions over 12 mo. Laboratories ground the samples and ran 1 to 3 replicates of each sample within fermentation run and analyzed 2 or 3 sets of samples. Laboratories used 1 of 2 NDFD procedures: 8 labs used procedures related to the 1970 Goering and Van Soest (GVS) procedure using fermentation vessels or filter bags, and 2 used a procedure with preincubated inoculum (PInc). Means and standard deviations (SD) of sample replicates within run within laboratory (lab) were evaluated with a statistical model that included lab, run within lab, sample, and lab × sample interaction as factors. All factors affected mean values for 30-h NDFD. The lab × sample effect suggests against a simple lab bias in mean values. The SD ranged from 0.49 to 3.37% NDFD and were influenced by lab and run within lab. The GVS procedure gave greater NDFD values than PInc, with an average difference across all samples of 17% NDFD. Because of the differences between GVS and PInc, we recommend using results in contexts appropriate to each procedure. The 95% probability limits for within-lab repeatability and among-lab reproducibility for GVS mean values were 10.2 and 13.4%, respectively. These percentages describe the span of the range around the mean into which 95% of analytical results for a sample fall for values generated within a lab and among labs. This degree of precision was supported in that the average maximum difference between samples that were not declared different by means separation was 4.4% NDFD. Although the values did not have great precision, GVS labs were able to reliably rank sample data in order of 30-h NDFD (Spearman correlation coefficient = 0.93) with 80% of the rankings correct or off by only 1 ranking. A relative ranking system for NDFD could reduce the effect of within- and among-lab variation in numeric values. Such a system could give a more accurate portrayal of the comparative values of samples than current numeric values imply.

Isotrichid protozoa influence conversion of glucose to glycogen and other microbial products1

The goal of this in vitro study was to determine the influence of isotrichid protozoa (IP) on the conversion of glucose (Glc) to glycogen (Glyc) and transformation of Glc into fermentation products. Treatments were ruminal inoculum mechanically processed (blended) to destroy IP (B+, verified microscopically) or not mechanically processed (B-). Accumulated microbial Glyc was measured at 3h of fermentation with (L+; protozoa+bacteria) or without (L- predominantly protozoa) lysis of bacterial cells in the fermentation solids with 0.2 N NaOH. Two 3-h in vitro fermentations were performed using Goering-Van Soest medium in batch culture vessels supplemented with 78.75 mg of Glc/vessel in a 26.5-mL liquid volume. Rumen inoculum from 2 cannulated cows was filtered through cheesecloth, combined, and maintained under CO(2) for all procedures. At 3h, 0.63 and 0.38 mg of Glc remained in B- and B+. Net microbial Glyc accumulation (and Glc in Glyc as % of added Glc) detected at 3h of fermentation were 3.32 (4.69%), -1.42 (-2.01%), 6.45 (9.10%), and 3.65 (5.15%) mg for B-L-, B+L-, B-L+ and B+L+, respectively. Treatments B+ and L+ gave lower Glyc values than B- and L-, respectively. Treatment B+L- demonstrated net utilization of α-glucan contributed by inoculum with no net Glyc production. With destruction of IP, total Glyc accumulation declined by 44%, but estimated bacterial Glyc increased. Microbial accumulation of N increased 17.7% and calculated CH(4) production decreased 24.7% in B+ compared with B-, but accumulation of C in microbes, production of organic acids or C in organic acids, calculated CO(2), and carbohydrates in cell-free medium did not differ between B+ and B-. Given the short 3-h timeframe, increased N accumulation in B+ was attributed to decreased Glyc sequestration by IP rather than decreased predation on bacteria. After correction for estimates of C from AA and peptides utilized by microbes, 15% of substrate Glc C could not be accounted for in measured products in B+ or B-. Approximately 30% of substrate Glc was consumed by energetic costs associated with Glc transport and Glyc synthesis. The substantial accumulation of Glyc and changes in microbial N and Glyc accumulation related to presence of IP suggest that these factors should be considered in predicting profiles and amounts of microbial products and yield of nutrients to the cow as related to utilization of glucose. Determination of applicability of these findings to other soluble carbohydrates could be useful.

Feeding, Evaluating, and Controlling Rumen Function

Achieving optimal rumen function requires an understanding of feeds and systems of nutritional evaluation. Key influences on optimal function include achieving good dry matter intake. The function of feeds in the rumen depends on other factors including chemical composition, rate of passage, degradation rate of the feed, availability of other substrates and cofactors, and individual animal variation. This article discusses carbohydrate, protein, and fat metabolism in the rumen, and provides practical means of evaluation of rations in the field. Conditions under which rumen function is suboptimal (ie, acidosis and bloat) are discussed, and methods for control examined.

Responses of late-lactation cows to forage substitutes in low-forage diets supplemented with by-products1

In response to drought-induced forage shortages along with increased corn and soy prices, this study was conducted to evaluate lactation responses of dairy cows to lower-forage diets supplemented with forage substitutes. By-product feeds were used to completely replace corn grain and soybean feeds. Forty-eight late-lactation cows were assigned to 1 of 4 diets using a randomized complete block design with a 2-wk covariate period followed by a 4-wk experimental period. The covariate diet contained corn grain, soybean meal, and 61% forage. Experimental diets contained chopped wheat straw (WS)/sugar beet pulp at 0/12, 3/9, 6/6, or 9/3 percentages of diet dry matter (DM). Corn silage (20%), alfalfa silage (20%), pelleted corn gluten feed (25.5%), distillers grains (8%), whole cottonseed (5%), cane molasses/whey blend (7%), and vitamin and mineral mix with monensin (2.5%) comprised the rest of diet DM. The WS/sugar beet pulp diets averaged 16.5% crude protein, 35% neutral detergent fiber, and 11% starch (DM basis). Cows consuming the experimental diets maintained a 3.5% fat- and protein-corrected milk production (35.2 kg; standard deviation=5.6 kg) that was numerically similar to that measured in the covariate period (35.3 kg; standard deviation=5.0 kg). Intakes of DM and crude protein declined linearly as WS increased, whereas neutral detergent fiber intake increased linearly. Linear increases in time spent ruminating (from 409 to 502 min/d) and eating (from 156 to 223 min/d) were noted as WS inclusion increased. Yields of milk fat and 3.5% fat-and protein-corrected milk did not change as WS increased, but those of protein and lactose declined linearly. Phosphorous intakes were in excess of recommended levels and decreased linearly with increasing WS inclusion. Nutritional model predictions for multiparous cows were closest to actual performance for the National Research Council 2001 model when a metabolizable protein basis was used; primiparous cow performance was better predicted by energy-based predictions made with the National Research Council or Cornell Net Carbohydrate and Protein System models. Model predictions of performance showed a quadratic diet effect with increasing WS. Lactating dairy cows maintained production on low-forage diets that included forage substitutes, and in which by-product feeds fully replaced corn grain and soybean. However, longer-term studies are needed to evaluate animal performance and to improve model predictions of performance on these nontraditional diets.

Divergent utilization patterns of grass fructan, inulin, and other nonfiber carbohydrates by ruminal microbes1

Fructans are an important nonfiber carbohydrate in cool season grasses. Their fermentation by ruminal microbes is not well described, though such information is needed to understand their nutritional value to ruminants. Our objective was to compare kinetics and product formation of orchardgrass fructan (phlein; PHL) to other nonfiber carbohydrates when fermented in vitro with mixed or pure culture ruminal microbes. Studies were carried out as randomized complete block designs. All rates given are first-order rate constants. With mixed ruminal microbes, rate of substrate disappearance tended to be greater for glucose (GLC) than for PHL and chicory fructan (inulin; INU), which tended to differ from each other (0.74, 0.62, and 0.33 h(-1), respectively). Disappearance of GLC had almost no lag time (0.04 h), whereas the fructans had lags of 1.4h. The maximum microbial N accumulation, a proxy for cell growth, tended to be 20% greater for PHL and INU than for GLC. The N accumulation rate for GLC (1.31h(-1)) was greater than for PHL (0.75 h(-1)) and INU (0.26 h(-1)), which also differed. More microbial glycogen (+57%) was accumulated from GLC than from PHL, though accumulation rates did not differ (1.95 and 1.44 h(-1), respectively); little glycogen accumulated from INU. Rates of organic acid formation were 0.80, 0.28, and 0.80 h(-1) for GLC, INU, and PHL, respectively, with PHL tending to be greater than INU. Lactic acid production was more than 7-fold greater for GLC than for the fructans. The ratio of microbial cell carbon to organic acid carbon tended to be greater for PHL (0.90) and INU (0.86) than for GLC (0.69), indicating a greater yield of cell mass per amount of substrate fermented with fructans. Reduced microbial yield for GLC may relate to the greater glycogen production that requires ATP, and lactate production that yields less ATP; together, these processes could have reduced ATP available for cell growth. Acetate molar proportion was less for GLC than for fructans, and less for PHL than for INU. In studies with pure cultures, all microbes evaluated showed differences in specific growth rate constants (μ) for GLC, fructose, sucrose, maltose, and PHL. Selenomonas ruminantium and Streptococcus bovis showed the highest μ for PHL (0.55 and 0.67 h(-1), respectively), which were 50 to 60% of the μ achieved for GLC. The 10 other species tested had μ between 0.01 and 0.11h(-1) with PHL. Ruminal microbes use PHL differently than they do GLC or INU.

Technical note: effect of sample processing procedures on measurement of starch in corn silage and corn grain.

Methods for processing feedstuffs before analysis can affect analytical results. Effects of drying temperature (corn silage), preservation method (corn grain), and grinding method (corn silage and grain) on starch analysis values were evaluated. Corn silage samples dried at 55 or 105 degrees C and grain samples dried at 55 degrees C were ground to pass the 1-mm screen of an abrasion mill or cutting mill and analyzed for free glucose and starch corrected for free glucose. Starch analyses were performed in triplicate to assess the effect of treatment on precision of starch determination. Drying at 105 degrees C decreased free glucose and tended to decrease starch detected in silage. Decreased free glucose and starch values in silages dried at 105 degrees C may have been caused by the destruction of glucose and production of Maillard products through nonenzymatic browning. Maillard products with reducing activity could potentially interfere with the glucose oxidase-peroxidase glucose detection method used. Compared with the cutting mill, grinding samples through the abrasion mill increased the precision of starch measures in silage, likely due to the effect of the finer particle size produced by the abrasion mill allowing more accurate subsampling of a more homogeneous matrix. Starch values were greater for grain ground with an abrasion mill than with a cutting mill, with the difference greater for dry-rolled than for high-moisture corn. For starch analysis of corn silage and corn grain, drying at lower temperatures (55 degrees C) in forced-air ovens and grinding through the 1-mm screen of an abrasion mill or its equivalent is recommended.

Effect of variable water intake as mediated by dietary potassium carbonate supplementation on rumen dynamics in lactating dairy cows

Water is a critical nutrient for dairy cows, with intake varying with environment, production, and diet. However, little work has evaluated the effects of water intake on rumen parameters. Using dietary potassium carbonate (K2CO3) as a K supplement to increase water intake, the objective of this study was to evaluate the effect of K2CO3 supplementation on water intake and on rumen parameters of lactating dairy cows. Nine ruminally cannulated, late-lactation Holstein cows (207±12d in milk) were randomly assigned to 1 of 3 treatments in a replicated 3×3 Latin square design with 18-d periods. Dietary treatments (on a dry matter basis) were no added K2CO3 (baseline dietary K levels of 1.67% dietary K), 0.75% added dietary K, and 1.5% added dietary K. Cows were offered treatment diets for a 14-d adaption period followed by a 4-d collection period. Ruminal total, liquid, and dry matter digesta weights were determined by total rumen evacuations conducted 2h after feeding on d 4 of the collection period. Rumen fluid samples were collected to determine pH, volatile fatty acids, and NH3 concentrations, and Co-EDTA was used to determine fractional liquid passage rate. Milk samples were collected twice daily during the collection period. Milk, milk fat, and protein yields showed quadratic responses with greatest yields for the 0.75% added dietary K treatment. Dry matter intake showed a quadratic response with 21.8kg/d for the 0.75% added dietary K treatment and 20.4 and 20.5kg/d for control and the 1.5% added dietary K treatment, respectively. Water intake increased linearly with increasing K2CO3 supplementation (102.4, 118.4, and 129.3L/d) as did ruminal fractional liquid passage rate in the earlier hours after feeding (0.118, 0.135, and 0.141 per hour). Total and wet weights of rumen contents declined linearly and dry weight tended to decline linearly as dietary K2CO3 increased, suggesting that the increasing water intake and fractional liquid passage rate with increasing K2CO3 increased the overall ruminal turnover rate. Ruminal ammonia concentrations declined linearly and pH increased linearly as K supplementation increased. As a molar percentage of total volatile fatty acids, acetate increased linearly as dietary K increased, though propionate declined. Increasing dietary K2CO3 and total K in the diets of lactating dairy cows increased water consumption and modified ruminal measures in ways suggesting that both liquid and total ruminal turnover were increased as both water and K intake increased.

Corn bran versus corn grain at 2 levels of forage: Intake, apparent digestibility, and production responses by lactating dairy cows.

The objective of this study was to determine the effect of substituting corn bran (CB) for dried ground corn grain (CG) in the nonforage portion of high-forage (HF) and low-forage (LF) diets. Twelve multiparous and 12 primiparous Holsteins were assigned to 4 diets using six 4× Latin squares with 3-wk periods. Forage was 64 or 38% of the total mixed ration (% of dry matter). On a dry matter basis, the HFCG diet had 20% CG, the LFCG diet had 39% CG, the HFCB diet had 19% CB, and the LFCB diet had 38% CB. Digestible organic matter intake (OMI) and milk energy yield were lower for CB compared with CG within forage level. Digestible OMI was greater (1.9kg/d) for the LFCG compared with the HFCG treatment. When CB replaced forage (LFCB vs. HFCB), digestible OMI was not different but milk energy yield was greater with the LFCB diet. The LFCG diet supported the greatest milk, milk protein, and milk energy yield. Decreased concentration of milk protein and increased concentration of milk urea nitrogen when feeding CB compared with CG suggests that lack of fermentable energy in the CB diets may have limited rumen microbial protein synthesis. Total substitution of CG with CB in the nonforage portion did not support maximum milk production, even when forage was reduced at the same time (HFCG vs. LFCB). Predicted neutral detergent fiber (NDF) digestibility at 1 times maintenance, based on chemical analysis of the individual feeds, was 22 percentage units greater for CB than for the forage mix (68.9 vs. 46.9%). In vitro NDF digestibility (30h) was 19.4 percentage units greater for CB than for the forage mix (68.9 vs. 49.5%). However, in vivo NDF digestibility of the diet when CB replaced forage (HFCB vs. LFCB) was similar (44.1 vs. 44.5%). Similarly, predicted total digestible nutrients at the production level of intake, based on chemical analysis, were greater for the CB treatments and lower for the CG treatments than those observed in vivo.