Location: Dairy Forage Research2010 Annual Report
1a. Objectives (from AD-416)
1) Identify and measure plant chemical and physical characteristics and dietary interactions that may alter nutrient digestibility and excretion by lactating dairy cattle. 2) Determine the effects of level of intake and digestion kinetics on diet component digestibility with current industry-representative lactating cows. 3) Measure the impact of fermentative digestion on nutrient utilization, quantify the transformations of nutrients into end-products of fermentation, and use molecular techniques to characterize and quantify changes in populations of ruminal bacterial species as affected by diet and animal. 4) Develop an integrated system for evaluating forage genotypes and validate the usefulness of in vitro, in situ, and small ruminant digestibility in assessing the utilization of nutrients by lactating dairy cows representing current levels of production.
1b. Approach (from AD-416)
1) The effects of PPO-modified plants, silage inoculants and lauric acid on protein utilization will be studied. Digestibility of corn silage with altered lignin/phenolic characteristics and alfalfa with down-regulated COMT and CCOMT to modify lignin will be evaluated with lambs and lactating cows. 2) Intake and digestibility from lactating cow trials will be compiled and digestibility of dry matter, fiber and soluble organic matter will be regressed on intake. Digestion kinetics will be measured on ration ingredients from trials. 3) In vitro fermentations using mixed ruminal microbes will be used to measure changes in digestion kinetics and microbial populations associated with direct-fed microbials, monensin, non-fiber carbohydrate sources, forage species and pH. 4) In vitro, lamb and lactating cow digestibilities will be compared to develop an integrated system for evaluating new forage genotypes.
3. Progress Report
A lactation trial tested the value of supplementing dairy cows with rumen-protected methionine for optimizing production of milk and milk components on low protein diets. Adding 2 different forms of rumen-protected methionine increased yield of energy corrected milk, and tended to increase yield of fat and protein, compared to unsupplemented diets with either low or high protein contents. Results indicated that milk production was maintained or even improved when feeding rumen-protected methionine while urinary nitrogen excretion was reduced. Feces and urine were collected separately from three investigations into lactating cow response to (1) protein:energy balance, (2) physically-effective neutral detergent fiber source and interaction with non-fiber carbohydrate source, and (3) bmr corn silage. Dairy cows were fed varying levels of protein and energy. If significant differences in excreta chemistry are determined, then soil-incubations and greenhouse trial will proceed. Evaluation of glucose fermentation by rumen microbes in vitro showed an 18% change in microbial nitrogen accumulation associated with changes in energy use associated with storage of glucose as glycogen. Laboratory assays and data analysis of studies of pasture utilization by grazing heifers were completed. All sample and data analysis were completed for the lamb intake and digestibility trial for the reduced ferulate cross linking sfe corn mutant. Preliminary results indicate that selectivity against fiber was reduced and total fiber digested increased when the sfe corn mutant was fed to lambs, and that cows fed total mixed rations containing the sfe corn silage had increased feed intake and milk production. In vitro disappearance of fiber and gas production of isolated fiber and intact forages have been measured on several hundred samples, and kinetic analysis of these data is in progress. Nearly complete (~95%) exchange of ruminal contents between pairs of cows known to differ in ruminal bacterial community composition resulted in re-establishment of pre-feed ruminal pH and total concentrations within one day. The bacterial community returned to various extents to more closely resemble the pre-exchange community, although this re-establishment occurred gradually over a period of 2 to 9 weeks. A lactation trial comparing untreated and inoculated (Lactobacillus plantarum) alfalfa silage was performed. Omasal and ruminal samples were collected during sampling periods to estimate the effects of the inoculated silage on overall rumen microbial biomass production and on the prominent species within the bacterial community in the rumen.
1. Each cow has a unique bacterial community in her main stomach, the rumen. Performance differences in cows fed the same diet have been suggested to be due in part to a specificity of the ruminal bacterial community for the host cow, but this has not been demonstrated experimentally. We used molecular markers to characterize changes in bacterial community composition in pairs of cows following nearly complete (~95%) exchange of ruminal contents. Ruminal pH and organic acid concentrations in rumens generally returned to their pre-exchange profiles within one day, while the bacterial community returned to its pre-exchange composition over a longer time scale (generally 2 to 9 weeks). The data suggest that cows harbor specific communities adapted to the individual host, and will vary in their response to microbial inoculants (probiotics) intended to beneficially alter ruminal microbial populations. These results will help researchers as they seek to improve the efficiency of dairy cattle diets and minimize the output of greenhouse gases by dairy cattle.
2. The influence of fiber composition on corn silage digestibility. Corn silage is a major feed resource for livestock, but fiber digestibility of the stover fraction is poor. A corn mutant with reduced ferulate cross linking, an inhibitor of fiber digestion, was fed to lambs to measure feed intake and digestibility response. Lambs fed the mutant corn silage were less selective against fiber in the feed they consumed and digested more fiber. If the gene responsible for cross-linking can be isolated from this corn mutant, then the digestibility of all grass forages, not just corn silage, can be improved.
3. Canopy structure and nutritive value influence pasture utilization. Forage availability is generally considered the primary criteria influencing intake and performance of grazing dairy cattle. When carefully controlled by rotational grazing, however, forage availability often exceeds the cow’s daily requirement, and intake may be affected by other factors. A study of four diverse cool-season grasses demonstrated that canopy height and proportion of the leaf fraction relative to the poorer quality stem fraction often governed herbage consumption. Cows consumed more forage as both canopy height and the proportion of the leaf fraction increased. When grasses had similar canopy structure, pasture utilization was positively correlated with herbage nutritive value. The results provide guidelines for the selection and management of cool-season grasses in rotational grazing systems.
4. Feeding two different forms of rumen-protected methionine allowed cows to produce more milk on less protein. Dietary protein supplies lactating cows with amino acids, the building blocks needed to make the protein in milk and body tissues. About half of the amino acids are essential, meaning they cannot be made by the cow but must be absorbed in the intestine. Methionine, often the most limiting essential amino acid, is available in rumen-protected form (coated with materials that physically protect the methionine from breakdown by microbes living in the rumen but allowing absorption at the intestine). A new, cheaper type of protected methionine has just become commercially available. This compound is a chemically modified derivative of methionine that can resist microbial attack in the rumen; the compound is converted to methionine in the cow’s body after absorption. Lactating dairy cows being fed a 15.6% protein diet were given no methionine, methionine protected with a physical coating, or the new chemically protected form of methionine; other cows were fed a 16.8% protein diet without added methionine. None of the diets changed feed intake or actual milk production. However, feeding the chemically protected form of methionine increased yield of energy-corrected milk and milk protein concentration; moreover, cows tended to yield more fat and protein on either methionine source versus both the low or high protein diets without methionine. Feeding 16.8% protein without methionine elevated urinary nitrogen excretion and reduced nitrogen efficiency from nearly 35% to about 30%. Results with either methionine source were similar, indicating that the less expensive chemically protected form was as effective as the physically protected methionine. This research indicates that wastage of dietary protein and the potential for nitrogen pollution on U.S. dairy farms can be reduced by an inexpensive supplement of rumen-protected methionine.
5. Protein efficiency changes due to carbohydrate handling by microbes. When dairy cows are fed sugar, the efficiency with which they utilize dietary protein frequently declines, but we have not understood why. Unlike other feed carbohydrates, rumen protozoa and bacteria can convert substantial amounts of sugar to the storage carbohydrate glycogen. An ARS researcher at Madison, WI evaluated the influence of removing protozoa that make glycogen on the partitioning of glucose into products of the fermentation using a short (3- hour) in vitro fermentation to give maximum accumulation of glycogen by protozoa, but little digestion of the carbohydrates or microbes they consumed. Without the protozoa, glycogen decreased by 43% and microbial protein increased by 18%, and methane decreased by 25%; organic acid production did not change. The improved protein production was due to a decrease in energy use for storing the glycogen and an increase in sugar available to the bacteria, both of which allow greater efficiency of microbial protein production. Nutritionists can use this information to understand the differences among feed carbohydrates as they formulate dairy cow rations. They could improve production efficiency on rations containing sugars by providing more rumen undegradable protein that the cow will use directly or modify rate of passage to get better harvest of microbes and glycogen.
5. Significant Activities that Support Special Target Populations
Activities targeting small farmers with size-neutral technologies were developed, such as improved management systems for rotational grazing systems in the northern USA. We continued research on dairy farms having small to medium herd sizes (100 to 200 dairy cows/farm) to understand the impact of biophysical (soils, weather) and socioeconomic factors on nutrient (feed, fertilizer and manure) management practices and the opportunities and challenges to improvements in profitability and environmental impacts through enhanced nutrient use.
Weimer, P.J., Stevenson, D.M., Mertens, D.R. 2010. Shifts in Bacterial Community Composition in the Rumen of Lactating Dairy Cows Under Conditions of Milk Fat Depression. Journal of Dairy Science. 93:265-278.