Metabolic relationships in the supply of nutrients for milk protein synthesis: integrative modeling

A meta-analysis of the relationship between milk protein production and absorbed amino acids and digested energy in dairy cattle

Milk protein production is the largest draw on AA supplies for lactating dairy cattle. Prior NRC predictions of milk protein production have been absorbed protein (MP)-based and used a first-limiting nutrient concept to integrate the effects of energy and protein, which yielded poor accuracy and precision (root mean squared error [RMSE] >21%). Using a meta-data set gathered, various alternative equation forms considering MP, absorbed total EAA, absorbed individual EAA, and digested energy (DE) supplies as additive drivers of production were evaluated, and all were found to be superior in statistical performance to the first limitation approach (RMSE = 14%-15%). Inclusion of DE intake and a quadratic term for MP or absorbed EAA supplies were found to be necessary to achieve intercept estimates (nonproductive protein use) that were similar to the factorial estimates of the National Academies of Sciences, Engineering, and Medicine (2021). The partial linear slope for MP was found to be 0.409, which is consistent with the observed slope bias of -0.34 g/g when a slope of 0.67 was used for MP efficiency in a first-limiting nutrient system. Replacement of MP with the supplies of individual absorbed EAA expressed in grams per day and a common quadratic across the EAA resulted in unbiased predictions with improved statistical performance as compared with MP-based models. Based on Akaike's information criterion and biological consistency, the best equations included absorbed His, Ile, Lys, Met, Thr, the NEAA, and individual DE intakes from fatty acids, NDF, residual OM, and starch. Several also contained a term for absorbed Leu. These equations generally had RMSE of 14.3% and a concordance correlation of 0.76. Based on the common quadratic and individual linear terms, milk protein response plateaus were predicted at approximately 320 g/d of absorbed His, Ile, and Lys; 395 g/d of absorbed Thr; 550 g/d of absorbed Met; and 70 g/d of absorbed Leu. Therefore, responses to each except Leu are almost linear throughout the normal in vivo range. De-aggregation of the quadratic term and parsing to individual absorbed EAA resulted in nonbiological estimates for several EAA indicating over-parameterization. Expression of the EAA as g/100 g total absorbed EAA or as ratios of DE intake and using linear and quadratic terms for each EAA resulted in similar statistical performance, but the solutions had identifiability problems and several nonbiological parameter estimates. The use of ratios also introduced nonlinearity in the independent variables which violates linear regression assumptions. Further screening of the global model using absorbed EAA expressed as grams per day with a common quadratic using an all-models approach, and exhaustive cross-evaluation indicated the parameter estimates for BW, all 4 DE terms, His, Ile, Lys, Met, and the common quadratic term were stable, whereas estimates for Leu and Thr were known with less certainty. Use of independent and additive terms and a quadratic expression in the equation results in variable efficiencies of conversion. The additivity also provides partial substitution among the nutrients. Both of these prevent establishment of fixed nutrient requirements in support of milk protein production.

ASAS centennial paper: Lactation biology for the twenty-first century

Knowledge of general aspects of mammary gland function, including metabolic pathways and hormonal regulation of mammary gland development and lactation, in livestock species was obtained several decades ago. As basic biological information of growth factor action, apoptotic mechanisms, and signal transduction events has exploded, the mouse became the model of choice for studying fundamental mechanisms regulating mammary function. A complete sequenced genome also has made the mouse amenable for studies of mammary gene network expression. Advances in molecular biology techniques currently allow researchers to genetically modify mice to either overexpress or completely lack specific genes, thereby studying their function in an in vivo setting. Furthermore, the use of mammary-specific promoters has allowed genes related to mammary gland function to be eliminated from the mammary gland in a developmental and tissue-specific manner. These studies have demonstrated the complexity that underlies mammary gland development and function in rodents and may provide insight into the mechanisms that ultimately allow the ruminant or swine mammary gland to function in a coordinated fashion throughout puberty, pregnancy, lactation, and involution. The challenge facing animal scientists is how to obtain similar information in much larger and expensive livestock. One possible approach is to manipulate gene expression in vitro using mammary cell culture models derived from domestic animals (e.g., genes can be "knocked down" using small interfering RNA approaches). Ultimately, major advances in understanding lactation biology may come from coupling basic mechanistic information with functional genomics, proteomics, and metabolomics approaches. A strong foundation in bioinformatics will also be required to optimize use of these new technologies. Stem cell biology also represents an exciting area in the next decade that holds promise for improving lactation efficiency. Strong training of our future scientists in these areas should facilitate livestock-focused mammary gland research that will allow basic information to be gained at unprecedented amounts, ultimately leading to optimization of livestock production.

Long-chain omega-3 fatty acids regulate bovine whole-body protein metabolism by promoting muscle insulin signalling to the Akt-mTOR-S6K1 pathway and insulin sensitivity

The ability of the skeletal musculature to use amino acids to build or renew constitutive proteins is gradually lost with age and this is partly due to a decline in skeletal muscle insulin sensitivity. Since long-chain omega-3 polyunsaturated fatty acids (LCn-3PUFA) from fish oil are known to improve insulin-mediated glucose metabolism in insulin-resistant states, their potential role in regulating insulin-mediated protein metabolism was investigated in this study. Experimental data are based on a switchback design composed of three 5 week experimental periods using six growing steers to compare the effect of a continuous abomasal infusion of LCn-3PUFA-rich menhaden oil with an iso-energetic control oil mixture. Clamp and insulin signalling observations were combined with additional data from a second cohort of six steers. We found that enteral LCn-3PUFA potentiate insulin action by increasing the insulin-stimulated whole-body disposal of amino acids from 152 to 308 micromol kg(-1) h(-1) (P=0.006). The study further showed that in the fed steady-state, chronic adaptation to LCn-3PUFA induces greater activation (P<0.05) of the Akt-mTOR-S6K1 signalling pathway. Simultaneously, whole-body total flux of phenylalanine was reduced from 87 to 67 micromol kg(-1) h(-1) (P=0.04) and oxidative metabolism was decreased (P=0.05). We conclude that chronic feeding of menhaden oil provides a novel nutritional mean to enhance insulin-sensitive aspects of protein metabolism.

Integration of Ruminal Metabolism in Dairy Cattle

An important objective is to identify nutrients or dietary factors that are most critical for advancing our knowledge of, and improving our ability to predict, milk protein production. The Dairy NRC (2001) model is sensitive to prediction of microbial protein synthesis, which is among the most important component of models integrating requirement and corresponding supply of metabolizable protein or amino acids. There are a variety of important considerations when assessing appropriate use of microbial marker methodology. Statistical formulas and examples are included to document and explain limitations in using a calibration equation from a source publication to predict duodenal flow of purine bases from measured urinary purine derivatives in a future study, and an improved approach was derived. Sources of specific carbohydrate rumen-degraded protein components probably explain microbial interactions and differences among studies. Changes in microbial populations might explain the variation in ruminal outflow of biohydrogenation intermediates that modify milk fat secretion. Finally, microbial protein synthesis can be better integrated with the production of volatile fatty acids, which do not necessarily reflect volatile fatty acid molar proportions in the rumen. The gut and splanchnic tissues metabolize varying amounts of volatile fatty acids, and propionate has important hormonal responses influencing milk protein percentage. Integration of ruminal metabolism with that in the mammary and peripheral tissues can be improved to increase the efficiency of conversion of dietary nutrients into milk components for more efficient milk production with decreased environmental impact.

Estimation of parameters describing lipid metabolism in lactation: challenge of existing knowledge described in a model of metabolism

We have been conducting research to improve quantitative descriptions of metabolism in lactating dairy cows depicted in an existing mechanistic, computer-assisted model. The model is dynamic and deterministic and is based on biochemical equations describing ruminal fermentation and chemical interactions in body tissues. The objective was to challenge this model with data collected in in vivo and in vitro experiments on high producing dairy cattle fed a range of energy. Cows that varied in genetic propensity for milk production (7045 to 12,909 kg of milk/305 d), lactation number (1 to 4), stage of lactation (-30 to 345 d in milk), and rate of intake (14 to 29 kg/d of dry matter) and that were fed various energy-yielding feedstuffs were used. Dietary inputs, milk component outputs, body fat, nutrient concentrations in blood, and maximal velocity and substrate sensitivity of adipose tissue metabolic reactions were observed. Model simulations were conducted; simulated yields of milk components for a 305-d lactation were within 5% of observed means. Simulated lipid metabolism and accumulation of body fat were adequate in many situations; however, the model response to changes in energy intake was too sensitive. This inadequacy was especially noticeable in later lactation because of inadequate representation of dynamic responses over periods more than a few weeks long. The model behaves consistently with biochemical principles, behavior was in the correct direction, and precision was adequate for many variables. Lack of precision in long-term dynamic changes indicates that the parameters describing energy-utilizing reactions are inadequate. This severe challenge of the model supports its functionality. Further experiments must be designed to determine how nutrients in viscera, muscle, and adipose tissue are used; these experiments must encompass sufficient range in genetic ability, nutrient input, and time to adequately describe the dynamic and integrated nature of metabolic reactions.

Current concepts of amino acid and protein metabolism in the mammary gland of the lactating ruminant

Milk protein responses to protein nutrition are typically poor and, in part, may be due to the low efficiency (approximately 25 to 30%) of converting dietary N into milk. Posthepatic availability of amino acids (AA) is not limited, yet only approximately 30% is converted into milk. The poor capture of AA by the mammary gland may relate to the imbalanced and uncoordinated timing of nutrient delivery to the gland. The infusion of essential AA improves the efficiency of utilization (0.31); however, further catabolism of AA within the mammary gland suggests that AA transport is not a major limitation. These losses may serve ancillary or functional roles, but mammary oxidation of some AA occurs only when AA extraction exceeds the stoichiometric requirements for milk protein synthesis. Intracellular substrate supply may be more limiting than is the appartus for protein synthesis. Studies utilizing isotope labeling and conducted in vitro and in vivo now suggest that circulating peptides and proteins can serve as sources of perhaps all AA for casein synthesis, but the source of these remains elusive. Constitutive protein and casein turnover contribute significantly (42 to 72%) to mammary protein synthesis. All AA are extensively channeled through an intermediary protein pool or pools that have rapid turnover rates. The AA are then incorporated into casein, which appears to be fixed in association with protein turnover. The mammary gland is a major controller of its metabolism, and the mechanisms of AA extraction and conversion into milk protein are linked to secretion events. Blood flow may be a key point of regulation whereby mechanisms sense and respond to nutrient supply and balance to the gland via alterations in hemodynamics.

Efficiency of nutrient use and relationship to profitability on dairy farms

As cows eat more feed to support higher milk production, the proportion of digested energy that is captured in milk increases. In contrast, as cows consume more feed, digestive efficiency decreases, but the magnitude of depression in digestibility is not characterized for high producing cows. Despite the digestibility depression, biological efficiency increases considerably as Holstein cows produce more milk up to 15,000 kg/yr. Above 15,000 kg/yr, the gain in biological efficiency per unit of increase in milk production is expected to approach 0. At 21,000 kg of milk/yr, approximately 25% of the gross energy consumed by the cow during the first 5 yr of life likely would be captured as milk, conceptus, or body tissues; further gains in biological efficiency are not likely without major advances in feed digestion. Although feeds generally cost more as cows are fed for higher milk production, increased productivity also enhances profitability, partly because of increased efficiency but also because fixed costs are decreased relative to total costs. This relationship between productivity and profitability is expected to continue up to > 21,000 kg/yr per cow. Other issues related to the efficiency of nutrient use will become more important in the future, namely, efficiency of the use of feeds that can also be consumed by humans, efficiency of use of tillable land, and minimization of nutrient losses to the environment.

Effects of feeding nonforage fiber sources on site of fiber digestion

Although many nonforage fiber sources have high extents of neutral detergent fiber (NDF) digestion, most have rates of digestion similar to or slower than the rates of forage NDF digestion. Rates of NDF digestion vary considerably among and within sources of by-products. Digestion kinetics also vary because of the technique used (in vitro versus in situ) and because of high amounts of dietary concentrate. Based on available data for passage rate and specific gravity measurements, rates of passage of nonforage fiber sources from the rumen of high producing cows appear to be faster than those of forages. Therefore, the potential to shift NDF digestion to the hindgut has been discussed. To account for variability in ruminal and total tract digestibility of NDF, multiple regression analysis was used to indicate that nonforage NDF percentage in the diet had about two-thirds the positive response on total tract NDF digestion that forage NDF percentage did. Although the loss of potentially digestible NDF may occur, DMI does not appear to decrease much until forage NDF is below 14 to 16% of dietary DM. Conversely, replacement of starch with nonforage NDF appears to increase digestibility of fiber, mostly in diets with high concentrations of nonfiber carbohydrates, apparently because of reduced negative associative effects. Increasing the concentration of total NDF above 35% also can decrease DMI with little improvement in NDF digestibility. Increased knowledge of the kinetics of digestion and the passage of various nonforage fiber sources used to replace forage or concentrate should increase the accuracy and precision of dynamic models, thereby increasing the flexibility and utility of nonforage fiber sources in dairy rations.

Absorption and delivery of nutrients for milk protein synthesis by portal-drained viscera

The predictability of diet effects on milk composition is limited by the lack of understanding of the metabolic transformations that absorbed nutrients undergo within the portal-drained viscera and liver of high yielding dairy cows. The mass of splanchnic tissues increases dramatically in early lactation, but little is known about the regulation of gut growth and adaptation in early lactation, and further research may provide strategies for optimizing gut adaptation. Glucose is critical for milk synthesis, but portal-drained visceral tissues normally use rather than absorb glucose on a net basis. Dietary starch of low ruminal digestibility increases postruminal starch digestion and decreases net use of glucose by portal-drained viscera slightly, but increases in glucose absorption by portal-drained viscera never account fully for increases in starch disappearance from the small intestine and occur at the expense of VFA absorption. For cows in positive energy balance, greater glucose availability increases tissue energy balance and glucose oxidation, but has little effect on milk or milk protein yield. Similarly, chronic increases in propionate absorption have little effect on milk or milk protein yield. In contrast, casein infusion into the small intestine consistently increases milk and milk protein yield, but the mechanisms responsible remain unclear. There are few data describing the absorption and metabolism of AA by splanchnic tissues of lactating dairy cows, but, as for glucose and VFA, utilization of many AA by portal-drained viscera is substantial. In addition, the contribution of peptides to AA absorption and transport is uncertain and must be clarified. Therefore, measurements of nutrient disappearance from the lumen of the gut cannot be equated with nutrient appearance in the portal vein. Data describing metabolism of nutrients by portal-drained viscera and liver of high yielding dairy cows are needed to improve feeding standards.

Impact of metabolism by extragastrointestinal tissues on secretory rate of milk proteins

This paper reviews the current state of knowledge about the postabsorptive utilization of AA. Data on the duodenal entry of AA and the projected output of these same AA as milk protein demonstrate significant losses. Within the essential AA, these losses range from 70% for Thr to 40% for Met and Lys. For Val, almost half of the loss is due to its use for nonsecretory purposes by the mammary gland; values decrease to 0% for other AA. The liver is the other major organ, apart from the portal-drained viscera, that is responsible for significant AA disposal. Interactions of Met metabolism with other methyl sources and gluconeogenic precursors that can alter the Met requirement also are discussed. Data on the transhepatic and transsplanchnic AA balance in lactating cows, and the coordinated use of these balances with duodenal flow and mammary balance, are needed. Further research also is needed into the functionality of use of AA for purposes other than milk protein synthesis to determine whether excess catabolism of AA occurs; assuming it does, more efforts are needed to identify regulation of AA disposal in the crucial tissues.