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Objectives of this study were to quantitate metabolite fluxes in ruminant liver and to delineate effects of recombinant bST on patterns of nutrient metabolism by liver. Nineteen multiparous cows ranging in previous lactational performance from 6400 to 13,500 kg per 305-d lactation were treated with either placebo or bST (40 mg/d) from wk 11 to 18 of lactation. Liver tissue was collected at slaughter. Tissue slices were incubated with various 14C-labeled substrates, and rates of conversion of label to CO2 and metabolites were measured. In vivo recombinant bST treatment increased in vitro conversion of [1-14C]propionate and [2-14C]acetate to glucose more than twofold. At 2.5 mM propionate, bST-treated cows converted propionate to glucose at 90% efficiency. Recombinant bST increased [14C]bicarbonate incorporation into glucose five-fold. Overall, bST treatment resulted in greater C flow from propionate and acetate through the TCA cycle. Acetate had only small effects on propionate metabolism and no effects on lactate plus pyruvate metabolism. Unexpectedly, propionate decreased acetate conversion to ketone bodies. Suggested mechanisms for this observation include depletion of coenzyme A and allosteric regulation of carnitine palmitoyltransferase I by methylmalonyl-coenzyme A formed from propionate. In summary, bST treatment resulted in increased rates of gluconeogenesis and oxidation in liver in support of lactation.

Studies of mammary arteriovenous difference were conducted on multiparous lactating Holstein cows (n = 21) on d 35, 70, 105, and 126 of lactation to examine kinetic relationships between arterial concentration and mammary gland extraction of AA. Additionally, these cows were paired by previous lactational performance and assigned to bST-treated or control groups to examine the effect of bST treatment on AA concentration and extraction by lactating mammary glands. Treated cows were injected daily with 40 mg of recombinant bST from d 71 through 126 of lactation. Arterial concentrations of Asp, Ser, Asn, Gly, beta-aminoisobutyrate, and Met were increased. Concentrations of Val, Ile, Leu, Phe, Orn, and Lys were decreased in bST-treated cows compared with controls. Increased extractions of Asp and Met by mammary glands in treated versus control cows were correlated positively with treatment-induced changes in arterial concentrations of these AA. However, increased mammary extractions of Arg, cystathionine, Leu, and Lys by bST-treated compared with control cows were not correlated with bST-induced changes in arterial concentrations of these AA. Extractions of Asn, His, Thr, Arg, Tyr, Met, cystathionine, cystine, Ile, Phe, Orn, Glu, Gly, Tau, Cit, Leu, and Val were correlated linearly with arterial concentrations (r2 greater than .15) of each AA. Extractions of Asp, Glu, Ser, Asn, Gly, Gln, Tau, His, Cit, Thr, Pro, Tyr, Val, cystine, Ile, Leu, Trp, Orn, and Lys also were correlated with arteriovenous differences of Met.

Twenty-one multiparous lactating dairy cows with previous 305-d milk production records varying from 5900 to 13,600 kg were used to investigate effects of bST administration and stage of lactation on nutrient plasma arterial concentrations and arterial-venous differences across the mammary glands (uptake). On d 71 and continuing until d 126 of lactation, cows were injected with 40 mg of sometribove (bST group) or bicarbonate buffer (placebo group). Arterial and venous blood plasma samples were collected over a 12-h period on d 35, 70, 105, and 126 of lactation. Plasma concentration of glucose was 7% higher in midlactation compared with early lactation cows. Plasma concentration of acetate decreased from 2.11 to 1.87 mM in placebo versus bST-treated cows. Plasma arterial concentration and uptake of D-beta-hydroxybutyrate were .52 and .18 mM higher, respectively, in early versus midlactation cows. Concentration and uptake of NEFA were elevated in both early lactation and bST-treated cows. Triacylglyceride concentrations were 24 and 19% lower in early lactation and bST-treated cows compared with midlactation cows receiving placebo. Likewise, uptake of triacylglyceride was reduced in early lactation and with bST treatment compared with midlactation. The mediation of nutrient delivery and uptake by the mammary gland appears to be markedly similar between early lactation and bST-treated cows, suggesting a similarity between these physiological states and the homeostatic and homeorhetic mechanisms regulating nutrient partitioning in the lactating dairy cow. In early compared with midlactation cows receiving placebo, uptakes of D-beta-hydroxybutyrate and NEFA were reduced, and triacylglyceride uptake increased.

Twenty-one multiparous lactating dairy cows with previous 305-d milk production records varying from 5900 to 13,600 kg were used to examine patterns of nutrient uptake by the mammary glands. On d 71 and continuing until d 126 of lactation, animals were injected daily with 40 mg of sometribove (bST group) or bicarbonate buffer (control group). Arterial and venous blood plasma samples were collected over a 12-h period on d 35, 70, 105, and 126 of lactation. Regression equations developed to evaluate linear effects of plasma arterial concentrations on net arterial-venous difference across the mammary glands demonstrated that, for acetate, NEFA, and D-beta-hydroxybutyrate, plasma arterial concentration accounted for over 50% of variation in uptake by the mammary glands. Additionally, a sigmoidal equation fitted the relationship between D-beta-hydroxybutyrate plasma arterial concentration and mammary gland uptake (r2 = .70). Triacylglyceride concentration was less effective in predicting uptake (r2 = .25). Administration of bST did not alter patterns of nutrient uptake, but a fourfold increase in NEFA uptake was predicted for bST-treated cows from this study, using NEFA concentrations from the literature. These observations indicate that plasma concentrations of acetate, NEFA, D-beta-hydroxybutyrate, and triacylglyceride are major determinants of uptake by the mammary glands. Factors other than plasma concentration, such as mammary gland biosynthetic capacity, availability of other nutrients, and blood flow, determine uptakes of glucose, lactate, and total and free cholesterol (r2 less than or equal to .03).

The objective of the experiment was to determine the contribution of red blood cells to transport of individual amino acids to lactating bovine mammary glands. Blood samples were collected from coccygeal and subcutaneous abdominal veins of 21 lactating Holstein cows on d 35, 70, 105, and 126 of lactation. Samples were collected every 20 min for 12 h. Subsamples of whole blood and plasma were pooled by hour and day. Hourly plasma samples and daily whole blood and plasma samples were analyzed for amino acid concentration. Plasma glutamate concentration was stable throughout the 12-h collection period, indicating that sample collection did not perturb amino acid homeostasis. Therefore, data from pooled daily samples were used for subsequent comparisons. Whole blood arteriovenous differences of phosphoserine, aspartate, glutamate, hydroxyproline, phosphoethanolamine, serine, asparagine, glycine, glutamine, taurine, histidine, citrulline, threonine, alanine, beta-aminoisobutyrate, carnosine, arginine, proline, alpha-aminobutyrate, tyrosine, valine, methionine, cystine, isoleucine, leucine, phenylalanine, tryptophan, ornithine, and lysine differed significantly from plasma arteriovenous differences. Uptakes of individual amino acids from plasma were poorly correlated with uptake from whole blood. These data clearly indicate that uptake data derived from plasma do not adequately represent whole blood amino acid uptake.

We have demonstrated that the zinc (Zn) concentration of mouse milk declines significantly over the lactation period. Pups radiolabeled in utero with 65Zn were forward-fostered (FF) to nonradiolabeled dams at a later stage of lactation to study the effects of early milk deprivation. Other groups of radiolabeled pups were back-fostered at 5, 10 and 15 days of age to a nonradiolabeled dam who had just given birth; this provided additional colostrum during the suckling period. Litters fostered at birth to an unlabeled dam at d 0 of lactation were used as controls. Weight gain of FF litters decreased and signs of Zn deficiency increased as the foster dam's days of lactation increased. The Zn concentration of kidney, brain and plasma tended to decrease with increasing lactation days of the foster dam. Tibia Zn concentration declined progressively as the lactation days of the foster dam increased, and the concentration of calcium also was lower in all three groups of FF litters than in controls, indicating that bone calcification may have been impaired. Kinetic data proved to be a more sensitive index of Zn status than tissue Zn concentration. The biological half-life of whole-body 65Zn for FF suckling mice increased in a linear fashion with increasing lactation days of the foster dam; whole-body retention of 65Zn for back-fostered litters did not differ from that for controls. Brain, small intestine, kidney muscle, plasma and tibia of pups FF to late-lactating dams seemed to have greater retention of Zn than did controls. Thus, deprivation of early milk impaired growth and development of the mouse neonate despite some ability to conserve Zn.

The turnover of 65Zn was examined in mice. During pregnancy, lactation, and growth, the loss of 65Zn from the body was twice as great as that observed in mature, nonpregnant animals. The results of this study emphasize the need for consideration of zinc losses in the formulation of the minimum daily requirement for zinc in humans and other mammalian species.

Zinc metabolism of adult female C57BL/6J mice varying in age and reproductive status was studied using 65Zn. Animals were injected intraperitoneally with isotope, and whole-body and tissue turnover of 65Zn was measured. Biological half-life of whole-body 65Zn for pregnant females was 129% of that for nonpregnant, nonlactating females of similar age (25.7 and 20.0 d, respectively). Conversely, the half-life of zinc was less for lactating (11.1 d) and aged (15.3 d) females than for either pregnant or nonpregnant, nonlactating young adult females. Retention of 65Zn was generally lower for all tissues of pregnant and lactating dams than for nonpregnant, nonlactating females except for brain and tibia. In aged females, specific activity at 20 d postinjection did not differ from that of nonpregnant, nonlactating young adult females for any tissue except bone. While specific activity of tibia increased during pregnancy and lactation, it was lower in aged females than in nonpregnant, nonlactating young adult females. Increased organ content of zinc resulted from changes in zinc concentration or in organ mass or both, and was supported by increased food intake. Thus, greater food intake for these groups than for young adult females contributed to the higher turnover of tissue 65Zn. Differences in concentrations of Cu, Ca and Mg were also observed among groups for some tissues; the significance of these differences and their relationship to zinc metabolism are not clear.

Context: The efficiency of the cow–calf sector could be enhanced by matching cow biological type to the production environment; however, methods to estimate the biological efficiency of grazing beef cows are not available. Aims: This study utilised a mathematical nutrition model for ranking beef cows for estimated biological efficiency, determining energetic efficiency and evaluate relationships with other production traits. Methods: Cow live weight, calf birth and weaning weight, calf birth and weaning date, and forage nutritive value of hay and pasture were collected for 69 Brangus crossbred beef cows over a complete production cycle. The Cattle Value Discovery System for Beef Cow (CVDSbc) model was used to compute metabolisable energy required (MER) for the cow, and energy efficiency index (EEI) was computed as the ratio of MER to calf weaning weight. Pearson correlation coefficients were computed among performance traits. During late lactation and gestation, low (n = 8) and high (n = 8) EEI cows were individually fed ad libitum for 44 and 32 days, respectively, then fed 0.5× the estimated metabolisable energy required for maintenance for 7 days (gestation experiment only). Apparent nutrient digestibility, heat production, and milk yield were measured. Key results: EEI was strongly negatively correlated (P < 0.05) with model predicted peak milk (−0.62) and calf weaning weight (−0.65), but moderately correlated (P < 0.05) with cow live weight (0.46). Dry matter intake was not different (P > 0.75) between low and high EEI cows even though low EEI cows weighed less (P < 0.05) during late lactation and gestation experiments. Low EEI cows tended to have greater efficiency of metabolisable energy use for maintenance and gain (P < 0.10), and EEI was negatively correlated (P < 0.05) with the efficiency of metabolisable energy use for maintenance (−0.56) and gain (−0.57). Conclusion: The CVDSbc model identified cows that weaned heavier calves due to greater dry matter intake of cows relative to live weight allowing more energy apportioned towards lactation, and more efficient use of metabolisable energy for maintenance and gain. Implications: Energy efficiency index might provide a logical assessment of biological efficiency of beef cows in grazing production systems. Beef cows consume 70% of feed required for beef production and 70% of this feed is used just to maintain the breeding female. Measuring feed consumption in grazing beef cows is not accurate or practical. Modelling approaches can be used to identify cows that wean more weight of calf per unit of feed required allowing genetic selection of more efficient cows. [ABSTRACT FROM AUTHOR]