D.R. Ouellet

The effect of intake on protein metabolism across splanchnic tissues in growing beef steers

The contribution of the total splanchnic tissue (TSP; portal-drained viscera (PDV) plus liver) to whole-body protein metabolism was estimated in relation to intake (0.6, 1.0 and 1.6 x maintenance requirements), in six multicatheterized growing beef steers used in a double 3 x 3 Latin square design. At the end of each 21 d experimental period, [1-13C]leucine was infused into a jugular vein (1.05 mmol/h for 5 h, preceded by a priming dose of 1.05 mmol). Arterial, portal and hepatic blood samples were collected hourly during the infusion. The increment in TSP leucine irreversible loss rate (ILR) observed with increasing intake reached significance (P < 0.10) only for PDV, while whole-body ILR increased markedly (P < 0.001) with intake. The relative contribution of TSP to whole-body leucine ILR averaged 44% (25% from PDV and 19% from the liver). Although these proportions were not affected by intake, on an incremental basis more than 70% of the increase of whole-body leucine ILR between the 0.6 and 1.0 x maintenance originated from the changes in TSP ILR, while the corresponding value was below 13% between 1.0 and 1.6 x maintenance. Total whole-body leucine oxidation and fractional oxidation increased (P < 0.05) with intake. Protein retention increased with intake (P < 0.01), as a result of a greater increase in protein synthesis compared with protein degradation. Protein breakdown had a major impact on protein turnover as 65% of the protein synthesized was degraded when intake varied from 1.0 to 1.6 x maintenance. Net leucine portal absorption increased (P < 0.001) with intake and represented 1, 16 and 23% of whole body leucine ILR, for 0.6, 1.0 and 1.6 x maintenance, respectively. Although leucine oxidation was not a major component of whole body ILR (9.3-19.9%), it represented 69% of the net available leucine (portal absorption) even at 1.6 x maintenance. The lower relative contribution of the TSP to whole-body leucine ILR at higher intake indicates the proportional increase in the metabolic activity of peripheral tissues as the animals moved into positive protein balance.

Abomasal amino acid infusion in postpartum dairy cows: Effect on whole-body, splanchnic, and mammary amino acid metabolism.

Nine Holstein cows with rumen cannulas and indwelling catheters in splanchnic blood vessels were used in a generalized randomized incomplete block design with repeated measures to study the effect of increased early postpartum AA supply on splanchnic and mammary AA metabolism. At calving, cows were blocked according to parity (second and third or greater) and allocated to 2 treatments: abomasal infusion of water (CTRL; n=4) or free AA with casein profile (AA-CN; n=5) in addition to a basal diet. The AA-CN infusion started with half of the maximal dose at the calving day (1 d in milk; DIM) and then steadily decreased from 791 to 226 g/d until 29 DIM. On 5, 15, and 29 DIM, 6 sample sets of arterial, portal, hepatic, and mammary blood were taken at 45-min intervals. Over the whole period, increasing AA supply increased milk (+7.8 ± 1.3 kg/d) and milk protein yields (+220 ± 65 g/d) substantially. The increased milk yield was not supported by greater dry matter intake (DMI) as, overall, DMI decreased with AA-CN (-1.6 ± 0.6 kg/d). Arterial concentrations of essential AA were greater for AA-CN compared with CTRL. The net portal-drained viscera (PDV) release of His, Met, and Phe was greater for AA-CN compared with CTRL, and the net PDV recovery of these infused AA ranged from 72 to 102% once changes in DMI were accounted for. The hepatic removal of these AA was increased equivalently to the increased net PDV release, resulting in an unaltered net splanchnic release. The net PDV release of Ile, Leu, Val, and Lys tended to be greater for AA-CN, and the net PDV recovery of these infused AA ranged from 69 to 73%, indicating increased PDV metabolism with AA-CN. The fractional hepatic removal of these AA did not differ from zero and was unaffected by the increased supply. Consequently, the splanchnic release of these AA was approximately equivalent to their net PDV release for both CTRL and AA-CN. Overall, greater early postpartum AA supply increased milk and milk protein yields substantially based on increased mammary AA uptake. The PDV metabolism of branched-chain AA and Lys were increased, whereas it seemed to be unaffected for other essential AA when the intestinal AA supply was increased. On a net basis, the liver removed more group 1 AA (His, Met, Phe, and Trp) for anabolism and catabolism when the early postpartum AA supply was increased. Thus, increasing the postpartum AA supply increased splanchnic and mammary consumption of AA; hence, the protein deficiency persisted.

Amino acid composition of rumen bacteria and protozoa in cattle

Because microbial crude protein (MCP) constitutes more than 50% of the protein digested in cattle, its AA composition is needed to adequately estimate AA supply. Our objective was to update the AA contributions of the rumen microbial AA flowing to the duodenum using only studies from cattle, differentiating between fluid-associated bacteria (FAB), particle-associated bacteria (PAB), and protozoa, based on published literature (53, 16, and 18 treatment means were used for each type of microorganism, respectively). In addition, Cys and Met reported concentrations were retained only when an adequate protection of the sulfur groups was performed before the acid hydrolysis. The total AA (or true protein) fraction represented 82.4% of CP in bacteria. For 10 AA, including 4 essential AA, the AA composition differed between protozoa and bacteria. The most noticeable differences were a 45% lower Lys concentration and 40% higher Ala concentration in bacteria than in protozoa. Differences between FAB and PAB were less pronounced than differences between bacteria and protozoa. Assuming 33% FAB, 50% PAB, and 17% of protozoa in MCP duodenal flow, the updated concentrations of AA would decrease supply estimates of Met, Thr, and Val originating from MCP and increase those of Lys and Phe by 5 to 10% compared with those calculated using the FAB composition reported previously. Therefore, inclusion of the contribution of PAB and protozoa to the duodenal MCP flow is needed to adequately estimate AA supply from microbial origin when a factorial method is used to estimate duodenal AA flow. Furthermore, acknowledging the fact that hydrolysis of 1 kg of true microbial protein yields 1.16 kg of free AA substantially increases the estimates of AA supply from MCP.

Casein infusion rate influences feed intake differently depending on metabolizable protein balance in dairy cows: A multilevel meta-analysis

The effects of casein infusion have been investigated extensively in ruminant species. Its effect on responses in dry matter intake (DMI) has been reviewed and indicated no significant effect. The literature reviewed in the current meta-analysis is more extensive and limited to dairy cows fed ad libitum. A total of 51 studies were included in the meta-analysis and data were fitted to a multilevel model adjusting for the correlated nature of some studies. The effect size was the mean difference calculated by subtracting the means for the control from the casein-infused group. Overall, casein infusion [average of 333 g of dry matter (DM)/d; range: 91 to 1,092 g of DM/d] tended to increase responses in DMI by 0.18 kg/d (n=48 studies; 3 outliers). However, an interaction was observed between the casein infusion rate (IR) and the initial metabolizable protein (MP) balance [i.e., supply minus requirements (NRC, 2001)]. When control cows were in negative MP balance (n=27 studies), responses in DMI averaged 0.28 kg/d at mean MP balance (-264 g/d) and casein IR (336 g/d), and a 100g/d increment in the casein IR from its mean increased further responses by 0.14 kg/d (MP balance being constant), compared with cows not infused with casein. In contrast, when control cows were in positive MP balance (n=22 studies; 2 outliers), responses in DMI averaged -0.20 kg/d at mean casein IR (339 g/d), and a 100g/d increment in the casein IR from its mean further decreased responses by 0.33 kg/d, compared with cows not infused with casein. Responses in milk true protein yield at mean casein IR were greater (109 vs. 65 g/d) for cows in negative vs. positive MP balance, respectively, and the influence of the casein IR on responses was significant only for cows in negative MP balance. A 100g/d increment in the casein IR from its mean increased further responses in milk true protein yield by 25 g/d, compared with cows not infused with casein. Responses in blood urea concentration increased in casein studies (+0.59 mM) and the influence of the casein IR was greatest for cows in positive MP balance (0.26 vs. 0.11 mM per 100g/d increment). Responses in DMI were also correlated negatively with responses in blood urea concentration only for cows in positive MP balance. Together, these results suggest an association between satiety and deamination and oxidation of AA supplied in excess of requirements for cows in positive MP balance. Therefore, casein stimulated appetite in cows fed MP-deficient diets possibly via the supply of orexigenic AA or through a pull effect in response to an increased metabolic demand. Conversely, casein induced satiety in cows fed diets supplying MP in excess of requirements. Not precluding other factors involved in satiety (e.g., insulin, gut peptides), casein could have increased the supply of AA (e.g., Ser, Thr, Tyr), which might depress appetite at the brain level or increase the deamination and the oxidation of AA in oversupply in agreement with the hepatic oxidation theory.

Effect of method of conservation of timothy on endogenous nitrogen flows in lactating dairy cows

The effect of the method of conservation of forage on endogenous N (EN) secretion was studied using a 15N isotope dilution technique in 4 lactating Holstein cows selected from a replicated 3x3 Latin square. Cows were equipped with ruminal, duodenal (n=4), and ileal (n=2) cannulas. Diets comprised 44% concentrate plus first-cut timothy conserved either as hay or as restrictively (formic) or extensively (inoc) fermented silage. Crude protein contents of hay, formic, and inoc averaged 10.4, 13.6, and 14.8%, respectively. Total EN flow and free EN at the duodenum were increased with hay compared with silages but were similar when expressed as proportion of duodenal N flow, with total EN flow averaging 25.8, 23.9, and 23.9% for hay, formic, and inoc, respectively, and free EN at the duodenum averaging 11.5, 9.8, and 9.7% for hay, formic, and inoc, respectively. Flow of bacterial N at the duodenum originating from an endogenous source tended to be higher with inoc compared with formic. Overall, the proportion of bacterial N derived from endogenous sources and urea was similar between treatments, averaging 23 and 15%, respectively. In the feces, flow of EN was similar across treatments and averaged 31% of total fecal N. More than 70% of fecal EN originated from undigested secretions into the forestomach. Absorption of N from the forestomach tended to increase for silages compared with hay. In conclusion, EN represented an important fraction of N flowing at the duodenum and in the feces. The free EN and the total EN at the duodenum were altered by the different methods of forage conservation studied. Estimation of true dietary N supply and requirements of the dairy cow should allow for endogenous N flows and losses.

Estimation of allantoin flux using continuous infusion of [ 14 C]allantoin: sensitivity to plasma loading with unlabelled allantoin

Allantoin net flux through the plasma allantoin compartment was determined in sheep given a roughage diet by means of a continuous infusion of [4,5-14C]allantoin for 17 h. Unlabelled allantoin was infused intravenously during the last 7 h of the tracer infusion to increase the allantoin flux by approximately 75 %. When unlabelled allantoin was infused, the specific radioactivity of allantoin in plasma and urine declined exponentially to approach a lower plateau some 2-3 h later. The estimate of net flux during the infusion of unlabelled allantoin, estimated from blood plasma and urine, was on average 79 and 90 % of expected values. Expected values of allantoin net flux during infusion of unlabelled allantoin were calculated as the sum of allantoin net flux pre-loading plus the known rate of infusion of the allantoin load. It is probable that endogenous allantoin synthesis was decreased by the infusion of allantoin: allosteric inhibition of uricase appears a plausible explanation for this observation, and for lower estimates of net flux. Appearance of labelled allantoin-C in ruminal or blood bicarbonate was negligible. Our results indicate that net flux of allantoin through blood plasma is a good predictor of the entry rate of allantoin into the primary compartment and should be a better predictor of rumen microbial outflow than urinary allantoin excretion. However, measurements of allantoin-specific radioactivity, during continuous infusion, should be taken after a period of 24 h, at which time the true plateau specific radioactivity value of allantoin in plasma would be attained.

Relationships between postruminal casein infusion and milk production, and concentrations of plasma amino acids and blood urea in dairy cows: A multilevel mixed-effects meta-analysis

The relationships between postruminal casein infusion and production variables and concentrations of plasma AA and blood urea were evaluated using multilevel mixed-effects models derived from literature data collected in dairy cows. The data set contained 147 treatment means [i.e., 66 controls (CTL) and 81 casein-infused (CAS) means]. Each CAS mean was paired with its corresponding CTL mean to create 81 mean differences (CAS minus CTL), which were analyzed as absolute and percentage-based units (i.e., percentage increase or decrease in CAS relative to CTL). The primary variable of interest was the difference in estimated metabolizable protein (MP) supply (ΔMP) between CAS and CTL. The other explanatory variables were based on levels in CTL: MP supply, MP balance, the ratio of duodenal microbial protein (MCP) to MP supply (MCPMP), the stage of lactation (early or mid/late) and the type of forage (grass/legume- or corn silage-based). The MP supply and MP balance influenced negatively the relationship between ΔMP and the response of true protein yield. Responses of milk urea, blood urea, and plasma urea cycle AA concentrations were associated positively with ΔMP, indicating that a large amount of infused AA was catabolized to urea. Responses of plasma essential AA concentrations were related positively to ΔMP. The relative effect of ΔMP was highest for responses of plasma His concentration in cows fed grass/legume-based diets and at high MCPMP ratios. This relationship suggests that positive responses of plasma His concentrations are associated with diets relying heavily on microbial protein synthesis (high MCP), low in crude protein (low estimated MP supply), or both. The relationship between ΔMP and responses of plasma group 2 AA (Ile, Leu, Lys, and Val) concentrations was approximately 2 times greater than that for group 1 AA (His, Met, and Phe+Tyr) at mean MCPMP and MP supply. This could reflect the low hepatic removal group 2 AA compared with group 1 AA in dairy cows. Collectively, these results provide novel information on how dietary and cow conditions may alter responses to protein supplementation.