The maximum capacity of the liver of the adult dairy cow to metabolize ammonia

Slow-Release Urea as a Sustainable Alternative to Soybean Meal in Ruminant Nutrition

Three experiments were conducted to evaluate the feasibility of using a commercial slow-release urea product (SRU; Optigen®, Alltech Inc., Nicholasville, KY, USA) as a partial replacement for vegetable protein sources in cattle diets. The first experiment was an in vitro rumen fermentation that evaluated the effect of replacing soybean meal (SBM) nitrogen with nitrogen from either SRU or free urea in diets varying in forage:concentrate ratios. The second experiment examined the effect of replacing SBM with SRU on in situ dry matter and nitrogen degradability in the rumen. In the third experiment, a feeding trial was conducted to evaluate the effect of replacing SBM (0% as-fed SRU) with 1% or 3% as-fed SRU on feed carbon footprint (CFP; total greenhouse gas emissions associated with the life cycle of feed raw materials) and the toxicity potential of SRU in growing beef cattle. Results showed that replacing SBM with SRU up to 1.3% did not negatively affect in vitro rumen fermentation parameters. Supplementing SRU favourably decreased ruminal accumulation of ammonia and lactic acid when compared to free urea. There was no significant effect on effective rumen degradability of dry matter and nitrogen when one-third of SBM was replaced by SRU in the in situ study. Compared with the 0% SRU diet, feed CFP decreased by 18% and 54% in 1% SRU and 3% SRU diets, respectively. Additionally, feeding up to 3% SRU diet to beef cattle did not affect health and intake, and blood hematological and biochemical indices were within the physiological range for healthy bulls, suggesting no indication of ammonia toxicity. Overall, these results indicate that SRU can be used as a sustainable alternative to partially replace vegetable protein sources in ruminant diets without compromising rumen function and health of ruminants.

Rumen metabolism and nitrogen flow to the small intestine in steers offeredLolium perennecontaining different levels of water-soluble carbohydrate

Eight Hereford ✕ Friesian steers were used to investigate the effect of feeding Lolium perenne (L) forage containing elevated levels of water-soluble carbohydrate (WSC) on rumen metabolism and nitrogen (N) absorption from the small intestine. The steers were offered ad libitum access to one of two varieties with matched heading dates (Ba11353, high WSC, HS; AberElan, intermediate WSC, control) cut at different times of the day to accentuate WSC differentials, zero-grazed for 21 days. This was followed by a 14-day period where the animals were on grass silage to provide a covariate intake. Although the total N concentration was similar for the two grasses, all other measured values were significantly different. The dry matter (DM) concentration of HS was greater than that of the control (202 v. 167 g DM per kg; P 0·01). WSC and in-vitro dry matter digestibility (IVDMD) were 243 and 161 g/kg DM, and 0·61 and 0·56 for HS and control, respectively. In contrast, acid- and neutral-detergent fibre were 251 and 296 g/kg DM and 480 and 563 g/kg DM for HS compared with control, respectively. DM intake was increased (9·3 v. 6·7 kg/day; P 0·001) for HS animals and this contributed significantly towards higher flows of non-ammonia N to the duodenum as well as increased absorption of amino acids from the small intestine. This DM intake response was partly due to the elevation in DM concentration of HS. However fresh weight intake was increased proportionately by ca. 0·15 (P 0·05) in animals on HS compared with control. Rumen ammonia levels were lower (14·0 and 26·4 mg N per l; P 0·001) and concentrations of rumen propionate higher (P 0·01) and acetate lower (P 0·01; increasing the glucogenic: lipogenic volatile fatty acid ratio) in animals on HS compared with control. However, the efficiency of microbial protein synthesis (15·9 and 17·8 g microbial nitrogen per kg organic matter apparently digested) and flow of N to the duodenum per unit N intake (0·84 and 0·93) for HS and control, respectively, were similar across both diets.

The rate and pattern of urea infusion into the rumen of wethers alters nitrogen balance and plasma ammonia

Changes in N balance, urinary excretion of purine derivative (PD), urea, creatinine and ammonia and plasma ammonia, glucose, urea, insulin and IGF-1 were examined in four wethers (37 ± 2.6 kg BW). The animals were fitted with permanent ruminal catheters, fed lucerne hay (9.4 MJ/day; 23 g N/day; 7 g soluble N/day, 6 equal meals/day) and treated with contrasting rates of urea infusion into the rumen: first, a continuous infusion (CT), at 3.2 mg urea-N/min for 10 days and then a discontinuous infusion (DT) at 156 mg urea-N/min for 4 min; in 6 daily doses with the meals for 7 days. N balance was calculated from pooled samples of faeces and urine. Jugular blood samples were collected before and 1.5 h after the morning meal (M1) on days CT10, DT2, DT4 and DT6. N retention decreased during DT (p = 0.01) due to a significant increase of N excretion in urine (4 g/day; p = 0.009) and faeces (1 g/day; p = 0.02). Dry matter (p < 0.001) and N digestibility in vivo (p = 0.01) decreased significantly during DT. Urinary urea and PD excretion were not altered by treatment. Significant linear (p = 0.004) and quadratic (p = 0.001) effects were observed for plasma ammonia in M1 (from 170 CT10 to 235 μm DT2 and returned to 120 μm DT6). No changes were observed in plasma glucose, urea, insulin and IGF-1. Results indicate that changes from CT to DT reduced N retention in sheep due to enhanced urinary N excretion, but it was not associated with changes in urinary urea or PD excretion; or plasma concentrations of insulin and IGF-1. As the dry matter (DM) an N digestibility could account a 0.23 of the decrease in N retention; the largest fraction of the reduction in N retention remained unexplained by the results.

Ruminant glycogen metabolism

The biochemistry of glycogen metabolism is well characterised, having been extensively studied in laboratory rodents and humans, and from this stems the bulk of our knowledge regarding the metabolism of glycogen in ruminants. With respect to intermediary metabolism, the key tissues include the liver and muscle. The liver glycogen depot plays a central role in intermediary metabolism, storing and mobilising glycogen during the fed and fasted metabolic states, with these responses modulated during pregnancy, lactation, and exercise. Alternatively, the muscle glycogen depot is particularly important for local energy homeostasis, and is likely to be less important as a key post-prandial sink for blood glucose given the reduced absorption of glucose from the gut in ruminant animals. Yet similar to the liver, this depot is also in a constant state of turnover, with the muscle glycogen concentration at any point in time a reflection of the rates of glycogen synthesis and degradation. Muscle glycogen metabolism attracts particular attention given its importance for post-mortem acidification of muscle tissue, with a shortage at slaughter leading to dark cutting meat. Simplistically the concentration of muscle glycogen at slaughter is a function of two key factors, the on-farm starting levels of glycogen minus the amount depleted during the pre-slaughter phase. On-farm concentrations of muscle glycogen are largely a reflection of metabolisable energy intake driving increased rates of muscle glycogen synthesis. Compared with simple-stomached species the rate of glycogen synthesis within ruminants is relatively low. Yet there also appears to be differences between sheep and cattle when fed diets of similar metabolisable energy, with cattle repleting muscle glycogen more slowly after depletion through exercise. While metabolisable energy intake is the key driver, genetic and age-related factors have also been shown to influence glycogen repletion. The amount of muscle glycogen depleted during the pre-slaughter phase is largely associated with stress and adrenaline release, and several recent studies have characterised the importance of factors such as exercise, age and genetics which modulate this stress response. This paper presents a summary of recent experiments in both cattle and sheep that highlight current developments in the understanding of this trait.

The partial replacement of soyabean meal and rapeseed meal with feed grade urea or a slow-release urea and its effect on the performance, metabolism and digestibility in dairy cows

The objectives of the study were to determine the effect of the partial replacement of soyabean meal and rapeseed meal with feed grade urea or a slow-release urea on the performance, metabolism and whole-tract digestibility in mid-lactation dairy cows. Forty-two Holstein-Friesian dairy cows were allocated to one of three dietary treatments in each of three periods of 5 weeks duration in a Latin square design. Control (C) cows were offered a total mixed ration based on grass and maize silages and straight feeds that included 93 g/kg dry matter (DM) soyabean meal and 61 g/kg DM rapeseed meal. Cows that received either of the other two treatments were offered the same basal ration with the replacement of 28 g/kg DM soyabean and 19 g/kg DM rapeseed meal with either 5 g/kg DM feed grade urea (U) or 5.5 g/kg DM of the slow-release urea (S; Optigen®; Alltech Inc., Kentucky, USA), with the content of maize silage increasing. There was no effect (P > 0.05) of dietary treatment on DM intake, which averaged 22.5 kg/day. Similarly, there was no effect (P > 0.05) of treatment on daily milk or milk fat yield but there was a trend (P = 0.09) for cows offered either of the diets containing urea to have a higher milk fat content (average of 40.1 g/kg for U and S v. 38.9 g/kg for C). Milk true protein concentration and yield were not affected by treatment (P > 0.05). Milk yield from forage and N efficiency (g milk N output/g N intake) were highest (P < 0.01) in cows when offered S and lowest in C, with cows receiving U having intermediate values. Cows offered S also tended to have the highest live weight gain (0.38 kg/day) followed by U (0.23 kg/day) and C (0.01 kg/day; P = 0.07). Plasma urea concentrations were higher (P < 0.05) at 2 and 4 h post feeding in cows when offered U and lowest in C, with animals receiving S having intermediate values. There was no effect (P > 0.05) of treatment on whole-tract digestibility. In conclusion, the partial replacement of soyabean meal and rapeseed meal with feed grade urea or a slow-release urea can be achieved without affecting milk performance or diet digestibility, with the efficiency of conversion of dietary N into milk being improved when the slow-release urea was fed.

Experimental soybean meal intoxication in cattle

BACKGROUND

Cattle are commonly fed soybean meal (SBM) and accidental intoxication sometimes occurs.

OBJECTIVES

To describe the biologic and clinical features of SBM intoxication.

ANIMALS

Four steers with ruminal cannula.

METHODS

Controlled experimental trial. SBM was administered once at 1 and 2% of body weight (BW) via cannula at 2-month intervals.

RESULTS

This study showed a 2-phase pathogenic course for 2% BW SBM intoxication. The 1st phase (until 10 hours post-administration) is restricted to ruminal modification with volatile fatty acid overproduction and moderate ruminal ammonia concentration. In the 2nd phase (12-22 hours post-administration), ruminal pH returned to initial values and marked ammonia accumulation occurred in blood, inducing severe metabolic alkalosis with hyperglycemia, hyperinsulinemia, and delayed aciduria (30-40 hours post-administration). Among the clinical signs, nervous signs were only observed during the period with increased plasma ammonia concentration. At 1% BW, ruminal and blood modifications were less pronounced than at 2% BW, and clinical signs were not observed.

CONCLUSIONS AND CLINICAL RELEVANCE

Ammonia accumulation in blood during the second phase is the consequence of continued ammonia production, decreased carbohydrate fermentation, and overwhelming of hepatic detoxifying capacity. Because ammonia accumulation is associated with the clinical signs, treatment of SBM intoxication could be similar to treatment of urea intoxication, including rumenotomy, oral administration of cold water and vinegar, and measurement of ruminal pH.

Resposta de vacas leiteiras à substituição parcial de farelo de soja por ureia encapsulada

Avaliou-se a substituição parcial de farelo de soja na dieta-controle por ureia encapsulada ou por ureia, ambos acrescidos de polpa cítrica. Dezoito vacas foram alocadas a uma sequência de três tratamentos, em seis quadrados latinos 3x3, com períodos de 21 dias. O teor de proteína bruta nas dietas foi 15,5%, cerca de 1,5% oriundo de nitrogênio não proteico (NNP). O uso de NNP reduziu o consumo diário de matéria seca em 0,8kg (P=0,04), sem influenciar a produção de leite (31,5kg, P=0,98). A conversão do alimento em leite foi menor na dieta-controle (P<0,09). As fontes de NNP aumentaram o teor de N-ureico no plasma (NUP) duas horas após a alimentação da manhã (P=0,06). A frequência de NUP acima de 22,0mg/dL foi maior com ureia (P<0,01), similarmente ao observado para o teor e a secreção diária de N-ureico no leite (P<0,01). A substituição de farelo de soja por NNP e polpa cítrica melhorou a eficiência alimentar, sem afetar o balanço de nitrogênio.

The effect of short-term hyperammonaemia on milk synthesis in dairy cows

To test the hypothesis that ammonia detoxification in ruminants consumes amino acids to the detriment of milk protein production, we infused four lactating dairy cows with ammonium acetate or sodium acetate in switchback experiments. Plasma ammonia concentrations increased to 411 μm within 1 h of the start of infusion of ammonium acetate at 567 mmol/h. The rate constant for ammonia clearance from plasma was 0·054/min and the half-life was 12·9 min. Infusion at 567 mmol/h for 1 h followed by 1 h without infusion, repeated four times between am- and pm-milking, caused a decrease in feed intake. Compared with sodium acetate, continuous infusion of ammonium acetate at 360 mmol/h throughout an entire 10-h milking interval increased plasma ammonia concentrations to 193 μm and caused a 20% decrease in milk, protein and lactose production with no effect on percentage composition of milk or the yield of milk fat. Arterial concentrations of glucose and non-esterified fatty acids tended to increase; there was no effect on arterial acetate, β-hydroxybutyrate or triacylglcerol, and branched-chain amino acids, Lys and Thr decreased. Mammary plasma flow, estimated by assuming 100% uptake/output of Phe+Tyr, was significantly correlated with milk yield. Mammary uptakes of acetate tended to be reduced by hyperammonaemia, but uptakes of other energy metabolites and amino acids were not affected. Thus, while an increase in amino acid consumption during hyperammonaemia was apparent from the drop in circulating concentrations of Leu, Ile, Val, Lys and Thr, there was no evidence to support the hypothesis that milk yield is affected by the lower concentrations. An ammonia-induced depression in feed intake may have caused the decrease in milk synthesis.

The route of absorbed nitrogen into milk protein

A database reviewing the metabolism of nitrogen (N) compounds from absorption to milk has been compiled from 14 published and unpublished studies (33 treatments) that measured the net flux of N compounds across the splanchnic tissues in dairy cows. Apparent N digestibility averaged 0·65, with this then partitioned between 0·34 excreted in urine and 0·31 secreted as milk.

Nitrogen metabolites are absorbed from the lumen of the gut into the portal vein, mainly as free amino acids (AA) and ammonia; these represented 0·58 and 0·57 of digested N, respectively. All of the ammonia absorbed was removed by the liver with, as a result, a net splanchnic flux of zero. Detoxification of ammonia by the liver and catabolism of AA results in production of urea as an end-product. Hepatic ureagenesis is a major cross-road in terms of whole body N exchange, being the equivalent of 0·81 of digested N. Therefore, salvage of a considerable part of this ureagenesis is needed to support milk protein synthesis. This salvage occurs via transfer of urea from the blood circulation into the lumen of the gut. On average, 0·47 of hepatic ureagenesis was returned to the gut via the portal-drained viscera (equivalent to 0·34 of digested N) with 0·56 of this then used for anabolic purposes e.g. as precursor N for microbial protein synthesis. On average, 0·65 of estimated digestible AA was recovered in the portal vein. This loss (0·35) is due to oxidation of certain AA across the gut wall and non-absorption of endogenous secretions. The magnitude of this loss is not uniform among AA and varies between less than 0·05 for histidine to more than 0·90 for some non-essential AA, such as glutamine.

A second database (six studies, 14 treatments) was constructed to further examine the subsequent fate of absorbed essential AA. When all AA are aggregated, the liver removed, on average, 0·45 of portal absorption but this value hides the considerable variation between individual AA. Simplistically, the AA behave as two major groups: one group undergoes very little hepatic removal and includes the branched-chain AA and lysine. For the second group, removal varies between 0·35 and 0·50 of portal absorption, and includes histidine, methionine and phenylalanine. For both groups, however, the efficiency of transfer of absorbed AA into milk protein decreases with increasing supply of protein. This loss of efficiency is linked directly with increased hepatic removal of AA from the second group and, probably, increased catabolism by peripheral tissues, including the mammary gland, of AA from the first group. Therefore, we must stop using fixed factors of conversion of digestible AA to milk in our predictive schemes and acknowledge that metabolism of AA between delivery from the duodenum and conversion to milk protein will vary with nutrient supply. New information evolving from re-analysis of the literature and recent studies will allow better models to be devised for the prediction of nutrient-based responses by the lactating cow. Consideration of biological efficiency, rather than maximal milk yield, will lead to systems that are economically more sensible for the farmer and that have better environmental impacts.

Effects of ammonia load on methionine utilization by growing steers

Seven ruminally cannulated Holstein steers (194 +/- 16 kg) housed in metabolism crates were used in a 6 x 6 Latin square, with one additional steer, to study effects of ruminal ammonia load on methionine (Met) use. All steers received a diet based on soybean hulls (2.6 kg DM/d), ruminal infusions of 200 g/d of acetate, 200 g/d of propionate, and 50 g/d of butyrate, as well as abomasal infusion of 300 g/d of glucose to provide energy without increasing microbial protein supply, and abomasal infusions of a mixture (248 g/d) of all essential AA except Met. Treatments were arranged as a 3 x 2 factorial and included urea (0, 40, or 80 g/d) infused ruminally to supply metabolic ammonia loads and Met (2 or 5 g/d) infused abomasally. Supplementation with the greater amount of Met decreased (P < 0.05) urinary N excretion from 68.8 to 64.8 g/d and increased (P < 0.05) retained N from 22.0 to 27.5 g/d. Urea infusions linearly increased (P < 0.05) urinary N excretions, plasma urea concentrations, and urinary urea excretions, but retained N was not affected. The efficiency of deposition of supplemental Met, calculated by assuming that Met deposition is 2.0% of protein deposition (6.25 x retained N), ranged between 18 and 27% when steers received 0 or 80 g/d of urea, respectively. There were no (P > or = 0.40) effects of treatments on serum insulin or IGF-I concentrations. In our model, increasing ammonia load did not affect whole-body protein deposition in growing steers when Met was limiting.

Reviewing the low efficiency of protein utilization in heavy preruminant calves – a reductionist approach

The efficiency of protein utilization for growth in preruminant calves is decreasing with increasing body weight. In contrast to calves weighing less than 100 kg of body weight, heavy preruminant calves do not respond in protein retention to an increased intake of indispensable amino acids in dose-response studies. The marginal efficiency of protein utilization is low compared with pigs and milk-fed lambs at a similar stage of maturity. A reductionist approach was taken to perceive the potential mechanisms for the low protein utilization in preruminant calves. Neither an imbalance in the dietary protein to energy ratio nor a single limiting indispensable amino acid was responsible for the low efficiency. Also, amino acids were not specifically used to detoxify ammonia. Alternative hypotheses to explain the low efficiency are discussed and result in (i) a reduced post-absorptive supply of amino acids: e.g. by fermentation of milk in the (premature) rumen or preferential amino acid utilization by specific tissues; or (ii) a reduced post-absorptive amino acid utilization: e.g. by decreased insulin sensitivity, utilization of amino acids for gluconeogenesis or an asynchronous nutrient supply. In conclusion, several mechanisms for the low efficiency of protein utilization in heavy preruminant calves were excluded. Other physiological processes which are potentially involved remain to be studied, because the large potential for improving protein utilization in heavy preruminant calves asks for further exploration of their amino acid metabolism.

Ruminal ammonia load affects leucine utilization by growing steers

Six ruminally cannulated Holstein steers (initial BW = 189 +/- 11 kg) housed in metabolism crates were used in a 6 x 6 Latin square to study effects of ruminal ammonia load on Leu utilization. All steers received a diet based on soybean hulls (2.7 kg of DM/d), ruminal infusions of 200 g of acetate/d, 200 g of propionate/d, and 50 g of butyrate/d, as well as an abomasal infusion of 300 g of glucose/d to provide energy without increasing microbial protein supply and an abomasal infusion of a mixture (238 g/d) of all essential AA except Leu. Treatments were arranged as a 3 x 2 factorial and included Leu (0, 4, or 8 g/d) infused abomasally and urea (0 or 80 g/d) infused ruminally. Abomasal Leu infusion linearly decreased (P < 0.05) both urinary and fecal N excretions and linearly increased (P < 0.05) retained N, but the decreases in urinary N excretion in response to Leu tended (P = 0.07) to be greater, and the increases in retained N in response to Leu were numerically greater in the presence of the urea infusion. Although urea infusions increased (P < 0.05) plasma urea concentrations, urinary N excretions, and urinary urea excretions, retained N also was increased (P < 0.05). The efficiency of deposition of supplemental Leu ranged from 24 to 43% when steers received 0 or 80 g of urea/d, respectively. Under our experimental conditions, increasing ammonia load improved whole-body protein deposition in growing steers when Leu supply was limiting.

Effect of forage legumes and anthelmintic treatment on the performance, nutritional status and nematode parasites of grazing lambs

Recent studies in New Zealand and the UK have shown that certain forages reduce parasitic infection in sheep. The aim of this experiment was to investigate the effects of legume forages compared to ryegrass on interactions between production, nutritional status and nematodes in grazing lambs. Twenty-four male lambs per forage treatment, half of which were treated with anthelmintics on Day 0, grazed monocultures of lucerne (Medicago sativa), red clover (Trifolium pratense) and white clover (Trifolium repens) and were compared with lambs grazing perennial ryegrass (Lolium perenne). Individual faecal egg counts (FEC) and liveweight were determined every 7 days for 56 days, after which half the lambs were slaughtered to determine total nematode intensities (TNI). Results showed that lambs grazed on red or white clover, but not lucerne, had lower pooled mean FEC and improved liveweight performance compared to lambs grazing ryegrass. Lambs treated with anthelmintics had higher TNI compared to lambs not treated, due to a trend for more adult nematodes in lambs grazing lucerne and treated with anthelmintics than all other lambs, except those grazing red clover and also given anthelmintics. Lambs grazing white clover tended to have fewer adult nematodes than lambs grazing other forages. Examination of the nematode species showed a change in female T. circumcincta occurred in all lambs following anthelmintic treatment and that the forage species grazed by lambs affected individual parasite species. Lambs grazing white clover had fewer male and adult T. circumcincta compared to lambs grazing other forages, and lambs grazing lucerne had fewer adult T. circumcincta compared to lambs grazing ryegrass or red clover. Data on small intestine TNI showed that lambs grazing lucerne and given anthelmintics had more male adult nematodes than other lambs, except those grazing red clover and treated with anthelmintics. Results indicate that lucerne and red clover both increase the re-infection of grazing lambs with Trichostrongylus species compared to ryegrass following anthelmintic treatment. In conclusion, legume forages have the potential to contribute to the control of abomasal but not small intestine nematode parasites in finishing lamb systems.

Effect of elevated systemic concentrations of ammonia and urea on the metabolite and ionic composition of oviductal fluid in cattle

High dietary protein leads to elevated systemic concentrations of ammonia and urea, and these, in turn, have been associated with reduced fertility in cattle. The effect of elevating systemic concentrations of ammonia and urea on the concentrations of electrolytes and nonelectrolytes in bovine oviductal fluid were studied using estrus-synchronized, nulliparous heifers (n = 25). Heifers were randomly assigned to 1 of 3 treatments consisting of jugular vein infusion with either ammonium chloride (n = 8), urea (n = 8), or saline (n = 9). Oviducts were catheterized, and fluid was recovered over a 3-h period on either Day 2 or 8 of the estrous cycle. No difference (P > 0.05) was found in the concentrations of any electrolyte or nonelectrolyte between oviducts ipsi- or contralateral to the corpus luteum. Plasma and oviductal concentrations of urea were increased by infusion with urea (P < 0.001) and ammonium chloride (P < 0.05) but not by saline (P > 0.05). Plasma and oviductal concentrations of ammonia were elevated by infusion with ammonium chloride (P < 0.001) but not by infusion with urea or saline (P > 0.05). No effect (P > 0.05) of treatment was found on oviductal or plasma concentrations of glucose, lactate, magnesium, potassium, or sodium or on plasma concentrations of insulin or progesterone. The concentration of calcium in oviductal fluid was reduced by urea infusion and was negatively associated with systemic and oviductal concentrations of urea. Oviductal concentrations of sodium were higher on Day 8 than on Day 2 (P < 0.05). No effect of sample day was found on any of the other electrolytes or nonelectrolytes measured (P > 0.05). Elevated systemic concentrations of ammonia and urea are unlikely to reduce embryo survival through disruptions in the oviductal environment.

Liver nitrogen movements during short-term infusion of high levels of ammonia into the mesenteric vein of sheep

Four 40 kg wethers were used in a crossover design to quantify, by arterio-venous procedures, the mass transfer of NH3, urea and amino acids (AAs) across the portal-drained viscera and the liver during a 31 min infusion of either 0 (C0) or 1100 (C1100) micromol NH4HCO3/min into the mesenteric vein. In C1100, hepatic NH3 extraction remained stable at 1214 micromol/min (1.90 micromol/min per g wet liver weight), the capacity for hepatic NH3 removal was exceeded by 654 micromol/min and the incremental (C1100-C0) urea-N release: NH3 -N removal ratio increased progressively, from 0.52 to 0.90. The NH4HCO3 infusion reduced total branched-chain AA transfer across the portal-drained viscera and total AA-N and lysine extraction by the liver. Hepatic release of glutamate was augmented ornithine switched from net release to net removal and net splanchnic release of free essential AA (44 micromol/min (sed 9.2), ) and branched-chain AA (33 micromol/min (sed 2.0), ) were reduced to 0.58 of their basal rate. The study showed that conversion of excess NH3 to urea during a short-term hepatic NH3 overload required no additional contribution of AA-N to ureagenesis; essential AA and branched-chain AA supply to non-splanchnic tissues was, however, temporarily decreased.

Effect of long-chain fatty acids on triglyceride accumulation, gluconeogenesis, and ureagenesis in bovine hepatocytes

A relationship between increased lipid concentration in the liver and reduced hepatic function has been suggested; however, there is little direct evidence of change in specific hepatic functions. Hepatocytes were obtained from ruminating calves and were incubated as monolayers for 36 h. The media contained a mixture of nonesterified fatty acids (NEFA) at 0, 0.5, 1.0, and 1.5 mM NEFA with molar proportions of 0.435 oleate, 0.319 palmitate, 0.144 stearate, 0.049 linoleate, and 0.053 palmitoleate. Ureagenesis or gluconeogenesis was measured from 48 to 51 h after plating using hepatocytes that had only previous (12 to 48 h), only concurrent (48 to 51 h), or previous and concurrent (12 to 51 h) exposure to NEFA. A previous 36-h exposure to NEFA caused cell triglyceride accumulation, yielding triglyceride concentrations that corresponded with liver that is clinically described as normal to moderately fatty. Previous, prolonged exposure to NEFA reduced ureagenesis and increased gluconeogenesis. Concurrent exposure to NEFA did not significantly affect gluconeogenesis or ureagenesis and did not alter the residual effect of prolonged incubation with NEFA. Reduced ureagenesis was related to increased cell triglyceride accumulation independently of other direct NEFA effects. Decreased ureagenic capacity may play a role in the morbidity associated with periparturient fatty liver in dairy cows.

Characterization of sheep hepatocytes in primary culture

1. Primary cultures of isolated sheep hepatocytes were used to characterize metabolic functions of liver: gluconeogenesis, ureagenesis and protein synthesis. The rates of all three metabolic activities were linear over a 20 hr culture period. 2. Hepatocytes in the presence of glucagon increased the synthesis of urea by approx 30% (P < 0.05) and increased release of glucose into the medium by 60% (P < 0.05). 3. In the absence of insulin, significantly more (35%; P < 0.05) glucose was released in the medium than in the presence of insulin. 4. Results help evaluate the primary culture of sheep hepatocytes as an appropriate experimental model to study nutritional and hormonal regulation of liver in the ruminant species.

Effects of various supplements on voluntary intake and performance by growing cattle consuming forage moderate to high in crude protein

Growing cattle were supplemented with substances to lessen or increase ruminal ammonia level to determine if consumption of moderate- to high-crude protein (CP) forage is limited by high ruminal ammonia absorption. In Experiment 1 (5 x 5 Latin square), five Holstein steers (198 kg) had ad libitum access to alfalfa cubes (17% CP) without supplementation or with corn at 0.25 or 0.75% body weight (BW) alone (LC or HC) or with 0.06% BW of zeolite (chabazite; LC-Z and HC-Z). Total daily organic water (OM) intake was not affected by treatment (6.08, 6.14, 6.11, 6.19 and 6.09 kg for control, LC, LC-Z, HC and HC-Z, respectively; SE 0.25). In Experiment 2, Simmental crossbred beef calves grazed fescue-clover paddocks in the spring (clipped forage: 21 to 27% CP) for 84 days and were supplemented with 0.25 or 0.75% BW corn alone or with added zeolite (0.06% BW) or lasalocid (0.5 mg/kg BW). Daily gain did not differ among treatments (1.24 to 1.31 kg/day). In Experiment 3 (6 x 6 Latin square), six Holstein steer calves (168 kg) had ad libitum access to medium-quality, orchardgrass-clover hay (15% CP). Steers received one of the following six treatments: no supplement (Control), 0.35% BW ground corn (C), corn plus 0.049% BW urea (C-U), 0.35% BW soybean meal (S), 0.205% BW feather meal (F) or 0.103% BW feather meal, 0.052% BW blood meal and 0.067% BW corn gluten meal (F-BG). Total daily OM intake was 5.00, 5.14, 5.00, 5.11, 4.84 and 4.70 kg for control, C, C-U, S, F and F-BG, respectively (SE 0.13). In Experiment 4 (6 x 6 Latin square), six Holstein steer calves (226 kg) were fed 0.75% BW of alfalfa cubes (19% CP) plus ad libitum access to medium- to high-quality orchardgrass hay (16% CP). Steers received one of the following six treatments: no supplement (Control), corn at 0.25% BW alone (c) or mixed with 0.12% BW peanut skins (C-PSK), a mix of high-CP feedstuffs high in ruminal escape protein (0.029% BW blood meal, 0.038% BW corn gluten meal and 0.028% BW feather meal; C-REP), 0.139% soybean meal (C-S) or soybean meal plus peanut skins (C-PSK-S). Total daily OM intake was 6.16, 6.25, 6.13, 6.52, 6.50 and 6.60 kg for control, C, C-PSK, C-REP, C-S and C-PSK-S, respectively (SE 0.10).(ABSTRACT TRUNCATED AT 400 WORDS)

Nonprotein nitrogen and protein distribution in the milk of cows

The NPN content of milk represents only 5 to 6% of the total N in milk. However, the significance of this milk N fraction to energy and N metabolism in the dairy cow has not been well characterized. The single largest contributor to the NPN fraction of milk NPN is urea. Urea equilibrates in body water, and blood urea is the primary source of milk urea. The urea in milk can be derived from at least two sources: the end product of digestion and amino acid catabolism. Blood urea N was positively associated with intakes of ruminally degradable and undegradable protein and negatively associated with intake of net energy. Consequently, it might be possible to develop a system to evaluate the dietary protein and energy status of the lactating dairy cow employing milk urea in conjunction with milk true protein.

The effects of intraruminal infusions of urea on the voluntary intake and milk production of cows receiving grass silage diets

Responses of dairy cows given silage diets to the intraruminal infusion of urea in progressively increasing doses were studied in four experiments, two with non-lactating cows and two with lactating cows. No clinical symptoms of NH3 toxicity were observed in any of the experiments. When urea was infused continuously, silage intake was depressed (P less than 0.05) when the total supply of N exceeded the equivalent of 250 g crude protein (CP)/kg DM in the total diet. However, when the urea load was administered twice daily, as opposed to continuously, intake depression (P less than 0.05) occurred at the equivalent of 170 g CP/kg DM. At the higher doses of urea, concentrations of NH3 in peripheral blood increased and were accompanied by increased concentrations of glucose and reduced levels of insulin in plasma. In general, responses of milk production followed those of silage intake but there was evidence of greater proportional reductions in the yield of lactose relative to that of fat and protein. It is concluded that the voluntary intake of high-protein silages may be depressed by factors associated with high rates of absorption of NH3 from the rumen.

Impact of protein nutrition on reproduction in dairy cows

In most but not all published studies, feeding diets with high concentrations of crude protein decreased reproductive efficiency. We were able to resolve some of the inconsistencies among published reports by considering the fate of consumed protein in the rumen and then using logistic regression analysis to identify factors affecting pregnancy. Our models showed that type and amount of protein fed explained much of the variation in conception rate. Age and dietary concentration of energy were identified as modifiers of the impact of protein on reproduction. Protein nutrition can affect reproduction through toxic effects of ammonia and its metabolites on gametes and early embryos, through deficiencies of amino acids, and by exacerbations of negative balances of energy. Alterations in the hypothalamic-hypophyseal-ovarian axis may be responsible for many of the effects of protein on reproduction.

Net portal-drained visceral and hepatic metabolism of glucose, L-lactate, and nitrogenous compounds in lactating holstein cows

Net portal-drained visceral and hepatic flux of glucose, L-lactate, alpha-amino N, NH3N, urea N, glutamate, and glutamine were measured in four Holstein cows. Cows were fed a 60:40 corn silage: concentrate diet ad libitum and milked at 12-h intervals. Six to 16 d postpartum chronic catheters were established in hepatic portal, hepatic, and mesenteric veins and a carotid artery was elevated. Twelve Measurements of net flux, the mathematical product of blood flow (measured by p-aminohippurate dilution) and venous-arterial concentration difference, were obtained for each cow at hourly intervals during 1 d of wk 4 and 8 postpartum. Dry matter, N, and energy digestion trials began 1 to 2 d after blood sampling. Dry matter intake and milk yield averaged 15.6 and 32.2 kg/d. Portal-drained visceral blood flow averaged 80% of hepatic blood flow (2041 L/h). Net flux of NH3N, urea N, and alpha-amino N across portal-drained viscera represented 68, 54, and 51% of N apparently digested. There was net use of glucose by portal-drained viscera. Hepatic glucose production (3.1 kg/d) exceeded calculated mammary glucose requirements. Net hepatic removal of L-lactate, alpha-amino N, and NH3N represented 115, 43, and 101%, respectively, of their net absorption by portal-drained viscera. Net hepatic L-lactate and alpha-amino N removal could account maximally for 17.4 and 16.5% of glucose produced.

Cerebral ammonia metabolism in normal and hyperammonemic rats

Brain ammonia is generated from many enzymatic reactions, including glutaminase, glutamate dehydrogenase, and the purine nucleotide cycle. In contrast, the brain possesses only one major enzyme for the removal of exogenous ammonia, i.e., glutamine synthetase. Thus, following administration of [13N]ammonia to rats [via either the carotid artery or cerebrospinal fluid (csf)], most metabolized label was in glutamine (amide) and little was in glutamate (plus aspartate). Since blood-and csf-borne ammonia are converted to glutamine largely, if not entirely, in the astrocytes, it is not possible from these types of experiments to predict with certainty the metabolic fate of the bulk of endogenously produced ammonia. By comparing the specific activity of L-[13N]glutamate to that of L-[amine-13N]glutamine following intracarotid [13N]ammonia administration it was concluded that metabolic compartmentation is no longer intact in the brains of rats treated with the glutamine synthetase inhibitor L-methionine-SR-sulfoximine (MSO) and that blood and brain ammonia pools mix in such animals. In MSO-treated animals, recovery of label in brain was low (approximately 20% of controls), and of the label remaining, a prominent portion was in glutamine (amide) (despite an 87% decrease in brain glutamine synthetase activity). These data are consistent with the hypothesis that glutamine synthetase is the major enzyme for metabolism of endogenously--as well as exogenously--produced ammonia. The rate of turnover of blood-derived ammonia to glutamine in normal rat brain is extremely rapid (t1/2 less than or equal to 3 s), but is slowed in the brains of chronically (12-14-wk portacaval-shunted) or acutely (urease-treated) hyperammonemic rats (t1/2 less than or equal to 10 s). The slowed turnover rate may be caused by an increased astrocytic ammonia, decreased glutamine synthetase activity, or both. In the hyperammonemic rat brain, glutamine synthetase is still the only important enzyme for the removal of blood-borne ammonia. Hyperammonemia causes an increase in brain lactate/pyruvate ratios and decreases in brain glutamate and brainstem ATP, consistent with an interference with the malate-aspartate shuttle. In vitro, pathological levels of ammonia also inhibit brain alpha-ketoglutarate dehydrogenase complex and, less strongly, pyruvate dehydrogenase complex. The rat brain does not adapt to prolonged hyperammonemia by increasing its glutamine synthetase activity.(ABSTRACT TRUNCATED AT 400 WORDS)

The use of monensin or formaldehyde to control the digestion of the nitrogenous constituents of perennial ryegrass (Lolium perenne cv. Melle) and white clover (Trifolium repens cv. Blanca) in the rumen of cattle

Pure swards of perennial ryegrass (Lolium perenne cv. Melle) and white clover (Trifolium repens cv. Blanca) were harvested daily as primary growth (grass in May, clover in June) or regrowths (clover only in July) and offered, in the long form, to growing cattle at a feeding rate of 22 g dry matter/kg live weight. With each forage, two treatments were compared with the untreated forage (C): monensin (100 g active ingredient/kg, 250 mg/d) addition to the rumen (treatment M) or formaldehyde (30 g/kg crude protein (nitrogen X 6.25] application to the diet (treatment F). The objective of the experiment was to examine means of manipulating N metabolism in the rumen and the duodenal non-ammonia-N (NAN) supply derived from fresh forages. The apparent digestion of ingested organic matter (g/kg) in the rumen was unaffected by treatment M (C 509, M 497) but was significantly (P less than 0.01) reduced by treatment F (443). The extent of cellulose digestion in the rumen was not affected by any of the treatments imposed and the changes in organic matter digestion were due mainly to effects on N digestion and rumen microbial synthesis. On the untreated diets, duodenal NAN supply averaged 0.74 g/g N intake and treatment M caused a small but non-significant increase (M 0.79 g/g N intake). In contrast, the effect of treatment F was much larger (F 0.91 g/g N intake; P less than 0.01). These differences were accompanied by corresponding reductions in rumen NH3 concentrations (mg/l; C 350, M 310, F 220; P less than 0.001). Of the increased flow of NAN to the small intestine observed on the white clover only diets with treatment F, 0.70 was accounted for by an increased net synthesis of microbial N, while treatment M had no effect on microbial N synthesis and a marginal reduction in feed N degradability only with the regrowth white clover diet. Treatment F reduced feed N degradability to a limited extent on both clover diets (C 0.82, M 0.81, F 0.77). No corresponding measurements were made for the ryegrass diets. It is concluded that the extensive loss of N from the reticulo-rumen of cattle fed on fresh forages can be reduced by the use of agents to reduce protein solubility. However, the study demonstrated that treatment F may in some circumstances increase N supply to the small intestine more through enhancing microbial N synthesis within the rumen than through increasing the passage of undegraded feed N to the small intestine.(ABSTRACT TRUNCATED AT 400 WORDS)

Propionate and butyrate metabolism in rat or sheep hepatocytes

The capacities of isolated hepatocytes to metabolize volatile fatty acids have been compared in rat and sheep hepatocytes. In both species, acetate utilization in vitro was quite limited. Significant species differences for propionate and butyrate consumption were found: propionate utilization by rat hepatocytes was relatively limited and plateaued at about 0.8-1.0 mM, whereas butyrate utilization was approx. 2-times higher. In contrast, ruminant hepatocytes exhibited a lower rate of butyrate utilization, but propionate metabolism was much more active than in rat liver cells. With relatively low concentrations of substrates (max. 2 mM), only propionate, compared to lactate or alanine, had a significant glucogenicity with hepatocytes from fed sheep. In both species, butyrate inhibited propionate consumption, although to a larger extent in sheep. The conversion of [2-14C]propionate to glucose by sheep hepatocytes was inhibited by 2 mM butyrate (60%) or ammonia (30%); 1 mM oleate or 10 mM glucose were ineffective. The basal rate of ammonia utilization by sheep hepatocytes was much lower than in rat and was unaffected upon addition of ornithine. Ammonia metabolism was markedly enhanced by butyrate and, in contrast to rat liver cells, also by propionate.

Ammonia: its effects on biological systems, metabolic hormones, and reproduction

The physical, chemical properties of ammonia, its sources and detoxification, its effects in biological systems, its influence upon insulin action and glucose metabolism, and its possible effects on reproduction are discussed. Present chemical methods do not distinguish nonionic from ionic forms. At physiological pH, nonionic ammonia concentrations remain low but are primarily responsible for toxic effects. Thus, biologically significant changes of ammonia concentrations may not be revealed by determinations of ammonia in blood plasma. For these and other reasons the subacute toxicity of ammonia often is unrecognized, and its effects on intermediary metabolism and the hormonal milieu in normal and disease states remain poorly understood. Effects of ammonia may be stimulatory at low concentrations and inhibitory as concentrations rise or exposure is extended. Extensive experiments in eight ureotelic species, including man, show that urinary excretion of orotic acid becomes significantly elevated when the quantity of ammonia presented to the liver exceeds the capacity for normal detoxification. Present evidence with arginine and other intermediates of the urea cycle suggest that these substances influence glucose metabolism and insulin action. Recent studies of dairy cattle provide speculative evidence that high protein feeding or forms of protein that lead to elevated ammonia concentrations in tissue decrease conception rates and increase the calving to conception interval of dairy cows. Limited data concerning luteinizing hormone concentrations and steroid hormone metabolism are insufficient to establish whether differences in reproductive performance are due to changes of hormonal physiology, intrauterine environment, or metabolism.