N. A. Macleod

Flow of nitrogen from the rumen and abomasum in cattle and sheep given protein-free nutrients by intragastric infusion

1. Three experiments were conducted to determine the flow of nitrogen through the rumen and abomasum when cows, steers and lambs were totally nourished on volatile fatty acids infused into the rumen. 2. In two dairy cows (650-700 kg) and two large steers (370-405 kg) the daily flow of non-ammonia-N (NAN) from the rumen was 50.7 and 58 mg/kg live weight (W)0.75 respectively. 3. The flows of NAN through the rumen and abomasum in four young steers (240-315 kg) were 85.0 (SE 21.0) and 195 (SE 7.0) mg/kg W 0.75 respectively. 4. In the third experiment the effects of altering rumen pH and osmotic pressure on flow of NAN through the rumen and abomasum were investigated in lambs. While rumen pH and osmotic pressure influenced rumen volume and outflow they had no significant effect on NAN flow. The mean values for NAN outflow from the rumen and abomasum were 76 and 181 mg N/kg W 0.75 respectively. 5. Abomasal NAN flow increased with increasing abomasal pH. When osmotic pressure was greater than about 330 mosmol/l in the rumen there was a net inflow of water, while below this value there was net loss of water. 6. For all experiments the flow of N both from the rumen and abomasum was highly variable; this has to be considered if a constant value is used for endogenous N in estimating dietary N in the abomasum. 7. With N-free infusion the rumen NH3 concentration varied from 50 to 120 mg NH3-N/l. 8. The amino acid composition of rumen and abomasal N was also determined. Relative to tissue N it contained a higher proportion of cysteine.

The determination of the minimal nitrogen excretion in steers and dairy cows and its physiological and practical implications.

1. Cattle were maintained by intragastric infusion to see how much nitrogen was excreted on protein-free diets. 2. Minimal N excretion was estimated with two dairy cows in three periods, i.e. when they were non-pregnant and non-lactating, when they were between 117 and 133 d pregnant and when they were between 220 and 233 d pregnant. The minimal N excretion was also estimated on two occasions with two steers when their average live weights were 200 and 350 kg. 3. Average urinary N excretion without protein infusion was 298, 305 and 283 mg/kg metabolic live weight (W0.75) for the non-pregnant cows and for cows during the first and second periods of pregnancy respectively; total N excretion including the faecal N was 340, 329 and 319 g/kg W0.75. 4. For steers the urinary N values were 403 and 295 mg/kg W0.75 at 200 and 350 kg live weight respectively and total N excretion including faecal N was 408 and 320 mg/kg W0.75. 5. Urinary excretion of creatinine was the same for animals given casein via the abomasum as a source of protein or given no protein with mean values for the cows of 13.6 and 14.9 g/d for the first and second stages of pregnancy respectively. Mean values for the steers were 6.5 and 7.6 g creatinine/d at 200 and 350 kg live weight respectively. 6. It is suggested that the so-called metabolic faecal N in ruminants, estimated with N-free diets, is mainly, endogenous N derived from tissue breakdown of protein but incorporated in microbial debris and excreted in the faeces.

The effect of changes in the amount of energy infused as volatile fatty acids on the nitrogen retention and creatinine excretion of lambs wholly nourished by intragastric infusion

The nitrogen balance and creatinine excretion of wether lambs of 30-48 kg, wholly nourished by the intragastric infusion of nutrients, were measured in two experiments. Four lambs were used in each experiment. In Expt 1 a constant amount of casein was infused into the abomasum (640 mg N/kg body-weight (W)0.75 per d) and the amount of volatile fatty acids (VFA) infused into the rumen ranged from 0 to 670 kJ/kg W0.75 per d as six increments. Expt 2 was of similar design but two levels of casein were infused (530 and 1060 mg N/kg W0.75 per d) and, with each level of casein, VFA infused ranged from 0 to 700 kJ/kg W0.75 per d as seven increments. Daily creatinine excretion was not constant, but varied between 2-d means with standard deviations of between 7.1 and 16.5% (average 13.1%) of the over-all means. There was an apparent correlation between creatinine excretion and the amount of energy infused in six out of eight lambs. There was no effect of the amount of casein infused. In both experiments N balance was negative only when the amount of energy infused was substantially below published values for energy maintenance. In Expt 1, N equilibrium was calculated to be achieved at a gross (VFA plus protein) energy infusion level of 162 (SE 29) kJ/kg W0.75 per d. In Expt 2 it was observed that, at a given level of energy infusion, N retention was greater when the amount of energy had been increased from the previous level, than when it had been decreased. It is concluded that the animal appears to allocate available amino acids to protein synthesis, rather than to oxidation, even when in negative energy balance. It is also concluded that enhanced N retention observed when the amount of energy infused had been increased represented compensation after a period of energy restriction.

Intragastric infusion of nutrients in cattle

1. A method of continuous alimentation of cattle by total infusion of nutrients has been developed. Friesian steers within the weight range 100-400 kg live weight and dairy cows were used. 2. A multi-channel peristaltic pump was used to infuse solutions of volatile fatty acids (VFA), minerals, and buffer through a cannula in the rumen and a casein-vitamin solution into the abomasum. 3. The method described was successfully used with two cows and four steers in a series of trials over intervals of approximately 2 months. The levels of infusion were up to twice maintenance and with various relative proportions of VFA and protein. Blood metabolite levels, rumen osmotic pressure and pH were monitored and effectively controlled.

Basal urinary nitrogen excretion and growth response to supplemental protein by lambs close to energy equilibrium.

Two experiments are reported. In Expt 1, five male lambs of 26-33 kg were used to measure basal nitrogen excretion when the lambs were entirely sustained by an intraruminal infusion of 450 kJ/kg body-weight 0.75 per d of volatile fatty acid (VFA) and were receiving no protein. In Expt 2, which was a conventional growth trial, the response to fish meal (66 or 132 g dry matter/d) of lambs given a control diet of sodium-hydroxide-treated barley straw was measured. In Expt 1 the mean basal N excretion of the lambs was 429 (SE 21) mg N/kg body-weight 0.75 per d. This exceeds current UK standards for the amino acid N of microbial origin which would be made available to the normally-fed host animal at a maintenance level of metabolizable energy intake. In Expt 2 there was a clear growth response to the fish meal, which was greater (P less than 0.05, single-tailed test) than that to be predicted from the energy content of the fish meal. There was no effect of fish meal on the voluntary intake of the basal diet, but there was a suggestion that the digestibility of the basal diet was improved. It is concluded from Expt 1 that the basal requirement for amino acid N by lambs is three- to fourfold that currently recommended in the UK. This higher basal N requirement should have resulted in a marked response to supplemental protein in Expt 2. The fact that the growth response in Expt 2 was less than anticipated may have been due to a combination of a slightly lower basal N excretion than that found in Expt 1, a higher yield of amino acids of microbial origin than current UK standards predict, and possibly to a change in the body composition of the lambs.

Osmotic pressure, water kinetics and volatile fatty acid absorption in the rumen of sheep sustained by intragastric infusions

The effects of changing rumen osmotic pressure (OP) upon water kinetics and volatile fatty acid (VFA) absorption in the rumen of sheep were studied in two 4 x 4 Latin square experiments, each using four lambs with a rumen cannula and an abomasal catheter. In both experiments the lambs were sustained by the intragastric infusion of all nutrients (VFA, Ca, P, Mg and a buffer solution into the rumen, and casein, vitamins and trace elements into the abomasum). On experimental days, which were at least 1 week apart, drinking water and the casein infusion were withdrawn, and the ruminal OP was changed and held constant for 9.5 h, by incorporating NaCl at different concentrations in the buffer solution being infused. In Expt 1 the target OP values were 300, 340, 380 and 420 mosmol/kg, and in Expt 2 were 261 (no saline addition), 350, 420 and 490 mosmol/kg. Using soluble non-absorbable markers (PEG in continuous infusion and Cr-EDTA injected in pulse doses) rumen volume, liquid outflow rates, apparent water absorption through the rumen wall and VFA absorption rates were estimated at six sampling times corresponding to the 1.5 h intervals during the last 7.5 h following the change in rumen OP. Liquid outflow rate (F; ml/h) showed a significant and positive linear relationship with the rumen OP (mosmol/kg), resulting in the equation F = 1.24 OP (SE 0.096)-36.5 (SE 36.6) (r2 0.96). Similarly, water absorption rate (W; ml/h) was significantly affected by rumen OP, and this relationship was given by W = 395 (SE 39.9)-1.16 OP (SE 0.105) (r2 0.95), which means that for an OP of 341 mosmol/kg the net movement of water across the rumen wall would be zero, and either a net efflux or a net influx of water would be observed with lower or higher OP respectively. In Expt 2 there was a significant linear effect of OP on rumen volume (P < 0.01), with higher OP being associated with increases in rumen liquid contents of about 10-20%. As rumen OP was increased there was also a decline in the absorption rate of VFA (from 232 mmol VFA/h for OP 350 to 191 mmol/h for OP 490 mosmol/kg), resulting in the accumulation of VFA (especially acetate) in the rumen and a consequent fall in rumen pH. Rumen OP seems to be important in defining water movement across the rumen wall and, hence, partitioning between absorption and outflow.

Effects of long-term protein excess or deficiency on whole-body protein turnover in sheep nourished by intragastric infusion of nutrients.

The effect of long-term dietary protein excess and deficit on whole-body protein-N turnover (WBPNT) was examined in lambs nourished by intragastric infusions of nutrients. Ten sheep were given 500 mg N/kg metabolic weight (W0.75) per d from casein for 2 weeks and then either 50 (L), 500 (M) or 1500 (H) mg N/kgW0.75 per d for 6 weeks. Volatile fatty acids were infused at 500 kJ/kgW0.75 per d. Daily WBPNT was measured by continuous intravenous infusion of [1-13C]leucine 3 d before, and on days 2, 21 and 42 after the alteration in protein intake. Whole-body protein-N synthesis (WBPNS) was calculated as the difference between WBPNT and the protein-N losses as urinary NH3 and urea. Whole-body protein-N degradation (WBPND) was then estimated from WBPNS minus protein gain determined from N balance. Fractional rates of WBPNS and WBPND were calculated against fleece-free body N content. WBPNS rates at the L, M and H intakes were respectively 35.1, 41.5 and 63.7 g/d (P < 0.001) on average over the 6 weeks and WBPND rates were 39.5, 41.1 and 56.8 g/d (P < 0.001). The fractional rates of WBPNS were 5.01, 6.37 and 7.73% per d (P < 0.001) while those of WBPND were 5.64, 6.29 and 6.81% per d (P < 0.005) respectively. On days 2, 21 and 42, WBPNS rates at intake H were 54.0, 61.8 and 75.4 g/d (P = 0.03) respectively, and WBPND rates were 43.2, 56.4 and 70.9 g/d (P = 0.03); at intake L the amounts were 38.2, 34.2 and 32.8 g/d for WBPNS (P = 0.003) and for WBPND were 43.4, 38.0 and 36.9 g/d (P = 0.016) respectively. There were no significant (P > 0.05) differences in fractional rates of WBPNS and WBPND with time at either the L or H intake. We concluded that absolute protein turnover was affected both by dietary protein intake and body condition while the fractional rate of turnover was predominantly influenced by intake.

Undernutrition in sheep. Nitrogen repletion by N-depleted sheep

Wether lambs of 29-44 kg live-weight, totally nourished by the infusion of volatile fatty acids (VFA) into the rumen and casein into the abomasum, were given five treatments in consecutive periods. The treatments were (daily amounts per kg live weight (W)0.75): (a) high-protein for 7 d (2500 mg nitrogen, 650 kJ VFA); (b) low-protein for 7-15 d (525 mg N, 650 kJ VFA); (c) N-free for 7 d (no N, 450 kJ VFA); (d) very-low-protein for 24-28 d (300 mg N, 400 kJ VFA); (e) high-protein for 40 d (2500 mg N, 650 kJ VFA). Nine lambs were subjected to treatments (a), (b) and (c) (Expt 1) and four of the lambs additionally received treatments (d) and (e) (Expt 2). In Expt 1 all nine lambs had a positive N retention on treatment (a) but abrupt change to treatment (b) resulted in substantial negative N balances initially, and a period of approximately 5 d adaptation was required before N equilibrium was re-established. Animals again exhibited negative N balances when the N-free infusion (treatment c) was introduced and during that period there was no evidence of adaptation. Basal urinary N excretion was estimated to be 356 (SE 12) mg N/kg W 0.75. In Expt 2 all four lambs were depleted of N when receiving the very-low-protein treatment (d). The progressively decreasing N losses recorded during days 1 to 12 of the treatment period were slightly greater than those recorded during days 13 to 28 but the difference between the means was not significant (P greater than 0.05). There was no evidence of an adaptation in N retention between days 13 and 28 of the treatment. As assessed during days 13 to 28 of the treatment the efficiency of utilization of infused casein N was 1.0; this compared with a value of 0.66 recorded during treatment (b) in Expt 1. Live weight loss during the period of N depletion was 101 (SE 27) g/d. When lambs were given treatment (e) during the last period of Expt 2, N repletion was rapid and complete within a few days. Ten days after the introduction of the treatment the rate of N retention was estimated to be 1019 (SE 38) mg/kg W 0.75 per d and this value declined at a rate of 9.5 (SE 1.9) mg N/kg W 0.75 per d for the following 30 d.(ABSTRACT TRUNCATED AT 400 WORDS)

Investigation of nitrogen balance in dairy cows and steers nourished by intragastric infusion. Effects of submaintenance energy input with or without protein.

Two dairy cows were maintained by intragastric infusion of volatile fatty acids and casein. Except when fasting, the casein-nitrogen was held constant, while total gross energy supply was varied from zero during fasting to 650 kJ/kg body-weight (W)0 . 75. One cow was estimated to attain zero N balance at an energy intake of 255 kJ/kg W0 . 75 and the other at 307 kJ/kg W0 . 75, which was calculated to be substantially below the estimated energy required for zero energy balance. When the cows were later given an N-free infusion for a period preceding the trial, N balance occurred at 98 kJ/kg W0 . 75 for one cow and 115 kJ/kg W0 . 75 for the other. Four steers were similarly nourished by intragastric infusion and the energy nutrient increased from 0 at fasting to 450 kJ/kg W0 . 75. The protein was held constant at 1 g N/kg W0 . 75 except at fasting. The energy level at which N balance occurred was 154 (SE 38) kJ/kg W0 . 75 or approximately equal to the energy content of the protein. The practical implications of these findings are discussed.

The effect of intragastric infusion of glucose, lipids or acetate on fasting nitrogen excretion and blood metabolites in sheep

Two experiments are reported in which the effect of the intragastric infusion of non-protein energy on fasting nitrogen losses was studied. Expt 1 was a preliminary trial with two 35 kg lambs given 0, 144, 288 or 432 kJ/kg live weight (W)0.75 per d as lipid or glucose infused into the abomasum for periods of 3 d. Expt 2 was of a 4 X 4 Latin square design with four sheep of about 30 kg live weight. The four treatments were control (fasted with water infusion), or the infusion of 144 kJ/kg W0.75 per d as glucose or lipid into the abomasum or as acetic acid into the reticulo-rumen. Compared with the fasted control, glucose infusion reduced (P less than 0.05) N excretion to about 0.6 of that of the control, increased (P less than 0.05) plasma glucose, decreased (P less than 0.05) plasma urea and beta-hydroxybutyrate, and was without effect on plasma amino-N or creatinine excretion. Lipid and acetate infusions were without statistically significant effect on N or creatinine excretion or any of the blood indices measured, with the exception of plasma glucose which was reduced (P less than 0.05) with acetate infusion.