The digestibility of dietary protein in the ruminant: The digestibility of protein following a prolonged fast, with a detailed study of the nitrogen metabolism

Review: Impact of protein and energy supply on the fate of amino acids from absorption to milk protein in dairy cows

Making dairy farming more cost-effective and reducing nitrogen environmental pollution could be reached through a reduced input of dietary protein, provided productivity is not compromised. This could be achieved through balancing dairy rations for essential amino acids (EAA) rather than their aggregate, the metabolizable protein (MP). This review revisits the estimations of the major true protein secretions in dairy cows, milk protein yield (MPY), metabolic fecal protein (MFP), endogenous urinary loss and scurf and associated AA composition. The combined efficiency with which MP (EffMP) or EAA (EffAA) is used to support protein secretions is calculated as the sum of true protein secretions (MPY + MFP + scurf) divided by the net supply (adjusted to remove the endogenous urinary excretion: MPadj and AAadj). Using the proposed protein and AA secretions, EffMP and EffAA were predicted through meta-analyses (807 treatment means) and validated using an independent database (129 treatment means). The effects of MPadj or AAadj, plus digestible energy intake (DEI), days in milk (DIM) and parity (primiparous v. multiparous), were significant in all models. Models using (MPadj, MPadj × MPadj, DEI and DEI × DEI) or (MPadj/DEI and MPadj/DEI × MPadj/DEI) had similar corrected Akaike's information criterion, but the model using MPadj/DEI performed better in the validation database. A model that also included this ratio was, therefore, used to fitting equations to predict EffAA. These equations predicted well EffAA in the validation database except for Arg which had a strong slope bias. Predictions of MPY from predicted EffMP based on MPadj/DEI, MPadj/DEI × MPadj/DEI, DIM and parity yielded a better fit than direct predictions of MPY based on MPadj, MPadj × MPadj, DEI, DIM and parity. Predictions of MPY based on each EffAA yielded fairly similar results among AA. It is proposed to ponder the mean of MPY predictions obtained from each EffAA by the lowest prediction to retain the potential limitation from AA with the shortest supply. Overall, the revisited estimations of endogenous urinary excretion and MFP, revised AA composition of protein secretions and inclusion of a variable combined EffAA (based on AAadj/DEI, AAadj/DEI × Aadj/DEI, DIM and parity) offer the potential to improve predictions of MPY, identify which AA are potentially in short supply and, therefore, improve the AA balance of dairy rations.

Nitrogen balance studies with the milk-fed lamb. 6. Effect of starvation and realimentation

1. Five male cross-bred lambs at the age of 3 weeks were starved for a period of 4 days. The utilization of nitrogen, energy and minerals was compared with the pre-starvation and post-starvation periods, when the lambs were given reconstituted dried whole cow's milk.

2. The mean loss in weight during starvation was 253 g/24 h. The gain in weight during realimentation was significantly greater than that in the pre-starvation period, when adjusted for differences in energy intake.

3. The urinary loss of N during starvation was 478 mg N/kg live weight 24 h. During realimentation the urinary N loss was greater than during the pie-starvation period. A higher urea excretion during realimentation was responsible for the increased urinary N.

4. Excretion of ammonia, creatine, amino acid and purine base N increased during starvation. Excretion of creatinine N remained constant.

5. The increase in total sulphur excretion during starvation was due mainly to an increase in the inorganic sulphate fraction. The ratio of N to S of 27: 1 during starvation was much greater than would be expected from the katabolism of muscle tissue alone (ratio N to S = 16.2:1).

6. The titratable acidity and acetone excretion in the urine increased during starvation.

7. There was a marked reduction in the urinary excretion of chloride, potassium and sodium during starvation, an increase in the excretion of phosphorus, but little change in the excretion of calcium and magnesium. The loss of minerals was not closely related to the losses predicted from muscle tissue alone.

8. Pulse rate, respiratory rate, and rectal temperature all decreased during starvation. Heat production decreased after 1 day of starvation, but was relatively constant thereafter.

9. Blood urea concentrations did not change significantly during starvation.

10. The results for the lamb were in reasonable agreement with published results for the calf, except for the marked ketone acidosis and considerable increase in the excretion of purine N. The losses during starvation differed from those with the adult sheep in showing a relatively greater loss of body protein.