At least quantitatively a great deal is known about the digestion of food, the uptake of amino acids over time, their subsequent metabolism and the rate of accumulation of protein in body tissues (Moughan, 1999). However, it is difficult to measure the variation in the processes involved in protein digestion and protein metabolism. These processes are difficult to measure, e.g. protein synthesis and protein breakdown (Baldwin et al., 1994). Knowledge of variation would give possibilities to influence these processes and thus give the possibility to influence them. Some approaches have given enormous possibilities to reduce the losses in various processes. One of these approaches is to distinguish between processes of maintenance and those associated with the accretion of new body tissue (Moughan, 1999). So feed protein will be involved both in the processes of maintenance and growth. At zero nitrogen growth there are still processes associated with protein metabolism. Animals use dietary protein as sources of individual amino acids (AAs). The essential AAs especially need to be supplied by the diet, the

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non-essential AAs can be provided by the diet but the animals are also able to synthesize them. Ruminants can satisfy their additional AA requirements by digestion of microbial proteins which are synthesized in the rumen and subsequently pass into the small intestine.

In all species the efficiency of utilization of absorbed AAs for protein synthesis is mainly determined by the availability of the most limiting amino acid at the site of protein anabolism. Any excess of AAs over what is needed is degraded and the surplus N is excreted as urea or ammonia depending on the species. In animal production the larger portion of ingested N is excreted as waste in faeces and urine; this surplus is then dispersed into the environmental water, soil and air.

Figure 24.1 represents the nitrogen flow for growing finishing pigs in partially slatted floors, and with surface application of sluny. The N intake is assumed to be 55 g pig-1 day-1 (Aarnink, 1997). The branches represent partitioning. Losses of N between species are not equal and the proportion of N that is not retained in the body increases from poultry to pigs to cattle (see Table 24.1).

© CAB International 2003. Amino Acids in Animal Nutrition, 2nd edition (ed. J.RF. D'Mello)



Retention Faeces Urine

Retention Faeces Urine

Slurry after storage 31 g N / 57%

Slurry after storage 31 g N / 57%

  • _ Emission
  • 10 g N/18%

Slurry after application 21 g N / 39% Fig. 24.1. Nitrogen flow for growing-finishing pigs.

Table 24.1. Ratio of retained nitrogen in animals to N content in diet. (From Booms-Prins etai, 1996.)








There is abundant evidence that the figures of efficiency can increase considerably from those mentioned in Table 24.1 provided that: (i) a correct level of essential AAs, (ii) a proper level of total AAs, and (1) a proper amino acid /energy ratio are given. If the balançai AAs level is the limiting factor, e.g. at low protein and high energy level, about 15-20% of absorbed protein is catabolized (KyriazaMs and Emmans, 1992).

In a normal diet, such as maize-soybean about 25% of protein consists of unbalanced amino acids. These will appear in excreta. It has been estimated further that about 50% of N in excreta of, for example pigs, can be attributed to non-optimal amino acid balance (De Lange et ai, 1999). A simple means of correcting this is to replace some of the protein by adding crystalline AAs (e.g. l-lysine, l-threonine, dl-methionine and l-tryptophan) to the diets of animals.

The differences between N taken in and maintenance N excreted are the source of N-emission into the environment. At low (limited) levels of protein intake the relation between utilization of ideally balanced protein and protein ratio is constant, this means that maintenance probably does not vary much. In addition to the amount of N which is excreted any change in the ratio of N in urine to N in faecal matter can influence the rate at which NH3 is emitted into the air (Canh et ai, 1997).

The following sections will discuss a few aspects of the use of crystalline amino acids in the feed to influence N-excretion and N-emission. Two approaches are followed to make use of AAs in addition to feed protein: (i) considering the animals; (ii) considering the feed and adjusting feeding strategy to the animal.

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