Reducing the contamination of soils, water and air caused by excessive build-up of animal wastes is now the priority of many nutritionists, land managers and lawmakers! A recent evaluation of dairy farms in the eastern US states indicated that dairy farmers over feed protein by 7%, resulting in a 16% increase in urinary N excretion (Jonker et ai, 2002). The transfer of current nutritional information from research scientist to nutritionist to farmer may be the problem, but more than likely it results from farmers including a 'safety margin' to ensure that higher producers in the herd are not underfed protein. The goal is to determine the level of protein required to achieve optimal production, yet also improve protein efficiency. Feeding lower protein diets supplemented with limiting amino acids has been successful in the pig and poultry (mostly lysine and methionine) industry where growth rate can be maintained with 15-20% less protein and a 30-40% reduction in nitrogen excretion (Pieterse et ai, 2000). In ruminants, this is also possible but a major roadblock in ruminant nutrition is in the prediction of limiting amino acids. The success of whole-animal feeding models, especially for ruminants, will require model development based on knowledge of the pattern of amino acid delivered to and metabolized by tissues and organs.
The need to reduce animal wastes in the environment continues to stimulate interest in defining the ideal pattern and quantity of amino acids required to optimize animal productivity at all stages of the life cycle. The gross efficiency for converting dietary protein into protein gain, milk secretion and egg production is less than optimal and varies among animal species. Beef cattle are the lowest at 8%, followed by pigs (19%), dairy cows (21%), laying hens (24%), growing birds (31%) and fishes (up to 40%) (CAST, 1999). The low value for ruminants reflects the significant (25-40%) losses of dietary N as ammonia. Based on comparisons of the disappearance of amino acids from the small intestines with their accretion in carcass, the estimates are much higher; for pigs, 60-85% (Batterham et al., 1990) and for ruminants 50-59% (MacRae et al., 1993).
A significant source of 'inefficiency' is the apparent obligatory loss associated with high rates of tissue turnover. Protein turnover is particularly high in the visceral tissues (20-100%/day), and even in muscle where turnover is 40-fold lower (fractionally 1-4%/day), only 32-46% of the protein synthesized in young (suckling) animals is retained. In older animals, this value drops to 24%. Protein efficiency declines with maturity, mainly as a result of reductions in protein translational efficiency (Davis et al., 2000) and in the sensitivity of target tissues to hormone and amino acid signals (O'Connor et al., 2000). At intakes above maintenance, protein synthesis and degradation are both stimulated by energy intake. However, the response in protein degradation is 24%-33% less than for protein synthesis, and so net anabolism results. The energetic cost of protein turnover is high, requiring 4.5-7 mol of ATP per mole of peptide bond formed and 1-2 mol of ATP per mole of peptide bond breakage. In addition, although most tissues recycle 80% of the amino acid derived from protein breakdown, the remainder appears to be oxidized. Strategies that target protein degradation would appear to be more beneficial in terms of improving overall energy and protein efficiency.
Achieving optimal rates of protein deposition or secretion is a trade-off with lower partial efficiencies of utilization as dietary protein intake increases. In part, the diminishing returns on protein gain may reflect the point of energy limitation (use of amino acid carbon as energy) and/or the limits to genetic potential. Most research has focused on providing the ideal balance of amino acid, which may become more important at higher levels of production. The 'ideal protein' concept now used in formulating pig and poultry diets is based on balancing dietary essential amino acids to meet requirements for maintenance and growth. The requirement for dispensable amino acids, which make up over half of the N in proteins, is now being considered. Earlier, the vital roles of some of the non-essential amino acid were emphasized, and indeed it is has been demonstrated that at maintenance the requirement is dominated by non-essential amino acids (Fuller et al., 1989). The requirement for non-essentials could be met by supplying extra essential amino acids, but this would be wasteful and most studies indicate that essentials (e.g. arginine, lysine, methionine/cysteine) are poor precursors for the nonessentials. Chapter 6 discusses the implications of balancing the essential to non-essential ratio in pig diets when the goal is to achieve maximum rates of nitrogen retention versus maximum nitrogen efficiency.
Imbalances or excesses of dietary amino acids are often inevitable when protein sources are expensive or limited in availability (see Chapter 7). Most imbalances are manifested at the level of transport through competitive mechanisms. An example is the inhibitory effect of arginine on lysine transport and utilization. Imbalances or excesses of branched-chain amino acids affects the transport of their cohorts (Langer and Fuller, 2000). In addition, an excess of one branched-chain amino acid stimulates oxidation of the others because the branched-chain keto-acid dehydrogenase has a high affinity for all of them. Methionine is also recognized by the branched-chain dehydrogenase. Under methionine-limiting conditions, however, excesses of the branched-chain amino acids reduce methionine oxidation due to competitive inhibition of the branched-chain keto-acids with the methionine keto-acid. The net effect is an improvement in nitrogen retention (Langer and Fuller, 2000; Langer et al.,
2000). Similarly, addition of glutamate to a threonine-limiting diet improves pig growth rate (Le Floch et ai, 1994), but here the effect appears to be at the level of the intestines where threonine catabolism is spared (Le Floch et ai, 1999).
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