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Maximizing milk production has been the primary goal of the dairy industry for many years. In recent years, the shift in the milkmarketing sector towards a milk component pricing structure and the health concerns of the public, have placed greater emphasis on producing milk with a high proteimfat ratio, i.e. designer milk. At the same time, increased environmental concerns over possible contamination of the soil, waterways and air with nitrogen from manure has pressured the industry to devise ways of reducing nitrogen wastes. The lactating dairy cow is not particularly efficient at converting dietary nitrogen into milk nitrogen using current management schemes (Table 19.1). Balancing the goals of environmental stewardship and maximum milk production with desirable composition has proved to be a challenge. Most in the industry feel that these goals can be accomplished through diet manipulation by assuring that there is sufficient energy, and a proper pattern and supply of amino acids (AA). Although balancing diets for individual AA has been in place for decades in poultry and pig production, the information needed for such an achievement is not yet available in rumi

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nant nutrition. The large variety of feedstuffs used, combined with the complex remodelling by the rumen microflora of nutrients ingested, has made it a challenge to adequately predict availability of nutrient flows to the duodenum of ruminants, precluding a precise definition of AA requirements.

Each AA is an individual entity that is required as such to synthesize proteins, yet requirements are still expressed as 'metaboliz-able' protein (MP), i.e. an aggregate of AA. Requirements take into account the nitrogen demands of the lactating animal for maintenance, including urinary endogenous, scurf and metabolic faecal losses. The term also considers nitrogen for productive processes such as for conceptus and tissue growth and lactation. In the NRC (2001) guidelines, the digestible essential AA component of MP derives from a rumen sub model. Although the utilization of MP rather than crude protein (CP) is an improvement to prediction schemes, the lack of consistent observations of responses to AA supplementation may reflect the inaccuracies in these schemes (Kohn et al, 1998). One feature of present schemes that may account for some of the inaccuracy is the use of a fixed efficiency (0.65-0.85 depending on the feeding scheme) for converting

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

Table 19.1. Efficiency of conversion of dietary crude protein to product crude protein by commercial animal species under typical US management conditions.

Production animal

Production (yield, gain)

Protein intake (g day-1)

Protein produced (9 day-1)

Conversion

%

Lactating dairy cowa

35 kg day-1

4230

1085

26

Growing beefb

1.36 kg day-1

1033

270

26

Growing pig0

325 g day-1

387

127

33

Growing broiler0

47.8 g day-1

19.3

8.59

45

Egg production®

300 eggs year1

17.5

6.03

34

a31 g kg-1 milk protein, 23.5 kg dry matter intake day-1, 180 g dietary crude protein (CP) kg-1.

b3GG kg body weight, liveweight gain comprised of 180 g CP kg-1, 8.2 kg dry matter intake day-1, 126.1 g dietary crude protein kg-1 (NRC, 2000).

┬░50 kg body weight, 2.5 kg dry matter intake day-1, 155 g dietary CP kg-1 (NRC, 1998).

dG-7 weeks of age, mixed sex, 92 g feed intake day-1, 210 g dietary CP kg-1 (North and Bell, 1990), liveweight gain comprising 180 g CP kg-1.

e3GG eggs year1, 7.25 g crude protein egg-1 (Fisher, 1994), 103 g feed intake day-1, 170 g dietary CP kg-1 (North and Bell, 1990).

absorbed protein into milk protein. Postabsorptive metabolism of AA is essentially considered an inflexible 'black box'. Based on recent research, the NRC (2001) proposed a refinement to the MP model and, based upon empirically derived dose-response relationships, the amounts of lysine and methionine required in MP to optimize milk protein production are estimated. The ratio of 3:1 is regarded as the optimal ratio of lysine to methionine in MP. This adjustment is an improvement over previous schemes; however, it inherently limits the usefulness of the scheme to diets where lysine and methionine are limiting (e.g. maize-based diets). For example, the scheme would be less accurate when grass silage diets are fed and where histidine appears to be first-limiting (Vanhatalo et al., 1999). Additionally, there appears to be some flexibility in mammary transport systems with respect to AA use (Hanigan et ai, 2001a, 2002). Ideally, models of AA requirements should represent this flexibility and more essential AA. Current models generally require the user to specify the desired level of production (INRA, 1989; AFRC, 1993; NRC, 2001). This constraint limits the models' ability to predict maximal levels of production from unlimited inputs. Increasing model complexity must be carefully considered, however, to ensure that the desired level of accuracy is achieved with the minimal amount of added complexity.

In the last 20 years, there has been considerable research aimed at developing mechanistic models of postabsorptive metabolism (Waghorn and Baldwin, 1984; Baldwin et ai, 1987; Danfaer, 1994; Hanigan and Baldwin, 1994; Maas et ai, 1997; Cherepanov et ai, 2000; Hanigan et ai, 2001b). In theory, these efforts should complement rumen-based modelling efforts such that resulting models are better able to predict the pattern and supply of AA and energy substrates required to optimize milk protein synthesis by the mammary gland. This chapter will examine the current state of knowledge of AA metabolism in lactating ruminants (dairy cows and goats), the metabolic adaptations in tissue protein and AA metabolism that affect the quality and quantity of AA reaching the mammary gland, and current attempts to model these events, particularly at the level of the mammary gland.

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