Food intakemediated factors

Environmental temperature

March and Biely (1972) and McNaughton et al. (1978) investigated the effects of environmental temperature on the growth responses of cockerels fed graded levels of lysine. The results of March and Biely (1972), shown in Fig. 14.3, indicate that when growth is considered in relation to dietary lysine content, two discrete curves are obtained for 20°C and 31.1°C, implying a decrease in the efficiency of lysine utilization at the higher temperature. However, if the same responses are plotted against daily lysine intake (Fig. 14.4) it becomes apparent from the single response curve that lysine utilization has not changed. Chicks at 31.1°C merely consumed less food and higher dietary concentrations of lysine were therefore required to compensate for this reduction in appetite. The daily intake of lysine required to support a given rate of growth was similar at the two temperatures. It is likely that the data of McNaughton et al. (1978) would conform with this pattern when responses are considered as a function of lysine intake.

Dietary lysine concentration (g kg-1)

Fig. 14.3. The effects of two environmental temperatures (20°C, V, and 31.1 °C, ▼) on daily weight gain of chicks fed graded concentrations of lysine. (From D'Mello, 1979; source of data: March and Biely, 1972. Reproduced with permission from Butterworth-Heinemann Ltd.)

Dietary lysine concentration (g kg-1)

Fig. 14.3. The effects of two environmental temperatures (20°C, V, and 31.1 °C, ▼) on daily weight gain of chicks fed graded concentrations of lysine. (From D'Mello, 1979; source of data: March and Biely, 1972. Reproduced with permission from Butterworth-Heinemann Ltd.)

Immunological stress

Klasing and Barnes (1988) suggested reduced requirements of growing chicks for SAA and for lysine following immunological challenge with injected bacterial antigens. The immunogens injected Included Escherichia coli lipopolysaccharide, Salmonella typhimurium lipopolysacc-ahride and heat-treated Staphylococcus aureus singly or in rotation. On the basis of the growth results, it was suggested that the lysine requirements of saline-injected control chicks were in excess of 9.5 g kg 1 diet, whereas immunogen-injected chicks required lysine concentrations of 7-9.5 g kg-1 diet. However, when the growth responses are plotted against lysine intake (Fig. 14.5) it becomes apparent that the principal effect of the immunogens is to reduce food intake. Lysine utilization remains largely unaffected since much of the data, particularly those relating to lysine deficiency, appear along a single response curve. The data points for immunogen-treated chicks yield a lower plateau than that for the control group, but this is associated with factors other than lysine intake. If lysine utilization had been affected by immunogen challenge then any differences would have been reflected in discrete response curves over the entire range of lysine intake. Notwithstanding the evidence presented in Fig. 14.5, it should be recognized that growth and food utilization

T3 3

100 200

Lysine intake (mg day-1 )

Fig. 14.4. Daily weight gain and lysine intake of chicks maintained at two environmental temperatures (20°C, V, and 31.1 °C, ▼) (From D'Mello, 1979; source of data: March and Biely, 1972. Reproduced with permission from Butterworth-Heinemann Ltd.)

may not represent the most appropriate criteria for quantifying the effects of immunological stress on amino acid requirements (Swain and Johri, 2000). There are reports that amino acid deficiency impairs immune function in broiler chicks and that dietary cysteine and the branched-chain amino acids may exert particular effects in the modulation of immune responses (Konashi et al., 2000; Takahashi et al., 1997).

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