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Effects of food restriction, followed by refeeding, on energy and nitrogen metabolism, growth rates and blood levels of hormones and metabolites were studied in steers. During the restriction period, which lasted for almost 5 mo, allowance for energy and nitrogen were close to maintenance requirements. Heat production and growth rates were markedly lowered. In response to reduced food intake concentrations of thyroxine (T4), 3,5,3'-triiodothyronine (T3), insulin (IRI), glucose and alpha-amino-acid nitrogen (AAN) were reduced, those of growth hormone (GH) and nonesterified fatty acids (NEFA) were elevated, whereas 3,3',5'-triiodothyronine (rT3) and albumin were not different from levels measured in nonrestricted animals. During refeeding heat production and energy balances increased, nitrogen balances were transiently elevated and the animals exhibited compensatory growth. In response to refeeding, concentrations of T4, T3 and IRI increased within days. In contrast, GH decreased whereas rT3 did not change. Within 2 d of refeeding there was a rapid fall of NEFA, and an increase of glucose, and beta-hydroxybutyrate within 2 and 12 d, respectively. The data demonstrate the ability of growing ruminants to adapt rapidly to variations in food intake by closely linked metabolic and endocrine changes, which are associated with shifts in energy and nitrogen metabolism and, finally, by reduced or compensatory growth.

In a wild-type strain of Saccharomyces cerevisiae the tryptophan analogue DL-5-methyl-tryptophan (5MT) causes only a slight reduction of the growth rate. Uptake experiments indicate that the limited inhibition is partly due to low levels of 5MT inside the cell. On the other hand, this low concentration of 5MT leads to an increase in the activity of the tryptophan-biosynthetic enzymes. Evidence is presented that suggests that 5MT acts primarily through feedback inhibition of anthranilate synthase, the first enzyme of the pathway. A number of 5MT-sensitive mutants have been isolated, characterized, and assigned to one of the following three classes: class I, strains with altered activity and/or feedback sensitivity of anthranilate synthase; class II, strains with elevated uptake of 5MT; class III, mutants with altered regulation of the tryptophan-biosynthetic enzymes, which do not exhibit increases in activity in the presence of 5MT. This failure to exhibit increased enzyme activities in mutants of class III can also be observed after tryptophan starvation. Two mutants of class III show high sensitivity towards 3-amino-1, 2, 4-triazole. They can not exhibit derepression of some histidine-and arginine-biosynthetic enzymes under conditions that lead to an increase in these same enzymes in the wild-type strain.

SUMMARYTwo trials investigating compensatory growth are reported in which lambs and young cattle were placed on either a continuous (C) or a discontinuous (RR) growth path. RR animals were subjected to a phase of restricted feeding and then realimented at an equivalent level of feeding to C animals over the same live-weight range. Eight 4-month-old lambs and 30 9-month-old Swiss Brown steers were used. The restriction (I) and realimentation (II) phase covered the live-weight ranges 23-32 kg and 32-44 kg respectively in tho lamb trial and 236-310 kg and 310-460 kg respectively in the steer trial.Fifty-six total energy balances were made with lambs using open-circuit respiration calorimetry. Fifty determinations of diet digestibility and N balance were made with steers. Lambs received a pelleted concentrate diet and, except for restrictively fed steers which received hay alone, steers were offered a diet based on maize silage.The restriction phase of RR lambs and RR steers was longer, and the daily ME intake and daily live-weight gains were significantly lower than those of the C animals.Compared with C lambs a marked reduction in methane production of RR lambs occurred during feed restriction which persisted throughout realimentation.During recovery realimented lambs gained non-significantly, but realimented steers significantly, more than C animals from a similar ME intake and required less ME/kg daily live-weight gain. Realimented lambs retained more protein at the start of recovery compared with C lambs but both C and realimented steeis retained similar amounts of nitrogen. Indirect evidence is presented that suggests improved utilization of ME for protein deposition, at least at the start of realimentation.Although the animals on the discontinuous growth path (RR) took longer to reach slaughter weight, their total intake of gross energy and overall energy conversion ratio (MJ ME/kg live-weight gain) was similar to those of animals on the continuous growth path (C). _ maintenance requirement; second, a fall in the 1JN1KODU ON energy value of the body-weight gains and third, The ability of animals to express compensatory an increased efficiency of feed utilization. The close growth following a period of nutritional limitation interdependency between these three factors, is well documented (Wilson & Osbourn, 1960; together with level of feeding differences, and their AUden, 1970). Little doubt remains that increased individual effects on efficiency (Bickel, 1977), often appetite, and the associated gut-fill effect, is an makes it difficult to interpret the results of studies important factor responsible for compensatory on compensatory growth. For example, the growth. However, disagreement remains concerning frequently but not universally observed higher the causal involvement of the following three feed intake of compensating animals (Allden, 1970) factors in the complex of changes resulting in prevents the separation of the effects on growth compensatory growth. First, a reduction of rate of the higher feed...

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