Metabolic Modifiers: Effects on the Nutrient Requirements of Food-Producing Animals

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formulations of pST (Knight et al., 1988, 1989; Klindt et al., 1992), and this is an area of active investigation.

Ruminants Studies examining the effect of ST on growth performance of sheep and cattle have been less extensive than those with pigs. Enright (1989) compiled the results of studies in which cattle and sheep were treated with ST. In general, it appears that the ST dose/growth performance response relationships are similar to those observed with pigs but the magnitude of the responses, in particular the effects on rates of protein and lipid accretion, are substantially less. Possible explanations for these differences are discussed in Chapter 4.

Chickens The effects of cST on chicken growth performance are equivocal (see Table 2-2 and Chapter 6) and differ from those observed for other domestic animals. In some studies, administration of cST increases adipose tissue accretion (Leung et al., 1986; Cogburn et al., 1989a), a response that is the opposite of results for growing pigs and cattle. In addition, some studies have shown that cST decreases protein deposition. The most encouraging results are from Vasilatos-Younken and colleagues (1988) in which episodic administration of cST to older birds markedly improved growth performance whereas continuous infusion had no effect. These authors suggested that the pattern of cST administration is important to attain a stimulatory effect.

TABLE 2-2 Effects of Chicken Somatotropin (cST) on Chicken Response

cST Dose (µg)	Average Daily Gain (%)	Feed: Gain (%)	Carcass Fat^a (%)	Carcass Protein^b (%)	Reference
50	7	NR^c	8	NR	Leung et al., 1986
500 per kg, 3 × per day	4	4	5	NR	Burke et al., 1987
100 per day	NR	—	—	—	Bowen et al., 1987
150 per kg	21	-34^c	-33	NR	Vasilatos-Younken et al., 1988
200 per kg per day	NR	NR	17	-3	Cogburn et al., 1989a
NOTE: NR, no response; — not tested. ^a A response represents a composite of percent of carcass fat, grams adipose tissue, or lipid accretion rates. ^b A response represents a composite of percent of carcass fat, grams muscle, or protein accretion rates. ^c Data were presented as body weight gain:feed consumed in this study.

TABLE 2-3 Increase in Milk Yield (kg milk/day above controls) in Response to Bovine Somatotropin (bST)

	bST (mg/day)
Controls	5	10-15	20-27	31-50	Reference
24.9	+3.6	+3.8	+5.0	+5.7	Chalupa and Galligan, 1989^a
26.0	+2.8	+4.1	+5.3	+6.2	Chilliard, 1988^b
^a Summarized from 7 studies. ^b Summarized from 20 studies.

LACTATION

The effects of bST on milk yield have been documented and discussed in several hundred studies (see reviews by Peel and Bauman, 1987; Chilliard, 1988; Chalupa and Galligan, 1989; Bauman, 1992). Administration of bST following peak milk production (approximately 60 days postpartum) results in a substantial increase in milk yield, and a marked improvement in the persistency of lactation is generally observed. Results from 27 lactation studies are summarized in Table 2-3. Milk yield increases in a dose-dependent manner and the composition of milk is unaltered. Increases of 4 to 6 kg/day have been most frequently observed in long-term studies. In addition, responses have been observed for all dairy breeds examined and in animals of different parity and genetic potential (see Chapter 3).

SUMMARY OF EFFECTS OF SOMATOTROPIN ON GROWTH AND LACTATION

Studies show that ST markedly increases growth performance in pigs and ruminants and enhances milk production in dairy cows. Overall, this results in an impressive improvement in productive efficiency. The effects of ST on chicken growth performance are equivocal at the present time. It should be emphasized that ST is not magic and the greatest

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