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Ewe size, pregnancy nutrition and pregnancy rank are known to affect the productive performance of ewes and their offspring. The aim of the present study was to compare the effects of two nutritional regimens, from day 21 to day 140 of pregnancy, in singleton-and twin-bearing ewes of two different mean weights, taken from a single flock. The study included Large ewes offered Ad lib (Large-Ad lib, n= 151), Large ewes offered Maintenance (Large-Maintenance, n = 153), Small ewes offered Ad lib (Small-Ad lib, n = 155) and Small ewes offered Maintenance (Small-Maintenance, n = 153) that were either singleton-or twin-bearing. During the period day 21 to day 140 Maintenance and Ad lib feeding regimens included Large and Small ewes and singleton-and twin-bearing ewes. At days 1 and 140 of pregnancy Large ewes were heavier (P < 0.05) and of greater (P < 0.05) body condition than Small ewes. Similarly, at days 1 and 140 of pregnancy twin-bearing ewes were heavier (P < 0.05) than single-bearing ewes. At day 1 of pregnancy there was no difference in ewe liveweight and body condition scores between Maintenance and Ad lib-fed ewes, but by day 140 ewes in the Ad lib regimen were heavier (P < 0.05) and had greater (P < 0.05) body condition than Maintenance ewes. There was an interaction (P < 0.05) between ewe nutritional regimen and lamb birth rank for lamb birth weight such that twin lambs born to Maintenance regimen ewes were lighter (P < 0.05) than their counterparts born to Ad lib ewes (4.52 ± 0.06 versus 5.23 ± 0.06 kg respectively). This relationship was not observed in singletons. There was also an interaction (P < 0.05) between ewe size and nutritional regimen for lamb birthweight. Birth weights did not differ between lambs born to either Small or Large ewes on the Ad lib feeding regimen. However, lambs born to Small ewes on the Maintenance feeding regimen were lighter (P < 0.05) than their counterparts born to Large ewes (5.09 ± 0.07 versus 5.37 ± 0.07 kg respectively). At day 100 of lactation lambs born to Large ewes were heavier (P < 0.05) than those born to Small ewes (32.65 ± 0.37 versus 31.16 ± 0.35 kg respectively) and those born to Ad lib ewes were heavier (P < 0.05) than those born to Maintenance ewes (32.77 ± 0.37 versus 31.03 ± 0.36 kg respectively). This study shows that level of dam pregnancy nutrition has a greater effect on twin-born lambs than their singleton-born counterparts. Furthermore, the data suggests that the heavier liveweights at weaning of lambs born to Large ewes may not have compensated for their dams' greater nutritional requirements compared to their smaller counterparts.

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1. The proportions of carbon in individual milk constituents derived from feed and body tissues in dairy cows, were estimated by taking advantage of the natural variations which occur in the ratio, 13C:'2C present in C,-and C,-plant species.2. Four cows, which had previously grazed C, plants (ryegrass (Loliurn spp.) and white clover (Tvi/oliurn repens)), were accustomed to indoor feeding on a ration of C,-plant material (cut pastures and barley meal). The ration was then changed abruptly to one of C,-plant material (paspalum (Paspalurn dilatatum) hay, maize silage and meal) for a period of 8 or 9 d in early and again in late lactation.3. During early lactation it was estimated that 54% of the C in milk fat was derived from the body fat reserves of high genetic merit cows. Corresponding values for casein and lactose were 34 and 24% respectively, if it is assumed they were derived from body protein reserves. In contrast steam-volatile fatty acids in milk fat were almost entirely derived from dietary sources.4. The proportional contribution of body-tissue C to individual milk constituents varied considerably between animals, possibly associated with genetic merit or the size of the body reserves available for mobilization.5. In late lactation, when cows were close to energy and protein balance, contributions of body-tissue C to milk fat, casein and lactose ranged up to 19, 19 and 8 % respectively.6. Estimates of endogenous losses of C in faeces averaged 12 and 9% in early and late lactation respectively, and corresponding values for endogenous urinary C were 23 and 15%.

1988). Plasma metabolite and hormone concentrations as predictors of dairy merit in young Friesian bulls: effect of metabolic challenges and fasting. ABSTRACTResponses to metabolic challenges were measured in 8-month-old Friesian bull calves with a view to identifying possible predictors of genetic merit for milk fat production. Seven 'high' breeding index bulls (mean breeding index for milk fat = 128-9 (s.d. 4-8) percentage units) were compared with six 'low' breeding index bulls (mean 107-3 (s.d. 2-4) percentage units). Bulls were from two selection lines and breeding indices calculated as the average of parental breeding index values.Challenges involved intravenous injection of the following (dose rates per kg body weight): adrenalin (1 ng); glucose (0-17 g); glucagon (0-175 ^g); insulin (0-01 mg); and arginine (40 mg). Blood samples were withdrawn prior to and after each challenge (two challenges daily). Bulls were then fasted for 3 days before being refed.Relative to the low breeding index bulls, those in the high breeding index group exhibited: (a) greater pancreatic sensitivity to circulating glucose as indicated by increased insulin concentrations following the glucose challenge; (b) more rapid clearance of glucose from plasma following the insulin challenge; (c) reduced sensitivity to the glycogenolytic/gluconeogenic effects of glucagon as indicated by lower blood glucose concentrations following intravenous injection of this hormone; (d) elevated plasma insulin and growth hormone concentrations during the fasting period; (e) elevated plasma concentrations of insulin, glucose and urea during refeeding; (f) no difference in responses to intravenous arginine or adrenalin, or in circulating concentrations of Insulin-like Growth Factor-1 or alkaline phosphatase. Results are consistent with previous observations that metabolic differences between cattle in the Massey University high and low breeding index lines are most evident in the metabolism of glucose and insulin. Metabolic challenges offer a potentially useful means of predicting genetic merit for milk fat production but the conditions under which repeatable differences between the lines can best be demonstrated are yet to be determined.