In this review, "productive efficiency" in dairy cows is defined as the yield of milk obtained in ratio to the nutritional costs associated with maintenance, milk synthesis and loss of body condition during lactation. Improvements in efficiency could occur as a result of changes in digestion and nutrient absorption, maintenance requirement, utilization of metabolizable energy for production or nutrient partitioning. Digestibility can be greatly enhanced by appropriate dietary manipulation. Likewise, it may be possible to reduce maintenance requirements and improve the efficiency with which metabolizable energy is used for milk synthesis by manipulation of the pattern of nutrients presented to tissues. However, these factors apparently do not respond to selection for increased milk yield, and little variation is observed among cows. In contrast, individual cows differ substantially in feed intake and in the partitioning of nutrients among body tissues. Techniques associated with genetic engineering and the early prediction of genetic merit have the potential to improve productive efficiency by manipulation of these processes. However, changes in nutrient partitioning and feed intake during lactation are coordinated by a complex network of controls that accommodate the nutrient requirements of each tissue while maintaining homeostatic balance. Future improvements in productive efficiency will therefore depend on our ability to understand the manner in which these controls operate.

S U M M A R YFourteen young wether sheep were fed freshly cut Lotus pedunculatus as a sole diet to examine the effects of condensed tannins (CT; 55 g/kg lotus DM) on nitrogenous aspects of digestion. The experiment was carried out indoors at Palmerston North, New Zealand over 32 days with one group of sheep receiving an intraruminal infusion of polyethylene glycol (PEG; 100 g/day) to preferentially bind CT (PEG group) so that the lotus was essentially 'CT-free'. The other sheep, not given PEG, were termed the 'Tannin' group.The principal effects of CT were to increase the flow of feed nitrogen (N) to the abomasum despite a 12% reduction in DM intake of the Tannin sheep. Rumen microbial N turnover rate was slower in Tannin animals than in those receiving PEG (1-86 v. 2-63/day) but microbial N flux to the abomasum was similar in both treatments. The proportion of N intake disappearing from the rumen was lower in Tannin (013) than in PEG sheep (0-26) and the N digestibility was 0-67 and 0-81 for the respective treatments (P < 0001).The beneficial effects of CT in reducing rumen degradation of feed protein were negated in part by a reduction in fractional absorption of amino acids (AA) from the small intestine. Fractional absorption of essential AA was 0-66 in Tannin and 0-79 in PEG sheep; values for non-essential AA were 0'59 in Tannin and 073 in PEG groups. Amino acid concentrations in blood were similar for both groups, but Tannin sheep had lower plasma urea concentrations, a more rapid plasma urea turnover rate and a higher irreversible loss than those receiving PEG. Growth hormone concentrations in plasma were similar for both treatments.

Bovine growth hormone was administered to four Holstein cows (late lactation) in a Latin square. Treatments were 1) control, subcutaneous injection of placebo, 2) subcutaneous injection of 25 IU growth hormone on alternate days, 3) daily subcutaneous injection of 25 IU growth hormone, and 4) continuous subcutaneous infusion of 25 IU/d growth hormone. Intravenous challenges of epinephrine and thyrotropin-releasing hormone were administered separately on the 1st d after a 12-d interval of growth hormone treatment. Baseline concentrations of nonesterified fatty acids in plasma were not affected by growth hormone treatment. However, release of nonesterified fatty acids to epinephrine challenge was positively related to amount of growth hormone in plasma during treatment and was correlated with milk fat yield and milk energy secretion. Growth hormone release following thyrotropin-releasing hormone was negatively related to amount of growth hormone in plasma during treatment. Release of prolactin to thyrotropin-releasing hormone challenge was not related to average daily dose of exogenous growth hormone. The galactopoeitic action of growth hormone may be partly attributable to its effects on responsiveness of adipose tissue to a lipolytic stimulus but apparently is not associated with changes of pituitary sensitivity to thyrotropin-releasing hormone. Results are consistent with growth hormone functioning as a homeorhetic control of nutrient partitioning.

The objectives were to determine the effects of daily injection of bovine growth hormone (bGH) on the metabolism of [1-14C]leucine and [1-14C]palmitate and on hormone and metabolite concentrations in growing Hereford heifers. The experimental design was a 28-d single reversal with two 14-d injection periods of placebo or bGH. Energy intake was restricted to a level slightly above maintenance. Injection of bGH did not affect circulating concentrations of glucose, beta-hydroxybutyrate, urea nitrogen, prolactin, triiodothyronine or thyroxine. Plasma concentrations of insulin and nonesterified fatty acids (NEFA) were chronically elevated whereas leucine concentration was chronically decreased after 1 wk of bGH injection compared to placebo injection. Leucine oxidation was lower and whole-body protein synthesis was higher during bGH injection than during placebo injection. There were increases in both total irreversible loss and oxidation of NEFA during bGH injection compared to placebo injection. These results suggest mobilization of stored fatty acids and increased reliance on NEFA to provide energy for cellular processes. The dual and reciprocal effects of bGH on nitrogen and NEFA metabolism demonstrate its role as a homeorhetic regulator, affecting metabolism of several body tissues to support lean body accretion in Hereford heifers at near-maintenance intake of metabolizable energy.

Fetal growth is normally constrained by maternal factors. This constraint is demonstrated by the usual inverse linear relationship between litter size and mean fetal weight. Cross-breeding experiments between mice of lines selected for high or low plasma insulin-like growth factor (IGF-I) levels suggested that elevations in maternal IGF-I abolish (P less than 0.01) this constraining effect and reverse the usual positive relationship between fetal and placental size in late gestation. This was confirmed by treating mice and rats throughout pregnancy with IGF-I. In normal mice and in low IGF-I line mice treatment with IGF-I (10 micrograms 8-hourly s.c. from day 1 to 19 of pregnancy) abolished maternal constraint whereas 0.9% (w/v) NaCl treatment did not. In Wistar rats osmotic pumps were implanted to deliver IGF-I (1 microgram/g body weight per day), bovine GH (bGH; 0.6 microgram/g body weight per day) or saline from day 1 to 19 of pregnancy. IGF-I therapy but not bGH or saline abolished (P less than 0.01) maternal constraint and altered (P less than 0.01) the relationship between placental and fetal weight. When high or low IGF-I line mice embroys were transplanted into a normal line of mice, the expected negative relationship (P less than 0.05) between mean fetal weight and litter size was maintained. However, the embryos of the high line were heavier (P less than 0.05) than those from the low line irrespective of fetal number, suggesting a direct role for IGF-I in the regulation of fetal growth.(ABSTRACT TRUNCATED AT 250 WORDS)

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Controlled release capsules (CRC) provide for the continuous uniform release of chromic oxide (Crp3) into the rumen and so offer an improved means of estimating feed intake in grazing animals. Two experiments were conducted with sheep to measure the effect of herbage type and level of herbage intake on the pattern of Crp3 release from CRC. In Experiment 1, differences in CRC plunger travel within and between rumen-fistulated sheep, fed lucerne chaff indoors over 42 days at an allowance equivalent to 1.1 maintenance (M), were not significant. The relationship between plunger travel and time was almost perfectly linear (P < 0.001),but a slight reduction in the rate ofplunger displacement became more pronounced while the animals were at grazing between Day 43 and Day 100. Chromium (Cr) release from Day 3 to Day 42 averaged 62 ± 1 (mean ± SE) mg Cr/day, In Experiment 2, rumenfistulated and intact wethers, housed indoors, were offered ryegrass, white clover, ryegrass-white clover

This study compared the effect on foetal growth of shearing ewes in very early pregnancy (day 50) with the effect of shearing later in pregnancy (day 70 or 100). One hundred and sixty ewes were allocated at day 49 of pregnancy to four ‘time of shearing’ treatments — pregnancy day 50 (P50), P70, P100 (no. = 45 ewes per treatment) and unshorn (shearing of this group occurred 43 days after weaning) (no. = 25); two methods of shearing (by standard comb and cover comb); and two levels (1 or 2) of pregnancy/rearing rank. All ewes had been mated over a 28-day period and grazed pasture throughout the trial. The mid point of lambing was 27 August 1996. Live weights of ewes (corrected for fleece weight) were not influenced by shearing time at any stage except at day 45 of lactation when ewes shorn at P50, P70 or P100 were about 3⋅0 kg lighter than unshorn ewes. Shearing method had no effect on ewe live weight. Time of shearing and shearing method had no effect on ewe greasy fleece production or wool tensile strength. Shearing significantly (P < 0⋅05) influenced the birth weight of single- (by 0⋅7 to 0⋅8 kg) but not twin-born lambs. Maternal plasma triiodothyronine concentration increased proportionately by 0⋅47 to 0⋅85 (P < 0⋅05) and lasted for at least 20 days in response to shearing irrespective of time of shearing. No difference in plasma insulin concentration was detected between shearing groups but plasma glucose concentration proportionately increased by 0⋅06 to 0⋅16 (P < 0⋅05) for 10 to 20 days after shearing. These results indicate that shearing between day 50 and 100 of pregnancy can increase lamb birth weights and that the response is associated with an increase in thyroid hormone concentrations in the maternal circulation.