Milk production and composition, and progeny performance in young ewes with high merit for rapid growth and muscle and fat accumulation.

In ewe lambs, acceleration of growth and accumulation of both muscle and fat leads to earlier sexual maturity and better reproductive performance. The next stage in the development of this theme is to test whether these aspects of growth in young ewes affect milk production in their first lactation and the growth of their first progeny. We studied 75 young Merino ewes that had known phenotypic values for depth of eye muscle (EMD) and fat (FAT), and known Australian Sheep Breeding Values for post-weaning weight (PWT) and depths of eye muscle (PEMD) and fat (PFAT). They lambed for the first time at 1 year of age. Their lambs were weighed weekly from birth to weaning at 10 weeks to determine live weight gain and weaning weight. Progeny birth weight was positively associated with live weight gain and weaning weight (P0.05). The PWT of the sire was positively associated with live weight gain (P0.05). The concentrations of fat, protein, lactose and total solids in the milk were not affected by the phenotype or genotype of the mothers or of the sires of the mothers, or by the sex of the progeny (P>0.05). We conclude that selection of young Merino ewes for better growth, and more rapid accumulation of muscle and fat, will lead to progeny that are heavier at birth, grow faster and are heavier at weaning. Moreover, milk production and composition do not seem to be affected by the genetic merit of the mother for post-weaning live weight or PEMD or PFAT. Therefore, Merino ewes can lamb at 1 year of age without affecting the production objectives of the Merino sheep industry.

Relationships among Puberty, Muscle and Fat, and Liveweight Gain during Mating in Young Female Sheep.

Greater depths of muscle are associated with better reproductive performance in ewe lambs, but, in adult ewes, reproductive performance also seems to vary with liveweight gain during the mating period. Therefore, in a large field study with Merino ewe lambs, we tested whether the relationships among eye muscle depth (EMD), fat depth (FAT) and reproductive performance depend on liveweight gain during the mating period. We selected lambs with a wide range in phenotypic values for depths of eye muscle (EMD) and fat (FAT) and assigned them to dietary treatments designed to achieve low (LOW, n = 244) or high (HIGH, n = 237) rates of liveweight gain during a 28-day mating period. The LOW treatment maintained live weight, whereas the HIGH treatment gained 179 ± 3.8 g/day (p < 0.001). From those ewe lambs that attained puberty, first oestrus was detected at live weight 37.8 ± 0.2 kg and age 232 days. The proportion of ewes that attained puberty increased with EMD (p < 0.01). Ewes from the HIGH treatment were more fertile (pregnant ewes per 100 ewes exposed to rams) and had a higher reproductive rate (foetuses in utero per 100 ewes exposed to rams; p < 0.001) than those from the LOW treatment. Fertility and reproductive rate were positively correlated with weight gain during mating as well as live weight at the start of mating, FAT and EMD (p < 0.05 to <0.001). We conclude that faster growth, due to either extra nutrition during mating or higher phenotypic potential for fat and muscle, will increase reproductive performance in ewe lambs mated at 8 months of age.

Relationships among body composition, circulating concentrations of leptin and follistatin, and the onset of puberty and fertility in young female sheep.

The onset of puberty depends on the attainment of critical body mass, so should also be affected by increases in the rate of accumulation of muscle and adipose tissue. Adipose tissue and reproduction are linked by leptin. For muscle, a link has not yet been identified, although one possibility is follistatin. We assessed the relationships among circulating concentrations of follistatin and leptin and the rates of growth and accumulation of muscle and fat during pubertal development in female sheep. We used 326 animals with known phenotypic values for live weight (LW), depths of eye muscle (EMD) and fat (FAT), and known breeding values at post-weaning age for body mass (PWT) and depths of eye muscle (PEMD) and fat (PFAT). Leptin concentration was positively correlated with values for EMD, PEMD, FAT, PFAT, LW and PWT (P<0.001), whereas follistatin concentration was negatively correlated with values for EMD and PWT (P<0.001), and PEMD (P<0.01) and FAT (P<0.05). Leptin concentration was negatively related to age and positively related to live weight at first oestrus and the proportion of females that attained puberty (P≤0.05), and to fertility and reproductive rate (P<0.01). Follistatin concentration was negatively related to live weight at first oestrus and to fertility (P<0.01) and reproductive rate (P<0.05). There were positive correlations (P<0.001) between muscle accumulation and leptin concentration, and between muscle accumulation and reproductive performance. We conclude that leptin and follistatin are probably both involved in effects of accelerated accumulation of muscle and adipose tissues on the onset of puberty.

Ewe lambs with higher breeding values for growth achieve higher reproductive performance when mated at age 8 months.

We studied the relationships among growth, body composition and reproductive performance in ewe lambs with known phenotypic values for depth of eye muscle (EMD) and fat (FAT) and Australian Sheep Breeding Values for post-weaning live weight (PWT) and depth of eye muscle (PEMD) and fat (PFAT). To detect estrus, vasectomized rams were placed with 190 Merino ewe lambs when on average they were 157 days old. The vasectomized rams were replaced with entire rams when the ewe lambs were, on average, 226 days old. Lambs were weighed every week and blood was sampled on four occasions for assay of ghrelin, leptin and ß-hydroxybutyrate. Almost 90% of the lambs attained puberty during the experiment, at an average live weight of 41.4 kg and average age of 197 days. Ewe lambs with higher values for EMD (P < 0.001), FAT (P < 0.01), PWT (P < 0.001), PEMD (P < 0.05) and PFAT (P < 0.05) were more likely to achieve puberty by 251 days of age. Thirty-six percent of the lambs conceived and, at the estimated date of conception, the average live weight was 46.9 ± 0.6 kg and average age was 273 days. Fertility, fecundity and reproductive rate were positively related to PWT (P < 0.05) and thus live weight at the start of mating (P < 0.001). Reproductive performance was not correlated with blood concentrations of ghrelin, leptin or ß-hydroxybutyrate. Many ewe lambs attained puberty, as detected by vasectomized rams, but then failed to become pregnant after mating with entire rams. Nevertheless, we can conclude that in ewe lambs mated at 8 months of age, higher breeding values for growth, muscle and fat are positively correlated with reproductive performance, although the effects of breeding values and responses to live weight are highly variable.

Selection for superior growth advances the onset of puberty and increases reproductive performance in ewe lambs.

The reproductive efficiency of the entire sheep flock could be improved if ewe lambs go through puberty early and produce their first lamb at 1 year of age. The onset of puberty is linked to the attainment of critical body mass, and therefore we tested whether it would be influenced by genetic selection for growth rate or for rate of accumulation of muscle or fat. We studied 136 Merino ewe lambs with phenotypic values for depth of eye muscle (EMD) and fat (FAT) and Australian Sheep Breeding Values at post-weaning age (200 days) for live weight (PWT), eye muscle depth (PEMD) and fat depth (PFAT). First oestrus was detected with testosterone-treated wethers and then entire rams as the ewes progressed from 6 to 10 months of age. Blood concentrations of leptin and IGF-I were measured to test whether they were related to production traits and reproductive performance (puberty, fertility and reproductive rate). In total, 97% of the lambs reached first oestrus at average weight 39.4 ± 0.4 kg (mean ± s.e.m.) and age 219 days (range 163 to 301). Age at first oestrus decreased with increases in values for PWT (P < 0.001), and concentrations of IGF-I (P < 0.05) and leptin (P < 0.01). The proportion of ewe lambs that achieved puberty was positively related with increases in values for EMD (P < 0.01), FAT (P < 0.05) or PWT (P < 0.01), and 75% of the ewe lambs were pregnant at average weight 44.7 ± 0.5 kg and age 263 days (range 219 to 307). Ewe lambs that were heavier at the start of mating were more fertile (P < 0.001) and had a higher reproductive rate (P < 0.001). Fertility and reproductive rate were positively correlated with increases in values for EMD (P < 0.01), FAT (P < 0.05), PWT (P < 0.01) and leptin concentration (P < 0.01). Fertility, but not reproductive rate, increased as values for PFAT increased (P < 0.05). Leptin concentration increased with increases in values for EMD (P < 0.001), FAT (P < 0.001), PWT (P < 0.001), PEMD (P < 0.05) and PFAT (P < 0.05). Many of these relationships became non-significant when PWT or live weight was added to the statistical model. We conclude that selection for genetic potential for growth can accelerate the onset of puberty and increase fertility and reproductive rate of Merino ewe lambs. The metabolic hormones, IGF-I and leptin, might act as a physiological link between the growing tissues and the reproductive axis.

Phenotypic blood glutathione concentration and selenium supplementation interactions on meat colour stability and fatty acid concentrations in Merino lambs.

The interaction between blood glutathione (GSH) and supplementation of selenium (Se, 2.5 mg/kg diet) on meat colour and fatty acids concentrations was studied. Forty eight Merino lambs selected for high blood GSH (HGSH) or low GSH (LGSH) concentration were used. They were fed individually with or without Se supplement for 8 weeks. There were interactions (P<0.05) between GSH and Se on the colour stability (as w630 nm/w580 nm ratio) of m. longissimus (LD), m. semimembranosus (SM) and m. semitendinosus. Without Se supplementation the ratio was higher in HGSH than LGSH group. However, the difference was reduced with Se supplement. Polyunsaturated and n-3 fatty acids in SM and LD were higher in HGSH than in LGSH group (P<0.05), and did not change with Se supplement. Se supplementation increased Se content in LD (P<0.001) and the lungs (P<0.05), but had no influence in the heart.

Multiple effects of an additional growth hormone gene in adult sheep.

Molecular genetics provides an increasing capacity to modulate the function of individual genes, but the practical implications of these technologies are still poorly understood. This study examined adult Merino or Merino-cross sheep that had an additional copy of the ovine GH gene with a modified metal-lothionine promoter, which resulted in a doubling of the plasma concentration of GH. Previous work showed that up to the age of 18 mo, GH sheep grew faster and had less s.c. fat, with only minor effects on fleece production. The present paper describes characteristics of reproduction, wool production, and animal health of these sheep during the following 2 yr of adult life. Ewes with the GH gene had a greater ovulation rate (1.78 vs. 1.35; P <0.05), but bore fewer lambs, apparently due to greater fetal loss after mating. Grease fleece weight was increased (P <0.05) due mainly to a greater content of suint (9.1 vs. 7.7 +/- 0.4%, P <0.01), which was associated with a deeper color of the raw wool. Effects on clean fleece weight and fiber diameter were not consistent between years. The GH sheep had swollen metatarsal and metacarpal joints, which was associated with a need for more frequent hoof-trimming, and more GH than control sheep died during the experiment (P <0.001). All of these changes are consistent with previously reported effects of increased plasma GH. Results of this study show that increased activity of a single gene (GH) affected several production characteristics and predisposed the animals to a number of distinct health problems, some of which developed after the normal age of genetic selection.

Reproduction and plasma concentrations of leptin, insulin and insulin-like growth factor 1 in growth-hormone-transgenic female sheep before and after artificial insemination.

The transgenic sheep used in this study expressed an additional copy of the gene for ovine growth hormone (GH), so they had continuously high plasma concentrations of GH. They were used to test whether the GH transgene affected plasma concentrations of the metabolic hormones leptin, insulin-like growth factor 1 (IGF-1) and insulin, and whether these effects were associated with changes in conception, pregnancy or parturition following artificial insemination. Compared with control animals, the GH-transgenic sheep had higher bodyweight, lower body condition score and less subcutaneous fat (P < 0.05). These sheep also had lower plasma concentrations of leptin, higher plasma concentrations of insulin, and higher plasma concentrations of IGF-1 (P < 0.001). A similar proportion of GH-transgenic and control ewes came into oestrus, but the conception rate to artificial insemination was lower in GH-transgenic ewes than in the controls. Only four live lambs were recovered from 12 GH-transgenic ewes (33%) compared with 38 lambs from 43 controls (88%). This outcome was not associated with any difference in plasma progesterone profile in the period leading up to artificial insemination (Day 0). The GH-transgenic ewes had lower concentrations of FSH at all times measured (Day -19, Day -2 and Day 19). These results indicate that appropriate regulation of GH secretion from pituitary or peripheral tissues is necessary for normal reproduction and normal levels of metabolic hormones. Chronically high concentrations of GH were associated with increased levels of IGF-1 and insulin, and decreased levels of leptin.

Relationships between plasma concentrations of leptin and other metabolic hormones in GH-transgenic sheep infused with glucose.

To study the regulation of leptin secretion in sheep, we infused glucose (0.32 g/h/kg for 12 h) into GH-transgenic animals (n = 8) that have chronically high plasma concentrations of ovine GH and insulin, but low body condition and low plasma leptin concentrations, and compared the responses with those in controls (n = 8). In both groups, the infusion increased plasma concentrations of glucose and insulin within 1 h and maintained high levels throughout the infusion period (P < 0.0001). Compared with controls, GH-transgenics had higher concentrations of insulin, IGF-1, GH (all P < 0.0001) and cortisol (P < 0.05), but lower GH pulse frequency (P < 0.0001). Overall, leptin concentrations were lower in GH-transgenics than in controls (P < 0.01). A postprandial increase in leptin concentrations was observed in both groups, independently of glucose treatment, after which the values remained elevated in animals infused with glucose, but returned to basal levels in those infused with saline, independently of transgene status. In both GH-transgenics and controls, glucose infusion did not affect the concentrations of GH, IGF-1, or cortisol. In conclusion, GH-transgenic and control sheep show similar responses to glucose infusion for leptin and other metabolic hormones, despite differences between them in body condition and basal levels of these hormones. Glucose, insulin, GH, IGF-1 and cortisol are probably not major factors in the acute control of leptin secretion in sheep, although sustained high concentrations of GH and IGF-1 might reduce adipose tissue mass or inhibit leptin gene expression.

The impact of a transgene for ovine growth hormone on the performance of two breeds of sheep.

The effect of a transgene encoding ovine growth hormone and regulated by a metallothionein promoter was examined in progeny of 69 Merino ewes and 49 Poll Dorset ewes that were inseminated by rams heterozygous for the gene construct. The presence of the transgene had no effect on the progeny from one of the three rams used, as evinced by a normal concentration and secretion pattern of growth hormone and normal growth rate and fatness. In progeny from the other two rams that bore an actively transcribed and translated copy of the transgene, the mean concentration of growth hormone in the plasma was twice that of controls, but the pulsatility of secretion was lost. These animals grew faster (P < 0.001) and were leaner (P < 0.001), but had a greater parasite fecal egg count (P < 0.001). The impact of the transgene differed between breeds with greater wool growth rate (P < 0.01) and live weight increase (P = 0.06) in Merino progeny compared with Poll Dorset cross. At 18 mo of age, the depth of the eyemuscle was decreased (P < 0.001), particularly in female sheep (P < 0.01). The results indicate that the production effects of genetic manipulation may depend on the age, the breed, and the sex of the animal. Furthermore, the transgene may fail to be expressed in some progeny so that its activity cannot be detected, even though the sheep bear the DNA construct.

Metabolic hormones and tissue concentrations of mRNA for IGF-I in lines of sheep that differ in their protein synthesis response to feed intake.

The rate of protein synthesis in the skin and muscle of sheep that have been genetically selected for high wool staple strength (SS) is less dependent on the level of dietary intake than that of low SS sheep. This study examined potential hormonal mediators of this difference in responsiveness. Sheep from SS+ and SS- genotypes were fed at 0.4, 1.1 or 1.8 times maintenance. Circulating concentrations of metabolic hormones and tissue concentrations of the mRNA for IGF-I were measured and compared with rates of protein synthesis measured previously. Plasma concentrations of GH, insulin, cortisol, thyroxine and IGF-I responded similarly to dietary intake in both genotypes, but SS+ sheep had higher plasma concentrations of IGF-I at all levels of nutrition (P<0.05). There were no interactions between diet and genotype. The concentration of mRNA for IGF-I was higher in the liver of SS+ sheep (P<0.05), and tended to increase (P=0.06) with nutrient intake, but there were no significant effects of genotype or nutrition in skin, muscle or gut. Concentrations of mRNA for IGF-I were not related to the rate of protein synthesis in any tissue examined. It was concluded that IGF-I did not drive the rate of protein synthesis directly, but it may mediate the responsiveness of protein synthesis rate, or protein degradation rate, to nutrient supply.

Determinants of the annual pattern of reproduction in mature male merino and Suffolk sheep: modification of endogenous rhythms by photoperiod.

In mediterranean environments, pastures are very poor during the autumn and consequently small ruminants, such as sheep, would have been losing body mass for many months so, during mating, gamete production would be depressed in both sexes. Effectively, the nutritive requirements for a photoperiod-driven, annual reproductive cycle are out of phase with seasonal changes in food availability. The problem could be overcome through more flexible timing of reproduction, perhaps explaining variations in seasonality between breeds that originate from differing latitudes. To study these concepts and the mechanisms involved, the endogenous rhythms and responses to photoperiod were compared in rams of 'mediterranean origin' (Merino) and 'temperate origin' (Suffolk). Groups of 16 rams of each breed were given a constant food supply and subjected to 16 months of constant equinoctial photoperiod (12L : 12D) or simulated 'mediterranean' changes in daylength (from 14L : 10D to 14D : 10L). With nutritional and photoperiodic inputs held constant, Merino and Suffolk rams showed similar endogenous rhythms in reproductive activity. Under constant nutritional inputs and a mediterranean photoperiodic cycle, the endogenous rhythms were modified differently in the two breeds, with the Merinos starting and finishing their seasons about 2 months earlier than the Suffolks. These observations partially explain the patterns observed in rams kept under field conditions. It is now necessary to test whether the rhythms of reproduction in these breeds are also modified by changes in nutrition and social cues.

Level of nutrition modulates the dynamics of oestradiol feedback on plasma FSH in ovariectomized ewes.

The frequency of multiple ovulations in mature, cyclic ewes is strongly influenced by the level of nutrition, but it is difficult to demonstrate concurrent changes in plasma concentrations of gonadotropins. The failure to do so might be a consequence of rapid compensation by the homeostatic feedback mechanism linking secretion by the hypothalamus/pituitary gland and ovarian hormones. Most experimental models have examined the components of the homeostatic feedback system after steady state relationships had been established. We hypothesised that the effects of nutrition might be observed more readily if the system were disrupted and then examined while equilibrium was being re-established. This hypothesis was tested in three experiments in Merino ewes by allowing gonadotropin secretion to escape feedback for 5 days after ovariectomy and then replacing ovarian hormones and examining effects of feeding regimen on the return of plasma concentrations of FSH to baseline values. In all three experiments, oestrogen replacement caused plasma concentrations of FSH to decline more rapidly (P < 0.05) in ewes fed at 0.5x maintenance, than in ewes fed at 1.4x maintenance, with groups fed at maintenance being intermediate. No effect of diet was observed on plasma FSH concentrations in the absence of oestradiol, and neither progesterone nor charcoal-treated bovine follicular fluid influenced the effect of nutrition. Plasma concentrations of oestradiol were 9.8% lower on average (NS) in ewes fed above maintenance than in the sheep fed below maintenance over the three experiments, suggesting that there may have been a reduced clearance of oestradiol which contributed to the result. We conclude that feeding regimen affects the secretion or clearance of gonadotropins in mature ewes, as in the mature ram, and that this is one mechanism by which ovulation rate may be affected.

Effect of diet on the clearance of estradiol-17 beta in the ewe.

The twinning rate of ewes is poorly related to plasma concentrations of gonadotropins. In this study, we tested an alternative hypothesis for the control of twinning rate, by testing whether nutritional treatment sufficient to affect twinning could alter estrogen metabolism, with a particular focus on the enterohepatic recirculation. Groups of 5 ovariectomized ewes were fed either above maintenance (supplemented) or below maintenance (restricted). The metabolism of estradiol-17 beta was examined by following the fate of a single i.v. injection of 1.45 micrograms [3H]estradiol-17 beta. In both groups, 74% of the radioactivity was recovered from the feces within 10 days, predominantly as free estradiol-17 alpha, but excretion was slower in the nutritionally restricted ewes. A further 6% of injected radioactivity was excreted in the urine, mostly within 24 h, with no effect of dietary group. Radioactivity in plasma was characterized by ion-exchange chromatography and HPLC. Within 30 min of injection, the main circulating radioactive compound was estradiol-17 alpha sulfate. This remained at a greater concentration than free steroid for the next 48 h, and was greater after 16 h (p < 0.05) in plasma of nutritionally restricted ewes than in the supplemented group. At 0.5 and 2 h, the free steroid was almost entirely estradiol-17 beta, but a polar compound, which appeared by 4 h and probably arose by recirculation from the intestine, remained the major unconjugated metabolite in plasma for the next 24 h. Plasma concentrations of this compound were higher (p < 0.05) in the restricted ewes than in the supplemented ewes during this period.(ABSTRACT TRUNCATED AT 250 WORDS)

Frequent blood sampling changes the plasma concentration of LH and FSH and the ovulation rate in Merino ewes.

During earlier studies we observed that ewes housed and sampled intensively to measure pulses of LH in plasma had a higher ovulation rate than similar ewes housed outside. In Expt 1, we pursued this observation by testing whether the increase was due to effects of housing or collection of blood samples. Ewes sampled at intervals of 4 h for 2 days before progestagen sponge removal and 2 days after sponge removal, and every 20 min for 12 h the day before sponge removal and every 10 min for 4 h on the day of sponge removal had a higher ovulation rate than ewes that were not sampled (1.72 versus 1.41; P < 0.05). The ovulation rate of the ewes housed indoors but not sampled was similar to that of ewes that remained in the paddock (1.43). In Expt 2, we studied the effects of blood sampling in three groups of 20 ewes sampled every 20 min for different periods of 24 h. Ewes from all three groups were sampled the day before sponge removal (day -1) and, in addition, one group of ewes was sampled for the previous 48 h (i.e. days -3 to -1) and another group was sampled on day -8. The frequency of LH pulses was lower (P < 0.05) in ewes sampled for the first time on day -1 compared with the frequency of LH pulses in groups also sampled earlier in the cycle (day -8 or days -3 and -2). In ewes sampled on days -3 to -1, the frequency of LH pulses was low for the first 24 h and then increased.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunization of ovariectomized ewes against progesterone, oestrogen or cortisol to detect effects of adrenal steroids on reproduction.

Ovariectomized ewes were immunized against cortisol, oestrogen or progesterone to determine whether steroids of non-ovarian origin occur in sufficient amounts to influence the reproductive system. Ewes immunized against oestrogen (oestrone + oestradiol) had smaller uteri (P less than 0.05) and a lower concentration of oestradiol in uterine tissue (P less than 0.05), while the adrenal gland was heavier (P less than 0.05). In the peripheral plasma of immunized ewes, the concentration of FSH, but not LH, was increased (P less than 0.05). Ewes immunized against cortisol also had smaller uteri (P less than 0.05) and heavier adrenal glands (P less than 0.05), but gonadotrophins and the concentration of oestradiol in the uterus were unaffected. Immunization against progesterone, or injection with synthetic glucocorticoids, did not affect any of the characteristics measured. We conclude that the adrenal gland of the ewes produced sufficient oestrogen or aromatizable androgen to have significant effects on the reproductive system. In contrast, adrenal production of progesterone did not produce detectable biological effects. Immunization against cortisol did affect the reproductive system, but the mechanism of this action could not be determined.