The manipulation of nutrition during pregnancy in the heifer and its effect on metabolic hormones, placental development, milk production, post partum oestrus and the reproductive development of the offspring

The effects of varied protein content in the maternal diet during pregnancy are particularly relevant to the cattle industry in Australia, as protein is the most deficient rangeland nutrient, with the majority of replacement beef heifers requiring protein supplementation (Bortolussi et al., 2005a). This thesis aimed to provide information on the effects of breeder protein supplementation on placental and fetal growth, postpartum reproductive efficiency, milk production and potential fertility of the offspring. At artificial insemination (AI), to the same sire bull, two year-old composite beef heifers were divided into four dietary treatment groups: high/high (HH), high/low (HL), low/high (LH), low/low (LL), identified by the level of protein fed during the first and second trimesters, respectively, of gestation. High levels of dietary protein increased IGF-I, -II, IGFBP and leptin. Leptin was positively and IGF-I negatively associated with calf size at birth. We concluded that dietary protein affected maternal IGF and leptin impacting on fetal growth. To investigate the effects of gestational protein supplementation on the placenta at term, expelled placentas from the heifers were collected and weighed and cotyledons were dissected, counted, weighed and measured. The number of cotyledons and weight of the expelled placenta were influenced by the level of maternal dietary protein during gestation. Concentrations of maternal metabolic hormones negatively correlated with the number of cotyledons and placental weight. Calf birth weight, although closely positively associated with placental and cotyledonary weights and area, was not associated with the number of cotyledons. We concluded that dietary protein during gestation influenced placental measures at birth which were important contributors to fetal size. High levels of dietary protein decreased placental function as indicated by decreased bovine pregnancy associated glycoprotein (bPAG) and estrone sulphate (ES) concentrations in first trimester and decreased bovine placental lactogen (bPL) concentrations in the second trimester of gestation. In the third trimester, placental function was no longer associated with previous dietary treatments, yet placental ES corresponded to calf birth weight. We concluded that the placenta was able to compensate for altered maternal protein by changing function as needed to maintain normal fetal growth. Gestation length, placental expulsion time and the interval to the first oestrus postpartum (PPI) are considered to influence postpartum reproductive performance. Whilst the protein treatments imposed during gestation did not directly affect any of these measures in the heifers, high levels of protein during the first and second trimesters resulted in increased prepartum body condition (BCS), which in turn decreased the PPI. Calf size positively contributed to gestation length and PPI. Placental size negatively influenced placental expulsion time. Milk production was assessed monthly postpartum until weaning at 6 months. The milk yield was decreased in heifers receiving a high level of protein in first trimester. Milk production was negatively associated with BCS and leptin during lactation. Colostral protein content was influenced by dietary protein in the first trimester dependent upon genotype and positively associated with bPAG. This inferred that increased dietary protein during the early gestation of the adolescent heifer may have impaired mammary development and affected colostral protein via placental effects. Prenatal protein supplementation may be detrimental to the reproductive function of the offspring. Prepubertal gross testis measures, seminiferous tubule diameters, gonadotrophin, and testosterone concentrations were considered to be indicative of potential reproductive performance in the bull calves. Higher prenatal maternal dietary protein levels were associated with decreased prepubertal Follicle Stimulating Hormone (FSH) concentrations and paired testicular volumes. In the heifer calves, prepubertal measures were taken of gonadotrophin concentrations and ovarian parameters along with postpubertal concentrations of FSH and ovarian measures. Elevated dietary protein in the second trimester of gestation was associated with smaller sized prepubertal ovarian follicles and reduced densities of primordial plus primary follicles and healthy ovarian antral follicles in mature heifer offspring. This thesis constituted, to date, the most comprehensive study of the effects of nutritional protein in the beef heifer during gestation on placental and fetal development and their associations with placental markers and metabolic hormones. It also provided information on the influence of prenatal protein supplementation of heifers on their postpartum reproductive performance and the longer term effects on the reproductive potential of their calves. It conclusively demonstrated that maternal metabolic hormones and placental development respond to levels of nutritional protein during gestation affecting fetal growth. This thesis emphasised that the continuous monitoring of seasonally changing feed values and nutritional requirements of the pregnant growing heifer is essential to production. This thesis also provided the foundations on which to form subsequent studies to further explore long term effects of prenatal protein on reproductive function in offspring