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Maternal nutrient restriction during critical windows of fetal development alters postnatal growth, often in a sexually dimorphic manner. Intrauterine growth restriction is frequently characterized by accelerated growth and increased adiposity in later life. Thyroid hormones are implicated as part of the mechanism involved in this scenario via their actions within the hypothalamic-pituitary-thyroid axis. We fed high (H = 240%) and low (L = 70%) levels of recommended daily crude protein intake during the first and second trimesters of gestation to beef heifers to investigate effects to their progeny's plasma concentrations of free and total triiodothyronine (FT3 and TT3) and thyroxine (FT4 and TT4) from birth until weaning at 191 days of age (n = 68). The study design was a two-by-two factorial. For male progeny, exposure to maternal diets low in protein during the first trimester of gestation resulted in greater FT4 at birth (P < 0.05) which was subsequent to lower concentrations of leptin in maternal plasma at 271 days of gestation compared with their high-protein-exposed counterparts. These same animals went on to have greater milk intake during the latter half of the lactation period (P < 0.05) and exhibited faster rates of average daily gain (ADG) relative to birth weight during this time (P < 0.05). For all progeny, independent of sex, exposure to low-protein maternal diets during the second trimester of gestation resulted in greater FT3 relative to TT3 at birth. Because FT3 at birth and 29 days was positively associated with ADG (P < 0.05) and ADG relative to birth weight (P < 0.05), it is proposed that FT3 plays an integral role in catch-up growth in the bovine as per other species. Protein intake during the first and second trimesters of gestation has a sexually dimorphic effect on progeny plasma thyroid hormone concentrations, and these changes are associated with altered milk intake and postnatal growth pathway.

The effect of nutrition during the first and second trimesters of pregnancy in composite beef heifers on reproductive parameters of their female calves was determined in the present study. At artificial insemination, heifers were assigned to one of four treatment groups (i.e. HH, HL, LowH and LL) depending on the level of crude protein intake (H = high; L = low) for first and second trimesters of pregnancy. Gonadotrophin concentrations and ovarian parameters were measured in their female calves at 5 and 23 months of age. Crude protein intake was positively associated with dam plasma urea (P < 0.001). The density of healthy follicles in heifers at the time of death was negatively correlated with dam plasma urea at Day 179 (P = 0.009). Heifers from LowH dams had a smaller-sized prepubertal largest ovarian follicle (P = 0.03) and lower densities of primordial and primary follicles (P = 0.02) and healthy antral follicles (P = 0.009) when they were killed. There was a positive correlation between plasma FSH concentrations at 5 and 23 months of age (P = 0.02), as well as between the sizes of the largest ovarian follicles at 6 and 23 months of age (P = 0.01). In conclusion, the reproductive development of heifers may be affected by prenatal nutrition during early and mid-gestation.

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