Skeletal growth in cattle in response to nutritional and hormonal manipulation

Abstract

In northern Australia seasonal changes in the feed availability and quality result in periods of reduced liveweight gain or liveweight loss (dry season) and accelerated LWG (wet season) of cattle. However, hip height of growing cattle continues to increase, albeit at a slower rate, when nutrient (protein and energy) availability is reduced during the dry season. Skeletal growth drives muscle growth and, hence, liveweight production of cattle through passive stretch mechanisms. However, little is known about the physiological and morphological adaptation of the skeleton to the seasonal changes in diet experienced by cattle in northern Australia. Longitudinal bone growth in cattle is affected by nutrition but the extent to which protein and/or energy affect bone growth and the hormones involved is largely unknown. Exogenous hormones and sex-steroids increase skeletal growth of cattle but the efficacy under nutrient-restriction is unknown. Therefore, an understanding of the mechanisms that regulate bone growth will provide information to help develop strategies to increase the rate of skeletal growth (during the dry season) and muscle growth (during the wet season) in cattle.Two experiments were conducted to examine the nutritional and hormonal effects on liveweight, body dimension changes, growth plate and trabecular bone histology and changes in the concentration of hormones and bone turnover markers in the circulation. Experiment 1 examined the level of nutrition and bovine somatotropin (bST, 500 mg Sometribove zinc suspension, Elanco Animal Health) treatment in Brahman-cross steers. There were three dietary treatments, high crude protein (CP) and high metabolisable energy (ME) intake (HCP-HME) and high and low CP intake at a low ME intake (HCP-LME and LCP-LME); steers within each nutritional treatment were administered either bST or saline. Experiment 2 examined the entire growth curve of Brahman-cross steers from weaning to maturity under a high level of nutrition and the effects of oestrogen (E2, a Compudose100, containing 21.1 mg oestradiol 17b, Elanco, NSW) or androgen (TE2, Component TE200, containing 200 mg trenbolone acetate and 20 mg oestradiol 17b, Elanco) based steroidal implants on the growth curve.The results of experiment 1 showed that steers with restricted ME intake grew slower (liveweight gain 0 kg/day; hip height gain 34 mm/100 days) with the intake of additional CP having no influence on either LWG or hip height gain. Feed restriction resulted in shorter height of the proliferative and hypertrophic zones and smaller diameter of terminallyndifferentiated chondrocytes with lower formation and mineralisation rates in the growth plates of the bones of Brahman-cross steers compared with steers with high ME intake. Similarly, the trabecular bone of steers with restricted ME intake was thinner and less dense than steers with a high ME intake. Exogenous bST had no effect on growth plate morphology or trabecular bone architecture. Brahman-cross steers with a high ME intake had a higher concentration of insulin, insulin-like growth factor-1, thyroid hormones and leptin in the circulation compared with steers with a ME restricted intake, regardless of CP intake. The concentration of hormones in the circulation was generally unaffected by exogenous bST with the exception of insulin-like growth factor-1 which increased in response to bST administration. The concentration of the bone formation marker (bone alkaline phosphatase) was elevated in the circulation in response to high ME intake and an early stage of maturity but was unaffected by exogenous bST. In contrast, the bone resorption marker (C-terminal telopeptides of Type-1 collagen) was insensitive to nutrient supply and bST.The results of experiment 2 showed that liveweight and hip height of Brahman-cross steers from weaning to maturity were described using the equation of Brody [At=Amax*(1-Be-kt)] regardless of steroid implantation. However, steers implanted with TE2 had a higher maturation rate (k) and attained a mature liveweight and hip height earlier than non-implanted or E2 implanted steers. The histology data showed that implant treatment did not affect any of the growth plate and trabecular bone measurements. The growth plate length was shorter and trabecular bone showed a greater trabecular separation when steers were getting older. The implant did not affect insulin, IGF-1, PTH, thyroid and leptin whereas only CTX-1 was affected by the implant treatment.In conclusion, steers undergoing energy restriction, regardless of CP intake, had lower liveweight gain and hip height growth which was associated with smaller proliferative and hypertrophic zones in the growth plate, altered trabecular bone structure and a lower concentration of lgrowth mediating hormonesr. Bone elongation and remodelling was generally more sensitive to nutrient availability than treatment with exogenous hormones or sex-steroids. Nevertheless, sex-steroids (especially TE2) altered the growth curve of steers under a continuous high level of nutrition.