Feed restriction in young bulls alters the onset of puberty in relationship with plasma insulin-like growth factor-I (IGF-I) and IGF-binding proteins

Nutrition and Sexual Development in Bulls

This manuscript provides an overview of the effects of nutrition during different stages of bull sexual development. Nutrition during the prepubertal period can modulate the hypothalamic GnRH pulse generator. Increased nutrition results in greater LH secretion, earlier puberty, and greater testicular mass in yearling bulls, whereas low nutrition has opposite effects. Targeting average daily gain from birth to 24 weeks of age to > 1.2 kg/d and limiting gain after 24 weeks of age to < 1.6 kg/d is recommended to optimize bull sexual development.

Animal Welfare Assessment Protocols for Bulls in Artificial Insemination Centers: Requirements, Principles, and Criteria

Animal welfare is a complex subject; as such, it requires a multidimensional approach with the main aim of providing the animals with the "five freedoms". The violations of any one of these freedoms could have an influence on animal wellbeing on different levels. Over the years, many welfare quality protocols were developed in the EU thanks to the Welfare Quality^® project. Unfortunately, there is a lack of such summarized information about bull welfare assessment in artificial insemination stations or about how disturbed welfare can be reflected in their productivity. Animal reproduction is the basis for the production of meat and milk; therefore, factors contributing to reduced fertility in bulls are not only indicators of animal welfare but also have implications for human health and the environment. Optimizing the reproductive efficiency of bulls at an early age can help to reduce greenhouse gas emissions. In this review, welfare quality assessment will be evaluated for these production animals using reproduction efficiency as a key area, focusing on stress as a main effect of poor animal welfare and, thereby, reduced fertility. We will address various welfare aspects and possible changes in resources or management to improve outcomes.

Review: Importance of colostrum supply and milk feeding intensity on gastrointestinal and systemic development in calves

Feeding management of the postnatal and preweaning calf has an important impact on calf growth and development during this critical period and affects the health and well-being of the calves. After birth, an immediate and sufficient colostrum supply is a prerequisite for successful calf rearing. Colostrum provides high amounts of nutrient as well as non-nutrient factors that promote the immune system and intestinal maturation of the calf. The maturation and function of the neonatal intestine enable the calf to digest and absorb the nutrients provided by colostrum and milk. Therefore, colostrum intake supports the start of anabolic processes in several tissues, stimulating postnatal body growth and organ development. After the colostrum feeding period, an intensive milk feeding protocol, that is, at least 20% of BW milk intake/day, is required to realise the calf potential for growth and organ development during the preweaning period. Insufficient milk intake delays postnatal growth and may have detrimental effects on organ development, for example, the intestine and the mammary gland. The somatotropic axis as the main postnatal endocrine regulatory system for body growth is stimulated by the intake of high amounts of colostrum and milk and indicates the promotion of anabolic metabolism in calves. The development of the forestomach is an important issue during the preweaning period in calves, and forestomach maturation is best achieved by solid feed intake. Unfortunately, intensive milk-feeding programmes compromise solid feed intake during the first weeks of life. In the more natural situation for beef calves, when milk and solid feed intake occurs at the same time, calves benefit from the high milk intake as evidenced by enhanced body growth and organ maturation without impaired forestomach development during weaning. To realise an intensive milk-feeding programme, it is recommended that the weaning process should not start too early and that solid feed intake should be at a high extent despite intensive milk feeding. A feeding concept based on intensive milk feeding prevents hunger and abnormal behaviour of the calves and fits the principles of animal welfare during preweaning calf rearing. Studies on milk performance in dairy cows indicate that feeding management during early calf rearing influences lifetime performance. Therefore, an intensive milk-feeding programme affects immediate as well as long-term performance, probably by programming metabolic pathways during the preweaning period.

Effects of a short-term feed restriction on growth performance, blood metabolites and hepatic IGF-1 levels in growing rabbits

<p class="Abstract">A total of 144 weaned hybrid HYLA rabbits (40-day-old) were randomly divided into 4 groups, to investigate the effects of the intensity of one week’s feed restriction on short- and medium-term growth performance, blood metabolites and hepatic IGF-1 in growing rabbits. Restricted groups were fed with 30% (Group L30), 50% (Group L50) 70% (Group L70) of ad libitum feeding for 1 wk and then fed ad libitum until the end of the experiment (75 d of age). The control group (Group AL) was fed ad libitum throughout the experiment. Total feed intake (–15.8%) and feed conversion ratio (–13.2%) were lower in the L50 than in the AL group (P&lt;0.05), but no difference was found between the L30, L70 and AL groups (P&gt;0.05) for these parameters. Total weight gain did not significantly differ among the 4 experimental groups (38.5 g/d; P&gt;0.05). At the end of the feed restriction period, the total serum protein level (P=0.01) was higher in restricted rabbits than AL rabbits (P&lt;0.01), while the hepatic IGF-1 level was lower in L30 and L50 groups than in the 2 other groups (P&lt;0.001). However, no difference remained between groups at the end of the experiment. In contrast, calcium, triglycerides, alkaline phosphatase, urea nitrogen and total cholesterol levels were similar between groups (P&gt;0.05) throughout the experiment. In conclusion, a short-term feed restriction improves feed conversion ratio in a lasting way, transiently alters serum protein and IFG-1 levels and leads to compensatory growth in growing rabbits.</p>

Effects of Feeding Milk Replacer Ad Libitum or in Restricted Amounts for the First Five Weeks of Life on the Growth, Metabolic Adaptation, and Immune Status of Newborn Calves

The pre-weaning period is critical for calf health and growth, and intensive milk feeding programs may assist postnatal development by improving body growth and organ maturation. The aim of the present work was to study the effects of ad libitum milk replacer (MR) feeding on the growth, metabolic adaptation, health, and immune status of newborn calves. Twenty-eight newborn Holstein and Holstein x Charolais crossbred calves were fed ad libitum (ADLIB) or in restricted amounts (6 liters per day; RES) during the first five weeks of life. The MR intake in the ADLIB treatment was gradually reduced at weeks 6 and 7, and all calves then received 6 liters of MR per day until day 60. Blood samples were collected to measure the plasma concentrations of metabolites, insulin, insulin-like growth factor (IGF)-I and IGF binding proteins (IGFBP), immunoglobulins, and acute phase proteins. The expression of mRNA associated with both the somatotropic axis and gluconeogenic enzymes was measured in the liver on day 60. Intensive feeding improved MR intake and growth in ADLIB without influencing concentrate intake. Carcass weight, perirenal fat, and muscle mass were greater in ADLIB. Plasma concentrations of glucose, triglycerides, insulin, and IGF-I were greater, whereas plasma concentrations of β-hydroxybutyrate, total protein, albumin, urea, IGFBP-2 and -4, and fibrinogen were lower at distinct time points in ADLIB. The hepatic mRNA expression of cytosolic phosphoenolpyruvate carboxykinase was greater in ADLIB. Most metabolic and endocrine differences occurred during the MR feeding period, but a slightly greater concentrate intake was associated with increased plasma IGF-I and insulin at the end of the study. The immune and health status of the calves were not affected by MR feeding. However, increased plasma fibrinogen in the RES group suggested differences in the acute phase response.

Different modes of food restriction and compensatory growth in double-muscled Belgian Blue bulls: plasma metabolites and hormones

The effects of different sequences of food restriction and fattening have been studied on plasma metabolites and hormones in double-muscled Belgian Blue bulls. Twenty animals were divided into five groups. The first group (control, CG) was given,ad libitum, a fattening diet based on sugar-beet pulp. In G2 and G3, fattening was interrupted after 103 and 187 days, respectively, by a period of food restriction lasting about 2 months during which the animals received a maintenance ration. They were finished with the same diet as CG. The last two groups, G4 and G5, received a limited amount of the restriction diet to support 0·5 and 0 kg gain per day, respectively, for 4 months, before being fattened as CG. Plasma glucose, alpha-amino nitrogen, non-esterified fatty acids, urea, creatinine, thyroxine (T4), 3, 3’, 5’-tri-iodothyroxine (T3), and insulin-like growth factor-1 (IGF-1) were measured in blood samples taken every 2 weeks. Plasma GH and insulin profiles were measured in serial blood samples obtained at three different times during growth. Animals that showed compensatory growth had lower plasma urea, associated with high levels of T3, T4 and IGF–1. Animals from G2 and G3 failed to show compensatory growth. In Belgian Blue bulls, compensatory growth is markedly affected when food restriction is severe or fattening interrupted.

Endogenous LPS alters liver GH/IGF system gene expression and plasma lipoprotein lipase in goats

Endotoxin lipopolysaccharide (LPS) affects the ruminant health and animal performance. The main purposes of this study were to investigate the potential effects of GH/IGF system and lipoprotein lipase (LPL) concentration on resistance the circulating LPS concentration increased in liver with high concentrate diet treatment. Non-lactating goats were randomly allocated to two groups: a high-concentrate diet (HCD) or a low-concentrate diet (LCD) in cross over design and the blood collection at different time points after feeding at the end of the experiment. The average rumen pH was significantly reduced (P<0.05), but the duration with pH was not more than 120 min in the HCD group. The plasma LPL concentration was significantly raised (P<0.05). However, from 2 h onwards, LPS concentration was significantly reduced (P<0.01) in the HCD group compared with LCD group. In addition, the plasma IGF1 concentration and the hepatic insulin-like growth factor-1 receptor (IGF1R) mRNA expression were markedly reduced (P<0.05). However, growth hormone (GH) secretion at 15, 30, and 45 min after feeding and growth hormone receptor (GHR) mRNA expression in the liver was significantly increased (P<0.05) in HCD group. The correlation analysis showed that the plasma LPL concentration was positively correlated with hepatic GHR mRNA expression (P<0.05). Conversely, the plasma LPS concentration was negatively correlated with LPL concentration (P<0.05). These findings reveal that alterations in GH/IGF system function in response to a high-concentrate diet are accompanied by corresponding changes in systemic LPL in non-lactating goats' liver in presence of endogenous LPS stress.

Effects of road transportation or droving on the weight and metabolism of young bulls

This study evaluated the effects of the mode of transportation on weight and metabolism of young bulls, which are generally transported by two systems: droving and truck. This experiment consisted of two phases: transportation and refeeding and involved transportation by droving and truck. Fifty young Nellore bulls were separated in equal numbers in both groups, droving group and truck group. In the transportation phase, animals were moved, in a 640-km journey. The refeeding phase consisted of 84 days and commenced after the animals arrived at the fattening farm. The effect of the transportation system on animals' weight, metabolic hormones, and serum biochemistry was assessed after 28, 56, and 84 days. During the transportation phase, animals in the droving group lost 10.11 kg and animals in the truck group gained 13.1 kg. The truck group showed 20.5 kg of weight gain at the end of the refeeding phase. The highest triiodothyronine (T3) and thyroxine (T4) concentrations were observed on days 0 and 60 in the transportation phase in both droving and truck groups. The serum cortisol concentration was higher in the droving group than in the truck group on day 56 of the refeeding phase. The IGF-I concentrations were higher during the refeeding phase in the droving group than in the truck group. Our results showed that transportation by droving leads to increased weight loss and reduced further weight gain compared to transportation by truck and that serum concentrations of T4, cortisol, and insulin-like growth factor (IGF-I) hormones are altered by the transportation system.

Effects of energy and protein restriction, followed by nutritional recovery on morphological development of the gastrointestinal tract of weaned kids

Effects of energy, protein, or both energy and protein restriction on gastrointestinal morphological development were investigated in 60 Liuyang Black kids, which were sourced from local farms and weaned at 28 d of age. Weaned kids were randomly assigned to receive 1 of 4 dietary treatments (15 kids per treatment), which consisted of adequate nutrient supply (CON), energy restriction (ER), protein restriction (PR), or energy and protein restriction (EPR). The entire experiment included adaptation period (0 to 6 d), nutritional restriction period (7 to 48 d), and recovery period (49 to 111 d). Three kids from each group were killed at d 48 and 111, and the rumen, duodenum, jejunum, and ileum were harvested. On d 48 (end of nutritional restriction), lengths of the duodenum (P = 0.005), jejunum (P = 0.003), and ileum (P = 0.003), and weights of the rumen (P = 0.004), duodenum (P = 0.006), jejunum (P = 0.006), and ileum (P = 0.004) of kids in ER, PR, and EPR were less than those of kids in CON. Compared with CON, PR decreased papillae width (P = 0.03) and surface area (P = 0.05) of the rumen epithelium, villus surface area (P = 0.05), and N concentration (P = 0.02) of the jejunum mucosa on d 48. Compared with CON, EPR decreased papillae height (P = 0.001), width (P = 0.001), and surface area (P = 0.003), N concentration (P = 0.01), and the ratio of N to DNA (P = 0.03) of the rumen epithelium. Compared with CON, EPR also decreased villus height (P = 0.01), width (P = 0.006), and surface area (P = 0.006), N concentration (P < 0.001), and the ratio of N to DNA (P < 0.001) of the jejunum mucosa on d 48. On d 111 (end of nutritional recovery), lengths of the duodenum (P = 0.001), jejunum (P = 0.001), and ileum (P = 0.001), weights of the rumen (P < 0.001), duodenum (P = 0.001), jejunum (P < 0.001), and ileum (P < 0.001) of kids in ER, PR, and EPR were still less than those of kids in CON; N concentrations of rumen epithelium of kids in PR (P = 0.01) and EPR (P = 0.001), and the ratio of N to DNA of jejunum mucosa of kids in EPR (P < 0.001) were greater than those of kids in CON. Results indicate that nutritional restriction of 6 wk can retard gastrointestinal morphological development for kids weaned at 28 d of age and retarded development remains evident, even after nutritional recovery of 9 wk.

Effects of pre-weaning concentrate feeding on calf performance, carcass and meat quality of autumn-born bull calves weaned at 90 or 150 days of age

A study was conducted to evaluate the effect of pre-weaning concentrate feeding in early-weaned (EW, day 90) or traditionally weaned (TW, day 150) autumn-born beef calves on growth, feed intake and feed efficiency, and carcass and meat quality. Twenty-eight male calves were either EW or TW, and offered a starter concentrate (S) or no additional feed (NS) during the pre-weaning period. Therefore, four management strategies were tested: EWS, EWNS, TWS and TWNS. Growth patterns were affected by management strategy. From day 90 to 150, TWNS calves presented a substantially lower average daily gain (ADG) than their counterparts, which had similar performance. During the finishing phase (from day 150 to slaughter at 450 kg live weight), EWS calves had the lowest ADG. Daily feed intake or efficiency in the finishing phase was unaffected by previous management. Serum IGF-I concentrations at day 90 and slaughter did not differ with management strategy, but early weaning and pre-weaning concentrate feeding increased IGF-I concentrations at day 150. Circulating leptin concentrations were unaffected by age at weaning and pre-weaning concentrate feeding, except for leptin concentrations at slaughter, which were higher in S calves than in NS calves. Total concentrate intake from birth to slaughter and the concomitant feed costs were higher for EWS and EWNS calves than for TWNS and TWS ones. However, cow feed costs were lower for cows whose calves had been early weaned. Concerning carcass quality, early weaning improved dressing percentage and increased fatness score, and particularly TWNS calves presented a poorer conformation. Meat quality was not affected by management strategy. Considering the economic performance, TWS, EWNS and EWS strategies yielded a similar economic margin, whereas TWNS would be the least advisable strategy when calves are fattened in the farm until slaughter.

Effect of compensatory growth on performance, carcass composition and plasma IGF-1 in grower finisher pigs

A total of 48 female pigs (Large White × Landrace × Duroc cross) were used to determine whether a compensatory feed regime influenced performance, carcass composition and the level of plasma IGF-1. Pigs of initial age 73 days were fed a commercial diet at 0.70 of ad libitum (R) for 40 days followed by a return to ad libitum feeding for a further 42 days. The control group was fed ad libitum (A) throughout. Groups of animals on R and A feed regimes were slaughtered at the end of restriction period (SL1), 2 days after refeeding ad libitum (SL2) to establish the more immediate effects of refeeding on IGF levels, and after 42 days refeeding (SL3; n = 8 for each group). As expected, during the restriction period, average daily live weight gain in all the slaughter groups of R pigs was significantly lower than A pigs (P < 0.01); there was no significant difference in feed conversion ratios. In the re-alimentation period of SL3, R pigs grew 12.9% faster (P = 0.033), indicating compensatory growth. At SL1, there was a trend for carcass weight (P = 0.108) of A pigs to be higher than R pigs, but at SL2 live weight and carcass weight of A pigs were significantly heavier than R pigs (P < 0.05), but not at SL3. For killing-out percentage, there was no difference in SL1. After refeeding for 2 days (SL2) and 42 days (SL3), R pigs had significantly lower killing-out percentage than A pigs (P < 0.05). As a proportion of live weight, R pigs had smaller heart, kidney and liver (P < 0.05) than A pigs at SL1. At SL2, only the kidney was smaller in the restricted group (P < 0.05) and there were no significant differences in SL3. As a proportion of carcass weight, Longissimus dorsi was heavier in the R pigs at SL1 (P = 0.108) and SL2 (P < 0.05), but not at SL3. At SL1, there was a trend for intramuscular fat of A pigs to be higher than R pigs. The plasma IGF-1 level was lower in R pigs than A pigs (P = 0.010) at SL1, and slightly lower at SL2 (P = 0.110), with no significant differences at SL3. Dietary restriction period influenced plasma IGF-1 levels, which returned to the ad libitum group levels when animals were refed, as did live weight and carcass weight. It appears that the internal organs and possibly fat, but not muscles, underwent a compensatory response when animals were refed.

Effect of compensatory growth on forms of glycogen, postmortem proteolysis, and meat quality in pigs

The current experiment was designed to examine if a compensatory feed regimen influenced storage of glycogen forms, activity of proteolytic systems, and meat quality. Female pigs (Large White × Landrace × Duroc cross) with an initial age of 74 d were allocated to 6 feeding treatment groups (n=8 for each group). Groups then consumed feed ad libitum for 40 (A40), 42 (A42), or 82 d (A82). The compensatory growth groups were fed 0.70 of ad libitum intake for 40 d (R40) followed by refeeding for ad libitum intake for 2 (R40A2) or 42 d (R40A42). Pigs were slaughtered at the end of the restriction period (SL1), then after refeeding for 2 (SL2) and 42 d (SL3). The feeding regimen caused restricted animals at SL2 to have a decreased BW (P=0.039), with the refed animals undergoing compensatory growth by SL3 so BW was not different (P=0.829). At SL1 there was a trend for the R40 pigs to have less intramuscular fat than A40 (P=0.084). There was a trend for macroglycogen (MG; P=0.051) and a significant effect for proglycogen (ProG; P=0.014) to be greater at slaughter in R40 than A40, along with a greater postmortem decline in both MG (P=0.033) and ProG (P=0.022) over the first 2 h in R40, which was associated with the R40 having a lower pH at 24 h postmortem (P=0.043). After refeeding for 2 d (SL2), only MG of R40A2 was greater (P=0.030) than A42 and had a trend for a greater difference of decline at 24 h postmortem (P=0.091), which was associated with lower pH at 24 h (P=0.012). The data suggest that the concentrations of ProG are more labile and recovered to the concentrations of pigs fed for ad libitum intake sooner than MG. After full compensation in SL3, there was no difference for MG content (at 0 h, P=0.721; at 2 h, P=0.987; at 24 h, P=0.343), ProG content (at 0 h, P=0.879; at 2 h, P=0.946; at 24 h, P=0.459), and muscle pH (at 45 min, P=0.373; at 24 h, P=0.226). At all slaughter points, there was no difference in shear force (at SL1, P=0.101; at SL2, P=0.420; at SL3, P=0.167). There were no significant effects of the feeding regimen on micro- and milli-calpain large subunit gene expression (for micro-calpain at SL1, P=0.450; at SL2, P=0.171; at SL3, P=0.281; for milli-calpain at SL1, P=0.666; at SL2, P=0.123; at SL3, P=0.617) or the activity of the 2 proteolytic enzymes at any of the slaughter dates (for micro-calpain at SL1, P=0.238; at SL2, P =0.238; at SL3, P=0.222; for milli-calpain at SL1, P=0.296; at SL2, P=0.230; at SL3, P=0.615). In R40 there was a trend (P=0.070) for greater gene expression of caspase 3, whereas in R40A2 the increase was significant (P=0.009) relative to pigs consuming feed ad libitum. However, gene expression of the E3 ligase, MuRF1, at SL3 was less in R40A42 (P=0.019). Although compensatory growth does appear to influence the expression of various proteolytic systems, the changes do not appear to be associated with meat quality as measured by shear force.

A spline polynomial model to describe serum IGF-I concentration from birth to slaughter in calves: effects of weaning age, pre-weaning concentrate feeding and breed

The statistical analysis of hormones sampled throughout the production cycle is complicated because factors such as age and weight at the measuring date interfere. Spline curves constructed from pieces of low-degree, random-effects polynomials could be used for a more accurate analysis of data. Concentration of insulin-like growth factor-1 (IGF-1), weight gain, and concentrate intake of Parda de Montaña (PM) (n=27) and Pirenaica calves (n=14) were modeled with a spline model according to age at weaning, pre-weaning concentrate feeding, and breed. At birth, calves were randomly assigned to early weaning (EW) at 90 d or traditional weaning (TW) at 150 d. During lactation, half of PM calves received concentrates (S), whereas the remainder received no concentrates (NS). After weaning, calves received concentrates on an ad libitum basis until they reached a weight of 450 kg. The spline model had better likelihood than a polynomial of 6 degrees or a split-plot model. Serum IGF-1 concentration was greatly affected by age at weaning and pre-weaning concentrate feeding, but not by breed. In NS calves, IGF-1 concentration was greater in EW than in TW calves from 120 to 300 d, irrespective of breed. During lactation, S calves had greater IGF-1 concentration than NS calves. After weaning, EWNS calves reached the IGF-1 concentration of EWS calves after 4 mo on concentrates, whereas TWNS calves attained IGF-1 concentration of TWS calves after only 2 mo, because of their increased concentrate intake relative to TWS calves. Concentration of IGF-1 was positively correlated with the immediate weight gains and intake, but it was not a good predictor of performance in the long term.

Seasonal influence on the response of the somatotropic axis to nutrient restriction and re-alimentation in captive Steller sea lions (Eumetopias jubatus)

Fluctuations in availability of prey resources can impede acquisition of sufficient energy for maintenance and growth. By investigating the hormonal mechanisms of the somatotropic axis that link nutrition, fat metabolism, and lean tissue accretion, we can assess the physiological impact of decreased nutrient intake on growth. Further, species that undergo seasonal periods of reduced intake as a part of their normal life history may have a differential seasonal response to nutrient restriction. This experiment evaluated the influence of season and age on the response of the somatotropic axis, including growth hormone (GH), insulin-like growth factor (IGF)-I, and IGF-binding proteins (BP), to reduced nutrient intake and re-alimentation in Steller sea lions. Eight captive females (five juveniles, three sub-adults) were subject to 28-day periods of food restriction, controlled re-feeding, and ad libitum recovery in summer (long-day photoperiod) and winter (short-day photoperiod). Hormone concentrations were insensitive to type of fish fed (low fat pollock vs. high fat herring), but sensitive to energy intake. Body mass, fat, and IGF-I declined, whereas GH and IGFBP-2 increased during feed restriction. Reduced IGF-I and IGFBP with increased GH during controlled re-feeding suggest that animals did not reach positive energy balance until fed ad libitum. Increased IGF-I, IGFBP-2, IGFBP-3, and reduced GH observed in summer reflected seasonal differences in energy partitioning. There was a strong season and age effect in the response to restriction and re-alimentation, indicating that older, larger animals are better able to cope with stress associated with energy deficit, regardless of season.

Re-alimentation in harbor seal pups: effects on the somatotropic axis and growth rate

The metabolic hormones, growth hormone (GH) and insulin-like growth factor (IGF)-I, together with IGF binding proteins (IGFBP), have been well studied in domestic species and are the primary components of the somatotropic axis. This hormone axis is responsive to nutrient intake, associated with growth rate, and accretion of protein and adipose. However, this relationship has not been evaluated in species that rely heavily on adipose stores for survival, such as pinnipeds. The primary objectives of this research were to investigate the response of the somatotropic axis to reduced nutrient intake and re-alimentation in rehabilitated harbor seal pups, and to assess if these hormones are related to nutritional status and growth rate in harbor seals. Stranded harbor seal pups (n=24) arrived at the rehabilitation facility very thin after fasting for several days (nutritional nadir). Throughout rehabilitation nutrient intake increased and pups gained mass and body condition. Concentrations of GH and IGFBP-2 decreased with re-alimentation, while IGF-I and IGFBP-3 concentrations increased. Overall, GH and IGFBP-2 were negatively associated and IGF-I and IGFBP-3 were positively associated with growth rate and increased body condition of harbor sea pups. Further, the magnitude of the growth response was related to the magnitude in response of the somatotropic axis to varied levels of intake. These data suggest that multiple components of the somatotropic axis may be used to assess the energy status of individuals and may also provide information on the level of feed intake that is predictive of growth rate.

Increased degradation of insulin-like growth factor-I in serum from feed-deprived steers

Severe feed restriction decreases serum insulin-like growth factor I (IGF-I) concentration in animals, and this decrease is thought to be due to reduced IGF-I production in the liver. The objective of this study was to determine whether feed deprivation also increases degradation of serum IGF-I and serum levels of IGF binding protein 3 (IGFBP-3) and acid-labile subunit (ALS), which inhibit IGF-I degradation and increase IGF-I retention in the blood by forming a ternary complex with IGF-I, in cattle. Five steers had free access to pasture, and another five were deprived of feed for 60 h. Serum concentration of IGF-I and liver abundance of IGF-I mRNA at the end of the 60-h period were 50% and 80% lower, respectively, in feed-deprived steers than in fed steers. Less (125)I-labeled IGF-I remained intact after a 45-h incubation in sera of feed-deprived steers than in sera of fed steers, suggesting that serum IGF-I is more quickly degraded in feed-deprived animals. Serum levels of IGFBP-3 and ALS were decreased by 40% and 30%, respectively, in feed-deprived steers compared with fed steers. These decreases were associated with more than 50% reductions in IGFBP-3 and ALS mRNA in the liver, the major source of serum IGFBP-3 and ALS. Taken together, these results suggest that feed deprivation reduces serum concentration of IGF-I in cattle not only by decreasing IGF-I gene expression in the liver, but also by increasing IGF-I degradation and reducing IGF-I retention in the blood through decreasing IGFBP-3 and ALS production in the liver.

Circulating metabolic hormones during the peripubertal period and their association with testicular development in bulls

The objective of the present study was to characterize changes in serum metabolic hormones concentrations from 20 weeks before to 20 weeks post-puberty in bulls and to investigate the associations of metabolic hormones concentrations with testicular development. Leptin concentrations increased from 16 weeks before puberty to 8 weeks post-puberty and insulin concentrations increased from puberty to 8 weeks post-puberty. Growth hormone concentrations decreased after 4 weeks post-puberty, whereas IGF-I concentrations increased from 8 weeks before puberty to 8 weeks post-puberty. During this period, testicular growth was accelerated and testosterone secretion increased substantially, without any significant changes in gonadotropin secretion. Monthly circulating concentrations of leptin, IGF-I and insulin accounted for 63% of the variation in scrotal circumference and 59% of the variation in paired testes volume. In conclusion, the secretion of metabolic hormones was not associated with changes in gonadotropins concentrations. Furthermore, the associations of leptin, IGF-I and insulin concentrations with testes size indicated that these hormones might be involved in a gonadotropin-independent mechanism regulating the testicular development in peripubertal bulls.

Induction of precocious puberty in heifers III: Hastened reduction of estradiol negative feedback on secretion of luteinizing hormone1

Precocious puberty (<300 d of age) can be induced in beef heifers by early weaning and feeding a high-concentrate diet. The objective of this experiment was to determine whether precocious puberty occurs as a result of a hastened reduction of estradiol negative feedback on secretion of LH. Thirty crossbred Angus and Simmental heifers were weaned at 83 +/- 2 d of age and 114 +/- 3 kg of BW, blocked by BW, and randomly assigned to receive a high-concentrate (60% corn; H) or control (30% corn; C) diet and to receive ovariectomy (OVX), OVX plus an estradiol implant (OVXE), or to remain intact (INT). Residual ovarian tissue after OVX necessitated withdrawal of 6 heifers during the course of the experiment, resulting in the following treatment groups: OVX-C, n = 3; OVX-H, n = 5; OVXE-C, n = 4; OVXE-H, n = 2; INT-C, n = 5; INT-H, n = 5. To determine concentrations of progesterone and estradiol, blood samples were collected weekly beginning at a mean age of 160 d. To characterize LH concentrations, serial blood samples were collected at 12-min intervals for 12 h at mean ages of 119, 149, 188, 217, 246, 281, 323, 365, 407, and 449 d. By a mean age of 202 d, heifers fed the H diet were heavier (P < 0.05) than those fed the C diet. Heifers in the INT-H treatment attained puberty earlier (P < 0.05) than in the INT-C treatment (275 +/- 30 vs. 385 +/- 14 d of age, respectively). Overall mean concentrations of estradiol did not differ between OVXE-H and OVXE-C, between INT-H and INT-C, or between OVXE and INT treatments. The OVX treatments exhibited greater LH pulse frequency than the OVXE and INT treatments by the first serial blood collection (treatment x age, P < 0.05). The frequency of LH pulses was greater (P < 0.05) in the INT-H than the INT-C treatment by a mean age of 246 d and was greater (P < 0.05) in the OVXE-H than the OVXE-C treatment by a mean age of 281 d. In the OVXE-H treatment, LH secretion increased and subsequently "escaped" from estradiol negative feedback (detection of > or = 1 LH pulse/h) earlier (P < 0.05) than in the OVXE-C treatment (307 +/- 30 and 420 +/- 21 d of age, respectively). It is concluded that advancing the reduction of estradiol negative feedback on secretion of LH is the mechanism by which early weaning and feeding a high-concentrate diet results in precocious puberty in heifers.

Testicular development in Brahman bulls

Brahman breed bulls (Bos indicus) are widely used to introduce environmental resistance traits into meat-producing herds. However, their reproductive development is slower than European breeds (Bos taurus). The objective of this study was to assess the development of the seminiferous epithelium in Brahman bulls. Twenty-three prepubertal bulls were castrated and testicular samples taken for histological processing. Light microscopic images were digitized and cells of the seminiferous epithelium were assessed. Immature Sertoli cells gradually decreased in numbers and were no longer detected after approximately 14 months of age; concurrently, the numbers of mature Sertoli cells increased from 10 to 14 months. Spermatogenesis started during the ninth month; prior to that, only gonocytes and immature Sertoli cells were observed. Type A spermatogonia, spermatocytes, round spermatids, elongated spermatids and spermatozoa were first detected at 9.5, 11, 11, 13 and 16 months of age, respectively. The delay in the onset of puberty in Brahman bulls with respect to B. taurus was attributed to a longer duration of the prepubertal period (interval from start of spermatogenesis to puberty) and a later start of spermatogenesis.

The ontogeny of the somatotropic axis in male and female Hereford calves from birth to one year of age

Insulin-like growth factor-binding proteins-2 and -3 may play a role in age-dependent growth response to bovine ST (bST) treatment in cattle; however, samples have been collected at infrequent intervals and at limited time points. Therefore, the objective of this experiment was to examine the ontogeny of components of the somatotropic axis in Hereford calves from birth to 1 yr of age at weekly intervals to determine whether there is a certain age or time frame when the somatotropic axis may change and/or potentially become more responsive to exogenous bST administration. Blood samples and body weight measurements were collected from eight male and eight female Hereford calves once per week from birth to 1 yr of age. Serum concentrations of ST, IGF-I, IGFBP-2, and IGFBP-3 were determined. Males began to grow faster than females at approximately 16 wk of age (P < 0.05). Average concentrations of ST, IGF-I, and IGFBP-3 were greater in males than females (P < 0.01). Average concentrations of IGFBP-2 were greater in females than in males (P = 0.05). Concentrations of ST decrease with age (P < 0.01); however, the decrease occurred earlier in female calves. Concentrations of IGF-I and IGFBP-3 increased in males and females (P < 0.01), and concentrations of IGF-I began to plateau at approximately the same time as growth rate differences were observed (16 wk of age). Following an initial increase (birth to approximately 16 wk of age), concentrations of IGFBP-3 remained constant until approximately 43 wk of age. Concentrations of IGFBP-2 increased to approximately 10 wk of age (P < 0.05), followed by a decrease, and then, similar to IGFBP-3, remained constant until 43 wk of age. Correlations between average daily gain, ST, IGF-I, IGFBP-2, and IGFBP-3 were determined. Average daily gain was negatively (P < 0.01) correlated with ST and positively (P < 0.1) correlated with IGF-I. In females, ST was negatively (P < 0.01) correlated with IGF-I. Concentrations of ST were positively correlated (P < 0.01) with IGFBP-2 and IGFBP-3. Concentrations of IGFBP-2 were negatively correlated (P < 0.01) with IGF-I and positively correlated (P < 0.01) with IGFBP-3. In conclusion, serum concentrations of ST, IGF-I, IGFBP-2, and IGFBP-3 differed between male and fe-male calves. In addition, changes in components of the somatotropic axis occurred around the same time as males began to grow faster than females.

An energy-rich diet causes rumen papillae proliferation associated with more IGF type 1 receptors and increased plasma IGF-1 concentrations in young goats

We tested the hypothesis that the dietary energy-dependent alterations of the rumen papillae size are accompanied by corresponding changes in systemic insulin-like growth factor (IGF)-1 concentration and in rumen papillary IGF type 1 receptors (IGF-1R). Young male goats (n=24) were randomly allocated to two groups (n=12) and fed a high level (HL) metabolizable energy [1200 kJ/(kg(0.75).d)] or a low level (LL) [500 kJ/(kg(0.75).d)] diet for 42 d. The concentration of ruminal total SCFA did not differ between the groups, but the molar proportion of butyric acid was enhanced by 70% in the HL group (P<0.05). Both the length and width of the papillae were greater (P<0.05) in the HL group, and the surface was 50-100% larger (P<0.05) in the tissue sampled from the artrium ruminis, the ventral ruminal sac and the ventral blind sac. Transport of Na+ across the rumen epithelium, which is amiloride sensitive, was higher (P<0.05) in the HL than in the LL group. Furthermore, the plasma IGF-1 concentration was about twofold higher in the HL group (P<0.05), and the maximal rumen epithelial IGF-1R binding was also higher in the HL (P<0.05) than in the LL group. IGF-1R mRNA and IGF-1 mRNA were detected in rumen papillae; however, they were unaffected by dietary treatments. DNA synthesis and cell proliferation of cultured rumen epithelial cells were higher (P<0.05) after IGF-1 treatment (25 or 50 microg/L) compared with those in the medium without IGF-1. Thus dietary energy-dependent alterations of rumen morphology and function are accompanied by corresponding changes in systemic IGF-1 and ruminal IGF-1R.

Evaluation of serum insulin-like growth factor binding proteins (IGFBP) in Angus cattle divergently selected for serum IGF-I concentration

Postweaning serum insulin-like growth factor-I (IGF-I) concentrations and serum IGF binding proteins (IGFBP) were investigated in 68 (1992 Fall-born) and 84 (1999 Fall-born) Angus cattle selected for either high or low serum IGF-I concentrations since 1989. Relative serum levels of IGFBP were determined by [125I]IGF-I Western ligand blotting. IGFBP species of 38-42, 34, 30, and 24 kDa were identified. The 34 kDa species was identified as IGFBP-2 by immunoblot analysis. No significant line effects were observed for any of the IGFBP. In both 1992 and 1999, heifers had higher IGFBP-2 levels than bulls (P<0.0005). In 1992 calves, relative levels of the 38-42 and 24 kDa species were significantly correlated with serum IGF-I concentration. In 1999 calves, none of the IGFBP were correlated with serum IGF-I, although IGFBP-2 was negatively correlated with several measures of body weight. No significant line effects were observed for growth or serum IGF-I traits in 1992 calves. However, 1999 high line calves had higher serum IGF-I concentrations and body weights than low line calves (P<0.05). In both 1992 and 1999 calves, bulls had higher serum IGF-I concentrations and body weights than heifers (P<0.05). Thus, while selection for high versus low serum IGF-I concentrations has resulted in divergence between the selection lines and also in changes in body weights, it has not resulted in changes in serum IGFBP levels. Furthermore, circulating IGFBP-2 appears to be higher in heifers than in bulls, and also appears to be negatively correlated with body weights.

Role of the somatotropic axis in the mammalian metabolism

The metabolism in mammalian is regulated by multiple levels of hormone action, with complex feedback and control mechanisms. The somatotropic axis, essentially consisting of growth hormone (GH), insulin-like growth factors (IGF-I and -II), their associated carrier proteins, and receptors, plays a key role in the control of the regulation of metabolism and physiological process. Among this axis, other hormones like insulin, leptine, glucocorticoids or thyroid hormones are involved in this mechanism by modulating GH and/or IGF-I synthesis and availability. This review summarizes the complexity of the regulation of the metabolism by the somatotropic axis using different examples such as special nutritional situations or growth promoters administration.

Mechanisms of reduced and compensatory growth

Growth is an integrated process, resulting from the response of cells dependent on the endocrine status and nutrient availability. During feed restriction, the production and secretion of growth hormone (GH) by the pituitary gland are enhanced, but the number of GH receptors decreases. Changes of GH binding proteins induce GH resistance and are followed by reduced insulin-like growth factor-I (IGF-I) secretion. On the other hand, high circulating levels of GH enhance the mobilization of fatty acids, which are used to support energy requirements. Thus, when feed restriction in growing animals is moderate, there is mainly protein but barely fat accretion. By contrast, a severe feed restriction enhances the release of catabolic hormones and stimulates, from muscle cells, the liberation of amino acids, which are used by hepatocytes for gluconeogenesis. During refeeding and compensatory growth, the secretion of insulin is sharply enhanced and plasma GH concentrations remain high. This situation probably allows more nutrients to be used for growth processes. The role of plasma IGF-I during compensatory growth is not clear and must be explained in connection with changes of its binding proteins. Thyroxin and 3,5,3'-triiodothyronine seem to have a permissive effect on growth. The simultaneous occurrence of puberty with refeeding can exert a synergistic effect on growth. Initially, compensatory growth is characterized by the deposition of very lean tissue, similar as during feed restriction. This lasts for some weeks. Then, protein synthesis decreases and high feed intake leads to increased fat deposition.