Direct determination of optimal amino acid intake for maintenance and growth in broilers.

This study has used a direct approach to calculate the optimal dietary supply of amino acids for maintenance and growth in broiler chicks. Amino acid intake and accretion in the carcass in Ross 308 chicks was measured following the feeding of different experimental diets with controlled feed intake from hatching through 42 d. Increasing crude protein levels at constant amino acid:protein ratio improved growth and reducing the crude protein levels at constant amino acid:protein ratios depressed performance. Altering the protein at constant essential amino acid concentrations had little affect on growth. Plotting the amino acid intake per metabolic BW against amino acid accretion per unit of metabolic BW yielded a straight line whose slope represents the efficiency of utilization of the amino acid for growth; the intercept represents maintenance. These values were then used to calculate the amino acid intakes required for any BW and growth rate. For comparison, inputs were calculated for the growth targets recommended in the Ross manual and compared with the dietary recommendations. The measured results were similar to the overall empirical recommendations although values differed slightly, with threonine required at higher levels then recommended. Using the growth rates obtained in this trial, optimal amino acid supply was compared with the NRC (1994) recommendations. This indicated that optimal amino acid supply was higher then NRC (1994) recommendations for most amino acids. The calculation of optimal amino acid supply as a function of BW and growth will allow effective formulation of diets for maximal efficiency.

Mucin dynamics and microbial populations in chicken small intestine are changed by dietary probiotic and antibiotic growth promoter supplementation.

The mucous layer that covers the intestinal absorptive surface acts as a barrier against bacterial translocation. The chicken gut contains a diverse bacterial population which interacts with the mucous layer. In this report, we studied the effect of changing the intestinal microbial populations on mucin dynamics by feeding 1-d-old chicks a control diet or that diet containing either antibiotic growth promoter (AGP) or a probiotic product for 14 d. Dietary AGP increased the proportions of Bifidobacterium species in the duodenum compared with the other groups. In AGP-fed chicks, the villous surface area was increased in the jejunum, goblet cell density was greater in the jejunum and ileum, and mucin glycoprotein levels in the duodenum were lower than in the other groups (P < 0.05). Feeding AGP increased the expression of mucin mRNA in the jejunum and ileum compared with controls. The dietary probiotic increased the proportion of Lactobacillus species in the ileum compared with the controls (P < 0.05) and significantly enlarged the goblet cell "cup" area throughout the small intestine compared with the other groups. Expression of mucin mRNA and the levels of mucin glycoprotein were greater in the jejunum of the probiotic-fed chicks compared with controls (P < 0.05). Neither the probiotic nor AGP treatments affected the thickness of the mucous adherent layer. These results indicate that both probiotic and AGP altered processes of mucin biosynthesis and/or degradation mediated via changes in the intestinal bacterial populations. These modifications in mucin dynamics influence gut function and health and may change nutrient uptake.

Weight loading young chicks inhibits bone elongation and promotes growth plate ossification and vascularization.

The mechanical stimuli resulting from weight loading play an important role in mature bone remodeling. However, the effect of weight loading on the developmental process in young bones is less well understood. In this work, chicks were loaded with bags weighing 10% of their body weight during their rapid growth phase. The increased load reduced the length and diameter of the long bones. The average width of the bag-loaded group's growth plates was 75 +/- 4% that of the controls, and the plates showed increased mineralization. Northern blot analysis, in situ hybridization, and longitudinal cell counting of mechanically loaded growth plates showed narrowed expression zones of collagen types II and X compared with controls, with no differences between the relative proportions of those areas. An increase in osteopontin (OPN) expression with loading was most pronounced at the bone-cartilage interface. This extended expression overlapped with tartarate-resistant acid phosphatase staining and with the front of the mineralized matrix in the chondro-osseous junction. Moreover, weight loading enhanced the penetration of blood vessels into the growth plates and enhanced the gene expression of the matrix metalloproteinases MMP9 and MMP13 in those growth plates. On the basis of these results, we speculate that the mechanical strain on the chondrocytes in the growth plate causes overexpression of OPN, MMP9, and MMP13. The MMPs enable penetration of the blood vessels, which carry osteoclasts and osteoblasts. OPN recruits the osteoclasts to the cartilage-bone border, thus accelerating cartilage resorption in this zone and subsequent ossification which, in turn, contributes to the observed phenotype of narrower growth plate and shorter bones.

Microflora ecology of the chicken intestine using 16S ribosomal DNA primers.

The microflora in the gastrointestinal tract of broiler chickens influences digestion, health, and wellbeing. Analysis of chicken gut microflora has been mainly by culture-based methods. Studies using these techniques have been useful for identification and analysis of specific groups of bacteria, however, the use of enrichment medium precludes even relative quantitation of bacterial species. Recent advances in ribosomal DNA-based molecular techniques make it possible to identify different bacterial populations in environmental samples without cultivation. In this study, the intestinal microflora was examined using 16S ribosomal DNA (rDNA) targeted probes from bacterial DNA isolated from intestinal and cecal contents of chickens at 4, 14, and 25 d of age. The ribosomal gene sequence was amplified using PCR with universal primers to determine total bacterial DNA and specific primers directed at 6 bacterial species: Lactobacillus, Bifidobacterium, Salmonella, Campylobacter, Escherichia coli, and Clostridium. The use of universal primers extends these methods to allow determination of relative proportions of different bacterial species. The results indicated that in young chicks the major species present in the small intestines and ceca was Lactobacilli, with a Bifidobacteria population becoming more dominant in the ceca at older age. Clostridium was detected in some segments of the small intestine in young chicks. In older chickens, Salmonella, Campylobacter, and E. coli species were found in the ceca. This study has demonstrated the use of molecular techniques for determining relative proportions of bacterial species and monitoring pathogens in the chick gastrointestinal tract.

The role of prolactin in reproductive failure associated with heat stress in the domestic turkey.

Reproductive failure associated with heat stress is a well-known phenomenon in avian species. Increased prolactin (PRL) levels in response to heat stress have been suggested as a mechanism involved in this reproductive malfunction. To test this hypothesis, laying female turkeys were subjected to 40 degrees C for 12 h during the photo-phase daily or maintained at 24-26 degrees C. Birds in each group received oral treatment with parachlorophenyalanine (PCPA; 50 mg/kg BW/day for 3 days), an inhibitor of serotonin (5-HT) biosynthesis, or immunized against vasoactive intestinal peptide (VIP). Both treatments are known to reduce circulating PRL levels. Nontreated birds were included as controls. In the control group, high ambient temperature terminated egg laying, induced ovarian regression, reduced plasma luteinizing hormone (LH) and ovarian steroids (progesterone, testosterone, estradiol) levels, and increased plasma PRL levels and the incidence of incubation behavior. Pretreatment with PCPA reduced (P < 0.05) heat stress-induced decline in egg production, increase in PRL levels, and expression of incubation behavior. Plasma LH and ovarian steroid levels of heat stressed birds were restored to that of controls by PCPA treatment. As in PCPA-treated birds, VIP immunoneutralization of heat-stressed turkeys reduced (P < 0.05) circulating PRL levels and prevented the expression of incubation behavior. But it did not restore the decline in LH, ovarian steroids, and egg production (P > 0.05). The present findings indicate that the detrimental effect of high temperature on reproductive performance may not be related to the elevated PRL levels in heat-stressed birds but to mechanism(s) that involve 5-HT neurotransmission and the induction of hyperthermia.

Fat digestibility in Equus caballus follows increasing first-order kinetics.

The digestibility of ether extract varies greatly from forages to grains and further to added fats consisting mainly of triglycerides. This variation has been attributed to two main factors, the presence of nonhydrolyzable substances in the ether extract, especially in leafy foods, and the dilution of endogenous fecal fat. A compilation of results from 188 equine digestion balance observations on five basal feeds and 18 test feeds with added fats demonstrated a true digestibility of fat approaching 100% and an endogenous fecal fat of 0.22 g x d(-1) x kg BW(-1). The results revealed that nonhydrolyzable ether extract and endogenous fecal fat were insufficient to account for the difference between true digestibility and apparent digestibilities of ether extract in basal feeds and partial digestibilities of added fats in test feeds. A third possible contributing factor was demonstrated: an increasing first-order relationship between observed digestibilities (D, %) and the fat content of the feed (F, g/kg): D = 92.0 - 92.0e(-F/342). r2 = 0.81, P < 0.001. This equation indicates that 46% digestibility (half maximum) occurs at an ether extract or fat content of 24 g/kg, which is common in forages. It is consistent with fat digestibility or efficiency of absorption being a function of the rate of lipolysis, especially when residence time in the small intestine is limited. Consequently, we suggest that the kinetics of lipases, which are difficult to measure, may contribute to low digestibility when substrate concentration in the small intestine is low due to a low fat content in food. The status of vitamins A and E might be affected by low dietary fat contents and might be improved by fat supplementation.

Catabolism and deposition of amino acids in growing chicks: effect of dietary supply.

Amino acid (AA) deposition and catabolism were examined in broilers by determining intake and carcass deposition of AA, while defining catabolism as the difference between intake and deposition. The first trial examined the effects of increasing concentrations of a single limiting AA, lysine, on carcass deposition and catabolism. Carcass deposition of all AA increased to a plateau. Catabolism of lysine increased linearly, whereas other AA showed decreased catabolism as dietary lysine increased before reaching a plateau. Carcass AA composition was not influenced by the diet. In the second trial, different dietary ratios of AA were examined and these resulted in increased carcass deposition of lysine, threonine, and arginine before reaching a plateau, whereas other AA showed constant deposition. Catabolism of all AA tended to increase with dietary concentration. Efficiency of AA deposition decreased with age and catabolism comprised a smaller proportion of intake in the first week posthatch. A third trial examined changing AA ratios and composition. No correlation was observed between dietary AA concentrations and carcass deposition, whereas catabolism was linearly correlated with dietary composition. Multiple regression analysis indicated that the catabolic pathways of some AA are interrelated. These data are consistent with a model where carcass accretion is determined by the limiting AA until some maximal rate is achieved, whereby another AA may become limiting. Excess supply of any AA is catabolized and these catabolic processes interact with an accompanying energy cost.

The effect of a green and blue monochromatic light combination on broiler growth and development.

Previous reports have suggested that green light enhances broiler growth at an early age, whereas blue light enhances growth at older ages. The aim of this study was to examine the effect of a switch in monochromatic light at 2 ages on growth and development of broilers. Male chicks (Anak, n = 640) were used. After hatch, chicks were weighed, wing-banded, and blocked into treatment groups. Chicks were grown in 1-m2 pens in 8 isolated light-proof rooms (20 birds/pen). The light treatments were (1) Control white (mini-incandescent lamps), 2) blue light-emitting diode (LED) lamps, 3) green LED lamps, 4) blue LED switching to green at 10 d of age, 5) blue LED switching to green at 20 d of age, 6) green LED switching to blue at 10 d of age, and 7) green LED switching to blue at 20 d of age. There were 8 pens for treatment 1, and 4 pens for each of the other treatments. The light schedule was 23L:1D, and intensity was 0.1 watts/m2. BW and feed consumption were recorded. Green light birds were significantly heavier at 4 d of age. Switching light at 10 d of age from green to blue caused a further increase in BW. This improved growth was maintained until the end of the experiment. Light switching from blue to green at 20 d of age also improved growth as compared with white light. Average feed efficiency and mortality rate did not differ between groups. No association was observed among light treatment, performance, and plasma triiodothyronine concentration. We suggest that green light stimulated growth of birds at early age, and shifting birds to a different light environment at 10 or 20 d of age might further stimulate growth.

Number of oocytes obtained from cows by OPU in early, but not late lactation increased with plasma insulin and estradiol concentrations and expression of mRNA of the FSH receptor in granulosa cells.

The effects of lactation stage and hormonal profile on the quality and quantity of oocytes and the gonadotrophic sensitivity of granulosa cells (GC) from small antral follicles obtained by sequential aspirations from ovaries of high producing dairy cows were examined. Cows in late lactation (263(+/- 60) days postpartum) and 98(+/- 16) days pregnant in positive energy balance (EB) showed no significant changes in plasma concentrations of estradiol, progesterone, insulin, IGF1 or expression of mRNA of the FSH receptor in GC from small antral follicles during the 49 days experimental period. There were no changes in the number and quality of oocytes obtained from each aspiration. In cows in early lactation (72.8 +/- 6 days postpartum), plasma insulin concentrations increased and were positively correlated with plasma estradiol concentration. Due to the sequential aspirations progesterone blood concentrations were low in early lactation cows. Expression of mRNA of the FSH receptor increased in GC from small antral follicles of early lactation cows together with the number of oocytes obtained with aspiration sessions. No differences were found in morphological quality or function between oocytes obtained from small antral follicles from cows in early or late lactation. In early, but not late lactation, the number of oocytes was correlated with both insulin and E2 plasma concentrations. Improved EB and sensitivity of GC to FSH may be involved in oocyte recruitment in early lactation.

Morphological, molecular, and functional changes in the chicken small intestine of the late-term embryo.

The rapid development of the gastrointestinal tract posthatch has been described; however, little information exists concerning the development of the small intestine in the prehatch period. The present study examined the morphological, cellular, and molecular changes occurring in the small intestine toward the end of the incubation period by examining the expression of intestinal genes that code for brush border digestive enzymes and transporters, their biochemical activities, and the morphological changes in the mucosal layer. The results indicated that during the last 3 d of incubation the weight of the intestine, as a proportion of embryo weight, increased from approximately 1% on d 17 of embryonic age to 3.5% at hatch. At this time the villi could be divided into two main developmental stages, differing in their length and shape, with the larger villi often being pear-shaped and the smaller villi being narrower and having a rocket-like shape. However, on d 19 a further stage of villus development was observed. Activities of maltase, aminopeptidase, sodium-glucose transporter (SGLT)-1, and ATPase began to increase on d 19 and further increased on the day of hatch. The expression of mRNA for these brush-border membrane (BBM) enzymes and transporters was detected from d 15. Determining quantities relative to beta-actin indicated that expression of all parameters examined was low on d 15 and 17, increased 9- to 25-fold on d 19, and all decreased again on the day of hatch. Relative expression of mRNA of the different enzymes and transporters were correlated as were their activities (r = 0.75 to 0.96); however, expression was not correlated with enzymatic activities. The role of these parameters in the ontogeny of absorption is discussed. Thus, major changes in the expression and localization of the functional brush-border proteins prepare the framework for ingestion of carbohydrate- and protein-rich exogenous feed posthatch.

Heavier chicks at hatch improves marketing body weight by enhancing skeletal muscle growth.

This study examined some of the factors connected with the greater marketing weight observed in chicks hatching with higher BW. Examination of chicks hatching from maternal flocks of different ages indicated that BW at hatch increased quadratically and yolk sac weight linearly with age, whereas components of the gastrointestinal tract showed no significant trend. Growth of pectoralis muscles and gastrointestinal tract were compared in chicks hatching at the same weight from maternal flocks of 28 and 64 wk of age and in chicks from the same maternal flock (44 wk old) hatching at different weights. The results indicated that no differences were found among chicks hatching with the same weight from maternal flocks of different ages. In contrast, in chicks from the same maternal flock hatching at different weights the gastrointestinal tract tended to compose a smaller proportion of BW in large chicks, and its growth was not correlated with performance. Liver proportions were greater in heavier chicks. Pectoralis growth and satellite cell numbers and activity were greater in heavier chicks through 5 d posthatch, and pectoralis muscles were heavier at marketing. Examination of some of the growth factors involved suggested that in heavier chicks satellite cells underwent higher proliferation and earlier differentiation during their critical period of activity in the immediate posthatch days. To determine when these differences in activity were established, examination of 15-d embryonic myoblast activity indicated that at this stage activity was already greater in the heavier eggs. This finding suggests that programming of muscle growth may be completed in late embryonic stages. This study suggests that enhanced satellite cell activity is involved in increased growth of chicks hatching with higher BW.

Functional development and intestinal absorption in the young poult.

1. Absorption, secretion of digestive enzymes and intestinal morphology were determined in poults from hatching to 19 d. 2. Oleic acid was approximately 80% absorbed at hatch and this changed little with age and was not influenced by fasting. In contrast, glucose and methionine were 48 to 56% absorbed at hatch and this increased to 75 to 80% at 4 d; however, this increase in absorption was inhibited by fasting. Percentage absorption of protein did not change between 5 and 19 d, whereas absolute uptake of fat and protein and net secretion of digestive enzymes and fatty acids and N to the duodenum increased with age and body weight (BW). 3. The length and diameter and villus size in the small intestines increased rapidly after hatch whereas the number of villi per cross section did not change with age. Duodenum and ileum surface area reached a plateau after 11 d whereas jejunal surface area continued to increase until 19 d. Duodenal and ileal surface area were not related to BW, but jejunal surface area was significantly correlated. 4. While the major part of the absorption occurred in the duodenum and upper jejunum, uptake of both protein and fat increased in the distal segments of the small intestine with age. Overall absorption of both fat and protein was correlated with BW. 5. It appears that in the posthatch poult intestinal surface area is not a limiting factor in growth, which was correlated with digestive secretions, fat and protein uptake.

Nutrient transport in the small intestine: Na+,K+-ATPase expression and activity in the small intestine of the chicken as influenced by dietary sodium.

The Na+-K+-ATPase, localized in the basolateral membrane of enterocytes plays a major role in nutrient transport in the small intestine by transferring K+ ions into and Na+ out of the cell. Within the enterocyte, homeostasis is maintained by active exclusion of Na from the cell by the Na+,K+-adenosine triphosphatase (ATPase) or sodium pump. Because much of the intestinal nutrient transport is by Na cotransporters, Na+,K+-ATPase may be used to evaluate nutrient uptake. In this study, nutrient transport was evaluated by determining expression and activity of Na+-K+-ATPase in the jejunum of chicks fed diets with different concentrations of Na. Expression of the chicken Na+-K+-ATPase gene was examined following isolation of an 1,140 bp cDNA fragment of the alpha-subunit using a reverse transcription (RT)-PCR reaction with specific primers. This fragment was sequenced and showed 95 to 98% homology with the mammalian alpha-subunit of the Na+-K+-ATPase genes. This cDNA fragment was used as a specific probe in Northern blot hybridization for determination of expression in the chicken jejunum. Expression of mRNA of Na+-K+-ATPase was enhanced at low dietary Na but was unchanged at high dietary Na concentrations. In contrast, activity of the enzyme was low with low dietary Na and unchanged at high dietary Na. The Vmax of the Na+-K+-ATPase was unchanged, but affinity was altered by dietary Na concentrations. Thus, determination of expression and activity of intestinal Na+-K+-ATPase allows clearer understanding of changes in intestinal uptake due to dietary Na.

Crude protein and essential amino acid requirements in chicks during the first week posthatch.

1. This study examined optimal lysine and sulphur amino acid supply in the first week posthatch in broilers and the relationship between essential amino acids and dietary crude protein during the first week posthatch on performance at 7 d and through marketing. 2. The optimal supply during the 7 d posthatch using a 230 g/kg crude protein diet for sulphur amino acids was 9.1 and for lysine was 10.3-10.8 g/kg with maximal body weight (BW) or feed efficiency as the criteria. 3. Feeding diets with crude protein content ranging from 200 to 260 g/kg with either constant amounts of essential amino acids at different crude protein levels or constant ratios of essential amino acids to crude protein resulted in enhanced performance at 7 but not at 4 d with high protein intake and proportionally increased essential amino acids. 4. Performance on diets with crude protein ranging from 160 to 280 g/kg, with constant ratios of essential amino acid to crude protein, was much enhanced with the high crude protein diets at 7 d. All chicks were transferred to standard diets after 7 d and the BW advantage due to the balanced amino acid-high crude protein diet remained through marketing. 5. Thus increasing essential amino acids in a constant ratio to crude protein enhanced performance during the 7 d posthatch.

The effect of chronic feeding of diacetoxyscirpenol and T-2 toxin on performance, health, small intestinal physiology and antibody production in turkey poults.

1. The effects of feeding T-2 toxin or diacetoxyscirpenol (DAS) at levels up to 1 ppm for 32 d on performance, health, small intestinal physiology and immune response to enteral and parenteral immunisation were examined in young poults. 2. Slight improvement in growth was observed in some groups of poults fed T-2 or DAS mycotoxins for 32 d, with no change in feed efficiency. Feeding both T-2 and DAS resulted in oral lesions which had maximal severity after 7-15 d. 3. Mild intestinal changes were observed at 32 d but no pathological or histopathological lesions were found. Both mycotoxins altered small intestinal morphology, especially in the jejunum where villi were shorter and thinner. In addition, both DAS and T-2 mycotoxins enhanced the proportion of proliferating cells both in the crypts and along the villi. Migration rates were reduced in the jejunum of poults fed T-2 toxin but did not change in the duodenum or in poults fed DAS. 4. No significant effects of T-2 or DAS were observed on antibody production to antigens administered by enteral or parenteral routes. 5. This study indicates that tricothecene toxins at concentrations of up to 1 ppm for more than 30 d influenced small intestinal morphology but did not affect growth or antibody production.

Pre- and posthatch development of goblet cells in the broiler small intestine: effect of delayed access to feed.

Mucin glycoproteins play a key role in the regular function of the epithelium of the gastrointestinal tract, and in this study, the ontogenesis and development of mucin producing cells was examined in the broiler. Mucin-producing cells were observed in the small intestine from 3 d before hatch, and at this time contained only acidic mucin. After hatch and until Day 7 posthatch, the proximal, middle, and distal segments of the small intestine contained similar proportions of goblet cells producing acidic and neutral mucins. A gradient of goblet cell density was observed increasing along the duodenal to ileal axis. Delayed access to feed for 48 h posthatch resulted in an increase in intestinal intracellular mucins, which might have been due to impaired mucin secretion or enhanced mucin production. Changes in mucin dynamics could affect absorptive and protective functions of the small intestine.

Fat and carbohydrate use in posthatch chicks.

Metabolism of carbohydrate and fat was examined in the hatching chick by introduction of 14C labeled oleic acid, triolein, or glucose to the gastrointestinal tract or to the yolk sac. Label in plasma and exhaled air samples were determined over a 12-h period. The activity-time curves could be fitted to double exponential equations, and rate constants for appearance and disappearance of label were calculated. Following injection to the yolk, oleic acid and triolein label showed rapid appearance in the circulation and rapid utilization. The rate of appearance of oleic acid and triolein in the circulation from the gastrointestinal tract was high at hatch and did not change with age. However, the rate constant for disappearance from the plasma increased with age and after 6 d posthatch disappearance curves fluctuated with little decline over 12 h. Following the introduction of labeled lipids more label was found in acylglycerides with time after injection but age had no effect. Lipids introduced via the yolk sac were present in higher proportions in fractions corresponding to low-density lipoprotein (LDL) than lipids introduced via the gastrointestinal tract. Glucose appearance in the circulation from the gastrointestinal tract was low at hatch and increased more than two-fold by 3 d posthatch. The rate constant for disappearance of glucose from the circulation did not change with age. It appears that lipoproteins synthesized maternally or in yolk transport lipids at hatch, but their concentration decreases posthatch, and this could reduce utilization of circulating lipids. Uptake of glucose increases after hatch, thus, allowing glucose to become a major source of energy as the yolk lipids are depleted.

The effect of dietary fibre on the small intestines and apparent digestion in the turkey.

1. In order to determine the feasibility of using high fibre diets in turkey rations, three crude fibre dietary concentrations were fed to turkey hens at three ages and performance, fibre digestibility and small intestinal morphology were determined. 2. Growth rate and feed efficiency decreased when diets contained 80 to 90 g crude fibre/kg; however, growth did not change when 60 g crude fibre/kg was fed between 1 and 4 weeks or between 6 and 8 weeks and was enhanced between 11 and 14 weeks of age. 3. Digestibilities of crude protein, fat and gross energy (GE) were depressed at fibre intakes of 80 to 90 g/kg between 1 and 4 weeks but not at later ages. Crude fibre digestibility increased with age and decreased with dietary fibre content. 4. Total small intestinal length and surface area were increased by high dietary crude fibre intake between 11 and 14 weeks. Small but inconsistent changes in the length, diameter and number of villi, villus size and area were observed in the duodenum, jejunum and ileum at the different ages as a result of feeding the different crude fibre concentrations. 5. Crude fibre can be utilised to some extent by turkeys and concentrations of 60 g fibre/kg in the diet did not result in decreased performance after 6 weeks of age.

Antioxidant status and muscle cell leakage during endurance exercise.

Antioxidant status of 35 endurance horses was studied during an 80 (OD80) or 160 km (OD160) race. Packed cell volume (PCV), total plasma protein (TPP), plasma ascorbic acid (VIT C), plasma alpha-tocopherol (VIT E) and erythrocyte glutathione (GSH) concentrations, erythrocyte glutathione peroxidase (GPX), plasma aspartate aminotransferase (AST) and plasma creatine kinase (CK) activities were measured at 0, 40, 80 km and 60 min of recovery (REC) at OD80, and 0, 64, 106, 142, 160 km and REC at OD160. In both races, no changes were found in plasma VIT E concentration, but VIT C and GSH concentrations decreased (P<0.05), and mean GPX, AST and CK activities increased from 0 km (P<0.05). Indices of muscle cell leakage (plasma AST and CK) were correlated (r = 0.36 to 0.67; P<0.03) with indices of antioxidant status (VIT C, GSH and GPX). Associations between increased muscle leakage and decreased antioxidant status may, in part, reflect oxidative stress and suggest the testing of antioxidant supplements in endurance horses to improve performance and welfare.

Interactions between dietary crude protein and essential amino acid intake on performance in broilers.

1. The effect of diets with increasing concentrations of crude protein at either fixed essential amino acid concentrations or at fixed essential amino acid:dietary crude protein ratios on performance was examined in 1- to 4-week-old male Cobb chicks. Increasing crude protein intakes at constant essential amino acid concentrations was carried out at two dietary energy contents. 2. Increasing crude protein resulted in a linear decrease in feed intake while weight gain and feed efficiency changed quadratically with a smaller positive effect at the highest crude protein intakes. Feed intake decreased and feed efficiency increased with higher dietary energy and interactions between protein and energy were significant. Abdominal fat content and the efficiency of protein retention decreased with increasing dietary protein intake. 3. Using constant essential amino acid:crude protein ratios at increasing crude protein intakes resulted in (Trial 3) feed intake, weight gain and feed efficiency all increasing before reaching a plateau. Abdominal fat decreased with protein intake and the efficiency of protein retention was quadratic, decreasing at the higher protein intakes. 4. Multiple regression analysis of the results of the three trials indicated that partition of energy intake into maintenance, fat-free tissue growth, fat and the energy required to transform protein intake in excess of retention explained more than 98% of variation. 5. It is proposed that broiler performance at the lower protein intakes was limited by either non-essential amino acid (Trials 1 and 2) or essential amino acid (Trial 3) intake whereas at high protein intakes the decreased efficiency of amino acid utilisation after growth requirements are fulfilled resulted in poorer performance.