Egg Shell Quality, Medullary Bone Ash, Intestinal Calcium and Phosphorus Absorption, and Calcium-Binding Protein in Phosphorus-Deficient Hens

Genetic patterns and genome-wide association analysis of eggshell quality traits of egg-type chicken across an extended laying period

The industry of egg-type chicken has shown a trend of extending the rearing period, with the goal of breeding chicken breeds capable of producing 500 qualified eggs by 700 d of age. However, the rapid decline in eggshell quality during the late laying period is one of the major challenges. In this study, a total of 3,261 Rhode Island Red chickens were used to measure eggshell quality traits including eggshell strength (ESS), eggshell thickness (EST), eggshell color (ESC) and eggshell gloss (ESG) at seven age points ranging from 36 to 90 wk of age. Phenotypic variations increased with the aging process, especially during the late laying period (> 55 wk), and the heritability during this period decreased by 22.7 to 81.4% compared to the initial and peak laying periods. Then we performed genome-wide association study (GWAS) to identify the genomic variants that associated with eggshell quality, with a custom Illumina 50K BeadChip, named PhenoixChip-I. The results indicated that 2 genomic regions on GGA1(23.24-25.15Mb; 175.95-176.05 Mb) were significantly (P < 4.48E-06) or suggestively (P < 8.97E-05) associated with ESS, which can explain 9.59% and 0.48% of the phenotypic variations of ESS46 and ESS36, respectively. Three genes, FRY, PCNX2, and ENSGALG00000052468, were considered to be the candidate genes for ESS. For other traits, the genome-wide suggestive SNPs were identified at each age point, exhibiting a certain trend with aging process. Additionally, SNP enrichment analysis and functional annotation of cross-tissue regulatory elements to ESS36 revealed a high concentration of enhancer elements specific to shell gland and kidney tissues. This study, deepened our knowledge of eggshells and laying a valued scientific foundation for chicken molecular breeding.

A Method to Reduce the Occurrence of Egg Translucency and Its Effect on Bacterial Invasion

Translucent egg consumption is low due to consumer acceptance and quality concerns, which is a problem that egg producers need to address. This study was performed to evaluate the reasons for the high occurrence of egg translucency in summer, as well as whether the addition of mono-dicalcium phosphate (MDCP) to the diet can relieve eggshell translucency and whether eggshell translucency is associated with the risk of bacterial invasion. A total of 72 laying hens that were 36 weeks old were randomly divided into control (CON) and MDCP groups and fed in the same environment. Results showed that the number of translucent eggs increases in July and August as the temperature and humidity increase. Compared with the CON group, in July, August, and October, the translucent egg grade (TEG) of the MDCP group was lower than that of the CON group (p < 0.05). TEG was correlated with mastoid space height (MSH), width (MSW), and area (MSA) (correlation coefficients 0.63, 0.59, and 0.68, respectively, p ≤ 0.05). There was no significant difference in the invasion rate of E. coli between translucent and non-translucent egg groups (47.2% vs. 39.33%), and translucent area and non-translucent area (13.49% vs. 15.08%). In conclusion, our results show that dietary MDCP may alleviate eggshell translucency and that eggshell translucency would not increase the probability of E. coli cross-shell penetration rate.

Adjusting phosphate feeding regimen according to daily rhythm increases eggshell quality via enhancing medullary bone remodeling in laying hens

BACKGROUND

Body phosphorus metabolism exhibits a circadian rhythm over the 24-h daily cycle. The egg laying behavior makes laying hens a very special model for investigating phosphorus circadian rhythms. There is lack of information about the impact of adjusting phosphate feeding regimen according to daily rhythm on the phosphorus homeostasis and bone remodeling of laying hens.

METHODS AND RESULTS

Two experiments were conducted. In Exp. 1, Hy-Line Brown laying hens (n = 45) were sampled according the oviposition cycle (at 0, 6, 12, and 18 h post-oviposition, and at the next oviposition, respectively; n = 9 at each time point). Diurnal rhythms of body calcium/phosphorus ingestions and excretions, serum calcium/phosphorus levels, oviduct uterus calcium transporter expressions, and medullary bone (MB) remodeling were illustrated. In Exp. 2, two diets with different phosphorus levels (0.32% and 0.14% non-phytate phosphorus (NPP), respectively) were alternately presented to the laying hens. Briefly, four phosphorus feeding regimens in total (each included 6 replicates of 5 hens): (1) fed 0.32% NPP at both 09:00 and 17:00; (2) fed 0.32% NPP at 09:00 and 0.14% NPP at 17:00; (3) fed 0.14% NPP at 09:00 and 0.32% NPP at 17:00; (4) fed 0.14% NPP at both 09:00 and 17:00. As a result, the regimen fed 0.14% NPP at 09:00 and 0.32% NPP at 17:00, which was designed to strengthen intrinsic phosphate circadian rhythms according to the findings in Exp. 1, enhanced (P < 0.05) MB remodeling (indicated by histological images, serum markers and bone mineralization gene expressions), elevated (P < 0.05) oviduct uterus calcium transportation (indicated by transient receptor potential vanilloid 6 protein expression), and subsequently increased (P < 0.05) eggshell thickness, eggshell strength, egg specific gravity and eggshell index in laying hens.

CONCLUSIONS

These results underscore the importance of manipulating the sequence of daily phosphorus ingestion, instead of simply controlling dietary phosphate concentrations, in modifying the bone remodeling process. Body phosphorus rhythms will need to be maintained during the daily eggshell calcification cycle.

The temporal gene expression profiles of calcium and phosphorus transporters in Hy-Line Brown layers

Calcium and phosphorus homeostasis is crucial for the performance and bone health of laying hens. The calcium and phosphorus transporters play an important role in calcium and phosphorus absorption, reabsorption, and excretion. In the present study, Hy-Line Brown layers were sampled at brooding period (1, 4, 6 wk), growing and developing period (12, 18 wk) and laying period (20, 28, 80 wk) respectively, and the calcium transporters CaBP-D28k and PMCA1b and phosphorus transporters NPt2a and NPt2b were respectively measured in duodenum, jejunum, ileum and kidney. The result showed that serum calcium increased (P < 0.0001) and phosphorus level fluctuated (P = 0.0019), while alkaline phosphatase activity decreased with age (P < 0.0001). The mRNA and protein expressions of CaBP-D28k in small intestine elevated after maturity (P ≤ 0.0001). In contrast, the PMCA1b mRNA showed a trend to increase with age in jejunum (P = 0.0059) and ileum (P = 0.0825) whereas there was a decrease for PMCA1b protein in 12-18 wk (P ≤ 0.0009). The peak of NPt2b mRNA were observed at 28 wk in duodenum (P = 0.0001) and jejunum (P = 0.0622) and 1 wk in ileum (P < 0.0001). The NPt2b protein expression reached the top point at 4 or 6 wk and 20 wk and decreased to the lowest point around 12 wk (P ≤ 0.0850). In kidney, CaBP-D28k mRNA was not influenced by age (P = 0.4999), while PMCA1b highly expressed in 6-12 wk (P = 0.0003). The protein expressions of CaBP-D28k (P = 0.0148) and PMCA1b (P = 0.0003) decreased with age and lowly expressed in 12-18 wk and increased thereafter. In contrast, NPt2a expression increased steadily with age and decreased at 80 wk (P < 0.0001). In conclusion, the expressions of intestinal calcium and phosphorus transporters were changed by age, intestinal CaBP-D28k and renal NPt2a showed a dramatic increase after maturity, which coincide with the increased calcium and phosphorus requirement for egg production.

Prolonged scotophase within a 24 hour light regime improves eggshell quality by enhancing calcium deposition in laying hens

Calcium (Ca) and phosphorus (P) transporters are responsible for their absorption and transport in small intestine and kidney, contributing to eggshell formation. The light-dark cycle is a primary cue in the reproduction of laying hen. In this study, we investigated the effect of different light-dark programs on eggshell quality and the expression of genes related to Ca and P transportation in laying hens. Seventy-two 56-week-old laying hens were randomly divided into two groups and reared at 16-h light and 8-h dark (control) or 9-h light and 15-h dark regime (long dark phase, LDP). The expressions of calcium transporter calbindin-D28k (CaBP-D28k), plasma membrane Ca ATPase 1b (PMCA1b), and phosphorus transporter NaPi-IIb (NPt2b) and NaPi-IIa (NPt2a) were measured in the small intestine, kidney, and eggshell gland. The results showed that feed intake (P < 0.001) and egg weight (P = 0.05) were decreased by LDP treatment. Compared with control, the eggshell hardness was increased (P = 0.011) by LDP treatment, but the eggshell thickness and the percentage of eggshell were not changed. The Ca and P contents in eggshell were increased by LDP treatment. During the scotophase, LDP-hens showed higher serum Ca (P = 0.0056) and P levels (P = 0.079) but lower alkaline phosphatase (ALP) activity than that of control hens. In the duodenum, the relative higher expression of CaBP-D28k and PMCA1b in scotophase compared to photophase was masked by LDP treatment. The expression of CaBP-D28k and osteopontin (OPN) in the eggshell gland were increased by LDP treatment, compared to control hens. In the jejunum, the protein expression levels of CaBP-D28k and PMCA1b decreased during photophase in LDP-hens. The result indicates that the increased blood Ca and P concentration during scotophase by LPD treatment is beneficial to the deposition of Ca and P in the eggshell. The result offers an alternative strategy for managing laying hens with poor eggshell quality.

Organic trace minerals improve eggshell quality by improving the eggshell ultrastructure of laying hens during the late laying period

The objective of this study was to investigate the effects of low inclusion levels of organic trace minerals (iron, copper, manganese, and zinc) on performance, eggshell quality, serum hormone levels, and enzyme activities of laying hens during the late laying period. A total of 405 healthy hens (HY-Line White, 50-week-old) were randomly divided into 3 treatments, with 9 replications per treatment and 15 birds per replication. The dietary treatments included a basal diet supplemented with inorganic trace minerals at commercial levels (CON), a basal diet supplemented with inorganic trace minerals at 1/3 commercial levels (ITM), and a basal diet supplemented with proteinated trace minerals at 1/3 commercial levels (TRT). The trial lasted 56 D (8 wk). Compared with the CON group, the ITM group showed decrease in (P < 0.05) egg production, eggshell strength, eggshell palisade layer, palisade layer ratio, serum estrogen, luteinizing hormone, glycosaminoglycan concentration, and carbonic anhydrase activity and increase in (P < 0.05) egg loss and mammillary layer ratio. However, the TRT group almost kept all the indices close to the CON group (P > 0.05). Furthermore, hens fed with low inclusion levels of organic trace minerals had smaller mammillary knobs (P < 0.05) than those in the CON and ITM groups. In conclusion, hens fed with low inclusion levels of proteinated trace minerals had better performance and eggshell strength than those fed with identical levels of inorganic compounds; organic trace minerals improved eggshell quality by improving the eggshell ultrastructure of laying hens during the late laying period.

Effects of Dietary Phosphorus Level on the Expression of Calcium and Phosphorus Transporters in Laying Hens

The transport of calcium and phosphorus is mainly relied on their corresponding transporters. The aim of this study was to determine the effect of dietary phosphorus level on the expression of the relevant calcium and phosphorus transporters in laying hens, which has a large amount of calcium and phosphorus input from intestine and output from kidney and eggshell gland. Thirty-six 25-week-old Hy-line Brown hens were fed diets with different available phosphorus level (AP, 0.15, 0.41, and 0.82%), respectively. The expression of phosphorus transporters type IIa and type IIb Na/Pi co-transporter (NPt2a, NPt2b), calcium transporter calbindin-D28k (CaBP-D28k), and plasma membrane Ca ATPase 1b (PMCA1b) were measured in small intestine, kidney, and eggshell gland by RT-PCR and western blot. The results showed that serum calcitriol and PTH concentrations were not affected (P > 0.05) by dietary AP levels. Duodenum had the highest mRNA and protein expression level of NPt2b than jejunum and ileum (P < 0.05). The protein expression abundance of CaBP-D28k and PMCA1b were higher in duodenum than that in jejunum and ileum (P < 0.05). 0.15%-AP diet upregulated the ileal mRNA expression level of NPt2b and renal mRNA expression level of NPt2a (P < 0.05), while downregulated the protein abundance of NPt2b and CaBP-D28k mRNA expression in shell gland (P < 0.05). In conclusion, both the Ca and P transporters were highly expressed in duodenum. Low AP diet decreased protein expression abundance of NPt2b in duodenum while upregulated the mRNA expression level of NPt2a in kidney. The result suggests that both the phosphorus absorption in proximal intestine and its reabsorption in kidney are involved in the adaption to low AP diet.

Effect of dietary nonphytate phosphorus on laying performance and small intestinal epithelial phosphate transporter expression in Dwarf pink-shell laying hens

This study examined the effects of various levels of dietary nonphytate phosphorus on laying performance and the expression patterns of phosphorus metabolism related genes in Dwarf pink-shell laying hens. A total of 405 28-week-old Dwarf pink-shell laying hens were fed the same corn-soybean basal meals but containing 0.20%, 0.25%, 0.30%, 0.35% or 0.40% nonphytate phosphorus. The results showed that feed intake, egg production, and average egg weights were quadratically correlated with dietary nonphytate phosphorus content (P < 0.05), and the highest egg production, feed intake and average egg weights were achieved when dietary nonphytate phosphorus was at 0.3% (P < 0.05). mRNA expression of intestinal sodium phosphorus co-transporter linearly decreased when dietary nonphytate phosphorus increased. mRNA and protein expression of intestinal calbindin and vitamin D receptor correlated quadratically with dietary nonphytate phosphorus, and the highest expression was found when dietary available phosphorus was at 0.25% to 0.3%. In conclusion, the ideal phosphorus requirement for Dwarf pink-shell layer hens is estimated to be 0.3% in a corn-soybean diet. With this level of phosphorus supplementation, calbindin and vitamin D receptor reached their highest expression.

Calcium transport in strongly calcifying laying birds: mechanisms and regulation

Birds that lay long clutches (series of eggs laid sequentially before a "pause day"), among them the high-producing, strongly-calcifying Gallus gallus domesticus (domestic hen) and Coturnix coturnix japonica (Japanese quail), transfer about 10% of their total body calcium daily. They appear, therefore, to be the most efficient calcium-transporters among vertebrates. Such intensive transport imposes severe demands on ionic calcium (Ca2+) homeostasis, and activates at least two extremely effective mechanisms for Ca2+ transfer from food and bone to the eggshell. This review focuses on the development, action and regulation of the mechanisms associated with paracellular and transcellular Ca2+ transport in the intestine and the eggshell gland (ESG); it also considers some of the proteins (calbindin, Ca2+ATPase, Na+/Ca2+ exchange, epithelial calcium channels (TRPVs), osteopontin and carbonic anhydrase (CA) associated with this phenomenon. Calbindins are discussed in some detail, as they appear to be a major component of the transcellular transport system, and as only they have been studied extensively in birds. The review aims to gather old and new knowledge, which could form a conceptual basis, albeit not a completely accepted one, for our understanding of the mechanisms associated with this phenomenon. In the intestine, the transcellular pathway appears to compensate for low Ca2+ intake, but in birds fed adequate calcium the major drive for calcium absorption remains the electrochemical potential difference (ECPD) that facilitates paracellular transport. However, the mechanisms involved in Ca2+ transport into the ESG lumen are not yet established. In the ESG, the presence of Ca2+-ATPase and calbindin--two components of the transcellular transport pathway--and the apparently uphill transport of Ca2+ support the idea that Ca2+ is transported via the transcellular pathway. However, the positive (plasma with respect to mucosa) electrical potential difference (EPD) in the ESG, among other findings, indicates that there may be major alternative or complementary paracellular passive transport pathways. The available evidence hints that the flow from the gut to the ESG, which occurs during a relatively short period (11 to 14 h out the 24- to 25.5-h egg cycle), is primarily driven by carbonic anhydrase (CA) activity in the ESG, which results in high HCO3(-) content that, in turn, "sucks out" Ca2+ from the intestinal lumen via the blood and ESG cells, and deposits it in the shell crystals. The increased CA activity appears to be dependent on energy input, whereas it seems most likely that the Ca2+ movement is secondary, that it utilizes passive paracellular routes that fluctuate in accordance with the appearance of the energy-dependent CA activity, and that the level of Ca2+ movement mimics that of the CA activity. The on-off signals for the overall phenomenon have not yet been identified. They appear to be associated with the circadian cycle of gonadal hormones, coupled with the egg cycle: it is most likely that progesterone acts as the "off" signal, and that the "on" signal is provided by the combined effect of an as-yet undefined endocrine factor associated with ovulation and with the mechanical strain that results from "egg white" formation and "plumping". This strain may initially trigger the formation of the mammillae and the seeding of shell calcium crystals in the isthmus, and thereafter initiate the formation of the shell in the ESG.

Intestinal and eggshell calbindin, and bone ash of laying hens as influenced by age and molting

A series of trials was conducted in order to study the effects of age and molt on intestinal and eggshell gland (ESG) calbindin, and on bone ash. For this purpose an ELISA for chicken calbindin was developed. Age did not significantly affect duodenal or ESG calbindin. Bone ash increased (but not significantly in this study) from 8 to 16 months of age. During molt induction, egg laying was arrested, duodenal and ESG calbindin almost completely disappeared and ovary mass, plasma estradiol and total calcium (Ca) decreased markedly, whereas bone ash and body mass (BW) decreased moderately. During the non-laying period that followed the feed withdrawal period, duodenal and ESG calbindin remained low, whereas plasma estradiol and other estrogen-dependent variables, such as plasma total Ca and bone ash, increased slightly. At the onset of egg production following molting, duodenal and ESG calbindin levels were similar to pre molt level. Bone ash was higher than at the pre molt period. Body mass, small yellow follicles, ovary and oviduct mass and plasma estradiol were lower than their values prior to molt induction. Bone ash contents in the molted hens at the ages of 583 and 820 days were similar to or even slightly higher than those in the non-molted hens, whereas duodenal and ESG calbindin were not significantly different. These results suggest that the improvement of shell quality in the molted birds does not involve mechanisms associated with calbindin synthesis.

Re-evaluation of calcium and phosphorus requirements in aged laying hens

1. A series of 5 trials was conducted with 401- to 650-d-old Lohmann, Yafa (local breed with brown eggshells) and Yarkon (local breed with white eggshells) hens fed for 56 to 84 d with diets containing 25 to 50 g/kg calcium (Ca) and 4.5 to 7.6 g/kg phosphorus (P). 2. Increasing dietary Ca from 24-25 to 36-40 g/kg improved egg production, shell weight (SW) and shell thickness (ST), and decreased mortality. 3. Increasing dietary Ca to 48 to 50 g/kg did not affect egg production but increased SW and/or ST. In one out of the 4 trials, this effect was not significant, most likely because of the high shell quality of the eggs from the Yafa hens used for this trial. 4. Dietary P content of 4.5 g/kg (1.0 g/kg added inorganic P) appears to be sufficient for maintaining egg production and shell quality in aged laying hen given 36 to 40 g/kg Ca. 5. Increasing dietary Ca above 40 g/kg may require a higher dietary P content.

Effects of added dietary fat and phosphorus on the performance and egg quality of laying hens subjected to a constant high environmental temperature

Two experiments were conducted to study the combined effect of nonphytate P (NPP), fat, and temperature on the performance and egg quality of hens pre- and postpeak. In Experiment 1, 192 Single Comb White Leghorn layers, aged 22 wk, were individually housed in cages under ambient (AT) and constant high temperature (CHT) of 33 +/- 1 C. In Experiment 2, 28-wk-old birds were used with a CHT of 35 +/- 1 C. Diets contained 0 and 4% added vegetable fat with four dietary NPP levels, ranging from 0.20 to 0.50% in Experiment 1 and from 0.15 to 0.45% in Experiment 2, at increments of 0.1%. These diets were fed to hens for 84 d. Performance and egg quality criteria were measured in both experiments, whereas serum and tibia Ca and P were determined in Experiment 2. Results showed that there was no significant interaction among NPP, fat, and temperature, for any criteria measured. The CHT significantly reduced BW, feed intake, egg production, egg weight, egg mass, and shell thickness. Moreover, it decreased (P < 0.05) serum Ca, P, tibia ash, and tibia P. The lowest NPP levels resulted in the greatest shell thickness (P < 0.05), which decreased with increasing NPP levels in both trials. In the postpeak experiment, the feed intake was significantly reduced by 0.15 and 0.25% NPP. Moreover, 0.15% NPP significantly reduced egg production as compared to 0.35 and 0.45% NPP diets. Levels between 0.25 and 0.45% significantly improved the serum phosphorus content as compared to 0.15%. Added fat improved only serum P level (P < 0.05). The beneficial effect of supplemental fat on hen performance was evident in Experiment 1 but not in Experiment 2, indicating that nutrients were oriented more toward supporting egg production rather than maintaining the BW at the postpeak stage.

Evaluation of the toxicity of alum (aluminum sulfate) in young broiler chickens

Two experiments were conducted to characterize the toxicity and evaluate the efficacy of alum to increase intestinal strength in young broiler chicks. Cobb x Cobb male broiler chicks were placed in an experimental design consisting of six dietary treatments of alum (control, 0.23, 0.47, 0.93, 1.9, and 3.7%) with four replicate pens of 10 broilers per pen. The chicks were housed in electrically heated batteries and provided the treatments for ad libitum consumption from 1 d to 3 wk of age. Alum significantly (P < or = 0.05) decreased body weights at 1.9 and 3.7% in Experiment 1 and at 0.93, 1.9, and 3.7% in Experiment 2. Feed conversion and the relative weight of the gizzard were increased in both experiments at 3.7%. Serum phosphorus was decreased at 1.9 and 3.7% in Experiment 1 and at 3.7% in Experiment 2. Intestinal and bone strength were decreased in both experiments at 3.7%. Bone ash was reduced at 3.7% in Experiment 2, bone S levels increased at 1.9 and 3.7% in Experiment 1 and at 3.7% in Experiment 2, and bone Al levels were elevated in both experiments at 3.7%. Muscle levels of P and S decreased, and that of Ca increased at 3.7%. Aluminum levels were not elevated in muscle tissues. These data indicate that alum can be toxic to young broiler chicks, but at levels that would not be expected to be reached through litter consumption, and that alum did not increase intestinal strength.

The effects of various dietary phosphorus levels on the circadian patterns of plasma 1,25-dihydroxycholecalciferol, total calcium, ionized calcium, and phosphorus in laying hens

Six hundred and sixty 75-wk-old Hy-line W36 hens were allocated to one of three dietary levels of total phosphorus, .30, .60, or .90%. Birds were fed the diets for 3 days following which blood samples were collected at six different times, 2, 6, 10, 14, 18, and 22 h postoviposition (POP), and analyzed for 1,25-dihydroxycholecalciferol [1,25-(OH)2 D3], total calcium (TCa), ionized calcium (Ca++), percentage Ca++ to TCa (%Ca++/TCa), and phosphorus (P). Plasma TCa and P significantly (P less than .001 and .025, respectively) peaked at 10 to 14 h POP. The Ca++ and %Ca++/TCa significantly (P less than .001) decreased during eggshell formation and following completion of the shell (22 h POP) levels returned to resting concentrations. Plasma 1,25-(OH)2D3 results confirmed the existence and time of a circadian rhythm in laying hens. Peak concentrations of the metabolite occurred at 10 to 14 h POP, which resulted in a quadratic relationship (P less than .001). Plasma P decreased with decreasing dietary P and plasma 1,25-(OH)2D3 increased (P less than .025). Feeding low dietary P significantly (P less than .001) increased Ca++ and %Ca++/TCa. Results of feeding various levels of dietary P to laying hens indicate that low P stimulates an increase in plasma 1,25-(OH)2D3 as well as Ca++ and %Ca++/TCa, but high P actually suppressed this response.

Effects of vitamin or mineral deficiency on the morphology of medullary bone in laying hens

Adult laying hens were fed diets deficient in phosphorus, calcium, calcium and phosphorus, and vitamin D3 to determine their effects on bone histology and parathyroid gland size. The phosphorus deficient diet caused an insignificant decrease in parathyroid size while the other diets caused significant increases. A considerable amount of individual variation in medullary bone volume and osteoid seam width was observed in all groups but, despite this, the calcium, calcium and phosphorus and vitamin D3 deficient diets clearly resulted in increased osteoid. Birds receiving diets deficient in calcium and phosphorus, and in vitamin D3 for longer periods were observed to have partially or completely resorbed medullary bone. Osteodystrophia fibrosa was noted in vitamin D3 deficient birds which had no follicular activity.

Evaluation of the dietary interaction of calcium and phosphorus in the high producing laying hen

1. In a 6 x 7 factorial experiment using 2688 22-week-old laying hens of the Lohmann-SL strain kept in cages (4 birds/cage), diets containing six calcium (20, 25, 30, 35, 40, 45 g calcium/kg) and seven phosphorus concentrations (3.2, 4.2, 5.2, 6.2, 7.2, 8.2, 16.2 g total phosphorus/kg (Pt)) were combined orthogonally. The resulting 42 treatments were replicated 8 times so that a replicate consisted of a double cage of 2 x 4 hens. The experiment lasted 40 weeks (10 x 28 days). 2. The experimental diets, based on maize and soyabean meals contained 11.5 MJ metabolisable energy/kg and 175 g/kg protein. Different dietary calcium and phosphorus contents were obtained by substituting oat hulls with limestone and dicalcium phosphate. 3. Mortality, egg production, egg weight, egg mass, food intake and food conversion efficiency were determined as well as the breaking strength, thickness of shells and the percentage of eggs with defective shells. 4. All responses measured were significantly influenced by the variance sources (calcium, phosphorus, interaction). Most of the production traits responded asymptotically to increasing dietary phosphorus concentration, the greatest increases or decreases generally being seen between 3.2 and 5.2 g Pt/kg. Further but weaker increases were seen between 5.2 and 8.2 or 16.2 g Pt/kg. 5. Increases in dietary calcium content always resulted in curvilinear responses. In all cases optimal effects were obtained with diets containing 25 g calcium/kg and the worst values at 45 g calcium/kg. The interaction between calcium and phosphorus was recognised by strong performance depressions and a high mortality at combinations of the lowest phosphorus concentration (3.2 g/kg) with high calcium contents (35 to 45 g/kg). These were largely offset by increasing dietary phosphorus. Thus, between 7.2 and 16.2 g Pt/kg and 25 and 45 g Ca/kg a plateau was formed where only small differences in egg production were observed. 6. From the three egg shell characteristics measured, breaking strength and shell thickness responded differently to the percentage of eggs with defective shells. While breaking strength and shell thickness were respectively negatively and positively influenced by increasing dietary phosphorus and calcium contents, both elements affected the proportion of eggs with defective shells.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of vitamin D on calcium regulation in vitamin-D-deficient pigs given a phytate-phosphorus diet

1. Vitamin-D-deficient pigs were fed on a phytate-phosphorus diet and treated with vitamin D3 (+D) to examine the time-course of adaptative changes in plasma minerals, vitamin D metabolites, parathyroid hormone (PTH) and calcium balance and intestinal Ca-binding protein (CaBP). 2. The 5-week vitamin D repletion (25 micrograms cholecalciferol/kg diet) regimen restored plasma Ca, P and alkaline phosphatase (EC 3.1.3.1) to normal, decreased PTH and markedly and rapidly increased plasma 25-hydroxycholecalciferol (25-OHD, sevenfold after 4 d) and 1,25-dihydroxycholecalciferol (1, 25(OH)2D3, 1.8-fold after 4 d). 3. CaBP concentrations were markedly elevated all along the digestive tract, especially in the distal regions. 4. Ca absorption and retention were enhanced (fourfold and sixfold respectively) by the +D diet. 5. The improved Ca absorption, coupled with increased CaBP and 1,25(OH)2D3 levels, suggest that vitamin D metabolism in phytate-P-fed pigs is sensitive to the depressed Ca availability due to phytate feeding. It also indicates that CaBP may play an important role in the adaptation of Ca absorption. 6. Persistent hypercalciuria indicates that mineral metabolism was still affected by the phytate nature of the dietary P in spite of the vitamin D treatment.