Evaluation of dietary calcium level effects on the productivity, eggshell quality, and bone traits of laying hens
Creators: Batres, Daniella
Publisher: name: University of Alberta Library
Year: 2022

<p>Given the significance of calcium (Ca) in the metabolism of laying hens, the purpose of this thesis was to contribute to existing knowledge regarding Ca metabolism and requirements in these highly productive birds. <br>
Phosvitin (PV), is an egg yolk phospholipid protein that increases Ca bioavailability. Experiment 1 evaluated the effects of dietary phosvitin (PV) as a potential strategy for protecting the shell and bone quality of end-of-cycle laying hens fed a Ca-reduced diet. A total of eight experimental diets were used including a positive control (PC), negative control (NC; 21% less Ca than the PC) and six NC-based diets that contained either PV (37.4% purity), dephosphorylated phosvitin (DPV; 39.5% purity) or phosvitin peptides (PVP; 39.8% purity), each fed at a 1% or 0.01% of the diet. It was hypothesized that dietary PV products would protect laying hen bone quality via increased Ca digestibility, while maintaining productivity and shell quality in end-of-cycle laying hens. Overall, there was no effect of the NC diet on egg production and egg mass over the duration of the experiment. Additionally, bone and shell quality remained unaffected by dietary treatment. This suggests that commercial end‐of‐cycle laying hens can maintain egg production, shell quality and bone quality under substantial reductions in dietary Ca, at least in the short term.<br>
The impressive performance of hens fed substantially reduced Ca prompted investigating Ca requirements in laying hens via a meta-analysis performed with publications using varying levels of dietary Ca. Dietary Ca level was defined as the independent variable of interest and dependant variables included: egg production (%), Ca intake (g/hen/day), phosphorus (P) intake (g/hen/day), egg mass (g), feed intake (FI; g/hen/day), feed conversion (kg feed: kg eggs), feed conversion (kg feed: dozen eggs), egg weight (g), shell weight (%), eggshell thickness (mm), egg specific gravity, egg breaking strength (N), bone breaking strength (BBS; N), bone Ca (%), bone P (%) and bone ash (%). Regression models for each dependant variable were created with the inclusion of moderator (random) variables that specifically improved each model as reflected by the Bayesian information criterion. Ultimately a data set containing 792 observations was compiled from 57 published papers between 1981 and 2020. Seven moderator variables (Record No., Year, Strain, Molted, Ca Particle Size, Ca Source and Heat Stress) were considered for inclusion in each regression model.<br>
Ca intake (1.04 g/hen/day per 1% increase in dietary Ca) and FI (1.54 g/hen/day per 1% increase in dietary Ca) were each significantly affected by dietary Ca levels. Additionally, each of the feed conversion parameters (feed:eggs and feed:dozen eggs) decreased with increasing levels of dietary Ca (-0.022 feed:egg ratio and -0.051 feed:dozen per 1% increase in dietary Ca). Shell weight (0.24 % per 1% increase in dietary Ca), eggshell thickness (0.0075 mm per 1% increase in dietary Ca), specific gravity (0.0015 per 1% unit increase in dietary Ca) and egg breaking strength (1.23 N per 1% unit increase in dietary Ca) each increased with increasing dietary Ca, which agrees with others that show that adequate Ca is essential for shell quality. The positive relationship between dietary Ca levels and bone ash (1.03 % per 1% unit increase in dietary Ca) was unexpected given the small amount of research that supports this. Alternatively, BBS significantly increased with increasing dietary Ca levels (12.92 N per 1% unti increase in Ca) which supports the idea that increasing dietary Ca levels prevent hens from depleting skeletal Ca when adequate dietary Ca is available. The presence of Year as a significant random variable in this relationship suggests the relationship between dietary Ca and bone quality is changing over time. Using the NRC (1994) Ca requirements as a reference, there do not seem to be negative consequences associated with over-supplementation of Ca within the Ca ranges studied as illustrated by the lack of plateaus or decreases in the response variables tested.</p>