Search

ABSTRACT: A 305-d lactation followed by a 60-d dry period has traditionally been considered economically optimal, yet dairy cows in modern intensive dairy systems are frequently dried off while still producing significant quantities of milk. Managing cows for an extended lactation has reported production, welfare, and economic benefits, but not all cows are suitable for an extended lactation. Implementation of an extended lactation strategy on-farm could benefit from use of a decision support system, based on a mathematical lactation model, that can identify suitable cows during early lactation that have a high likelihood of producing above a target milk yield (MY) at 305 d in milk (DIM). Therefore, our objectives were (1) to compare the suitability of 3 commonly used lactation models for modeling extended lactations (Dijkstra, Wood, and Wilmink) in primiparous and multiparous cows under a variety of lactation lengths, and (2) to determine the amount of early-lactation daily MY data needed to accurately forecast MY at d 305 by using the most suitable model and determine whether this is sufficient for identifying cows suitable for an extended lactation before the end of a typical voluntary waiting period (50–90 d). Daily MY data from 467 individual Holstein-Friesian lactations (DIM >305 d; 379 ± 65-d lactation length [mean ± SD]) were fitted by the 3 lactation models using a nonlinear regression procedure. The parameter estimates of these models, lactation characteristics (peak yield, time to peak yield, and persistency), and goodness-of-fit were compared between parity and different lactation lengths. The models had similar performance, and differences between parity groups were consistent with previous literature. Then, data from only the first i DIM for each individual lactation, where i was incremented by 30 d from 30 to 150 DIM and by 50 d from 150 to 300 DIM, were fitted by each model to forecast MY at d 305. The Dijkstra model was selected for further analysis, as it had superior goodness-of-fit statistics for i= 30 and 60. The data set was fit twice by the Dijkstra model, with parameter bounds either unconstrained or constrained. The quality of predictions of MY at d 305 improved with increasing data availability for both models and assisting the model fitting procedure with more biologically relevant constraints on parameters improved the predictions, but neither was reliable enough for practical use on-farm due to the high uncertainty of forecasted predictions. Using 90 d of data, the constrained model correctly classified 66% of lactations as being above or below a target MY at d 305 of 25 kg/d, with a probability threshold of 0.95. The proportion of correct classifications became smaller at lower targets of MY at d 305 and became greater when using more lactation days. Overall, further work is required to develop a model that can forecast late-lactation MY with sufficient accuracy for practical use. We envisage that a hybridized machine learning and mechanistic model that incorporates additional historical and genetic information with early-lactation MY could produce meaningful lactation curve forecasts.

Abstract When two putatively cryptic species meet in nature, hybrid zone analysis can be used to estimate the extent of gene flow between them. Two recently recognized cryptic species of banded newt (genus Ommatotriton) are suspected to meet in parapatry in Anatolia, but a formal hybrid zone analysis has never been conducted. We sample populations throughout the range, with a focus on the supposed contact zone, and genotype them for 31 nuclear DNA SNP markers and mtDNA. We determine the degree of genetic admixture, introgression, and niche overlap. We reveal an extremely narrow hybrid zone, suggesting strong selection against hybrids, in line with species status. The hybrid zone does not appear to be positioned at an ecological barrier, and there is significant niche overlap. Therefore, the hybrid zone is best classified as a tension zone, maintained by intrinsic selection against hybrids. While the two banded newt species can evidently backcross, we see negligible introgression and the pattern is symmetric, which we interpret as supporting the fact that the hybrid zone has been practically stationary since its origin (while extensive hybrid zone movement has been suggested in other newt genera in the region). Our study illustrates the use of hybrid zone analysis to test cryptic species status.

The ability of four non-linear mixed models and one linear mixed model to describe phosphorus (P) retention as a function of dietary P intake, expressed on an available P (avP) basis, was assessed in growing and finishing pigs. Of the four non-linear models, the monomolecular and Michaelis-Menten describe diminishing returns behaviour, while the Richards and Morgan describe sigmoidal behaviour with the ability to also describe diminishing returns. Using a meta-analysis approach, models were fitted to avP intake vs. P retention data from P balance studies. Pig bodyweights (BW) ranged from 43.5 to 133 kg, P intake ranged from 0.055 to 0.468 g kg−1 BW0.75 d−1 for avP, and 0.151 to 0.806 g kg−1 BW0.75 d−1 for total P, with P retention ranging from 0.026 to 0.329 g kg−1 BW0.75 d−1. Models were evaluated using statistical measures of goodness-of-fit and inspection of residuals. The monomolecular and Michaelis-Menten best described the relationship between P retention and P intake. Endogenous P losses and P requirement for maintenance were found to be higher in finishing pigs compared to growing pigs as BW increased.

The cryosphere, which comprises a large portion of Earth’s surface, is rapidly changing as a consequence of global climate change. Ice, snow, and frozen ground in the polar and alpine regions of the planet are known to directly impact atmospheric composition, which for example is observed in the large influence of ice and snow on polar boundary layer chemistry. Atmospheric inputs to the cryosphere, including aerosols, nutrients, and contaminants, are also changing in the anthropocene thus driving cryosphere-atmosphere feedbacks whose understanding is crucial for understanding future climate. Here, we present the Cryosphere and ATmospheric Chemistry initiative (CATCH) which is focused on developing new multidisciplinary research approaches studying interactions of chemistry, biology, and physics within the coupled cryosphere – atmosphere system and their sensitivity to environmental change. We identify four key science areas: (1) micro-scale processes in snow and ice, (2) the coupled cryosphere-atmosphere system, (3) cryospheric change and feedbacks, and (4) improved decisions and stakeholder engagement. To pursue these goals CATCH will foster an international, multidisciplinary research community, shed light on new research needs, support the acquisition of new knowledge, train the next generation of leading scientists, and establish interactions between the science community and society.

Aim of study: The aim of the present study was to introduce a sinusoidal equation into poultry science by applying it to temporal growth data from quail. Material and methods: To examine the performance of the sinusoidal equation in describing the growth patterns of quail, four conventional growth functions (Gompertz, logistic, López and Richards) were used as reference in this study. Comparison of models was carried out by analysing model behaviour when fitting the curves using nonlinear regression and assessing statistical performance. Maximum log-likelihood estimation, mean squared error, Akaike and Bayesian information criteria were used to evaluate the general goodness-of-fit of each model to the different data profiles. Main results: The selected sinusoidal equation precisely describes the growth dynamics of quail. Comparison of the growth functions in terms of the goodness-of-fit criteria revealed that the sinusoidal equation was one of the most appropriate functions to describe the age-related changes of bodyweight in quail. Research highlights: To the best of our knowledge there are no studies available on the use of sinusoidal equations to describe the evolution of growth in quail. The sinusoidal equation used in this study represents a suitable alternative to conventional growth functions to describe the growth curves for a range of strains/lines of male and female Japanese quail.

Background/Methods: This study determines the co-expression of mammaglobin-A, vascular endothelial growth factor receptor-3 (VEGFR3) and Ki67 by immunohistochemistry (IHC) in tissue samples from 80 patients undergoing breast surgery (cancer or benign disease). The tissue expression was compared with the tumour histopathology and Kaplan Meier 5-year survival analysis was performed. Results: Positive breast tissue expression was observed in 53% samples for mammaglobin, 41% Ki67 and 65% VEGFR3 with a significant positive correlation between Ki67 and VEGFR3 co-expression. Ki67 and VEGFR3 expression correlated with the breast tumour grade and Ki67 expression also correlated with oestrogen receptor (ER) status. At 5 years post-operatively, 6/80 patients had died and 3 patients were alive but had cancer recurrence. High Ki67 expression significantly correlated with poor survival (disease-free and overall). Conclusions: In this study, VEGFR3 and Ki67 expression but not mammaglobin-A correlated with breast tumour pathology. Positive Ki67 expression was also associated with a poor 5-year survival outcome.

Because of the relatively long growing cycle and the high cost of research into turkey production and nutrition, the potential benefits from modelling growth in this avian species are considerable. Though there are many studies aimed at evaluating animal growth models, the number of studies targeting growth models in turkeys is quite limited. In this paper we present a sinusoidal function to describe the evolution of growth in turkeys as a function of time based on data published by Aviagen. The new function was evaluated with regard to its ability to describe the relationship between body weight and age in turkeys and was compared to four standard growth functions: the Gompertz, logistic, Lopez, and Richards. The results of this study show that the new sinusoidal function precisely describes the growth dynamics of turkeys. Fitting the functions to different data profiles nearly always led to the same or less maximized log-likelihood values for the sinusoidal equation, indicating its suitability in describing growth data from turkeys.

Gas production profiles typically show a monotonically increasing monophasic pattern. However, atypical gas production profiles exist whereby at least two consecutive phases of gas production or additional extraneous features that distort the typical profile are present. Such profiles are more likely to occur with the use of a fecal inoculum and are much less well described. The presence of multiple phases or non-descript extraneous features makes it difficult to apply directly recommended modeling approaches such as standard response functions or classical growth functions. To overcome such difficulties, extensions of the Mitscherlich equation and a numerical modeling option also based on the Mitscherlich are explored. The numerical modeling option uses an estimate of relative rate obtained from the smoothed data profile and an estimate of maximum gas produced together with any lag time information drawn from the raw data to construct a simple Mitscherlich equation. In summary, this article illustrates the analysis of atypical gas production profiles obtained using a fecal inoculum and explores the methodology of numerical modeling to reconstruct equivalent typical growth-like trends.

In vitro gas production systems are utilized to screen feed ingredients for inclusion in ruminant diets. However, not all in vitro systems are set up to measure methane (CH4) production, nor do all publications report in vitro CH4. Therefore, the objective of this study was to develop models to predict in vitro CH4 production from total gas and volatile fatty acid (VFA) production data and to identify the major drivers of CH4 production in these systems. Meta-analysis and machine learning (ML) methodologies were applied to a database of 354 data points from 11 studies to predict CH4 production from total gas production, apparent DM digestibility (DMD), final pH, feed type (forage or concentrate), and acetate, propionate, butyrate and valerate production. Model evaluation was performed on an internal dataset of 107 data points. Meta-analysis results indicate that equations containing DMD, total VFA production, propionate, feed type and valerate resulted in best predictability of CH4 on the internal evaluation dataset. The ML models far exceeded the predictability achieved using meta-analysis, but further evaluation on an external database would be required to assess generalization ability on unrelated data. Between the ML methodologies assessed, artificial neural networks and support vector regression resulted in very similar predictability, but differed in fitting, as assessed by behaviour analysis. The models developed can be utilized to estimate CH4 emissions in vitro.

Two new models, based upon the principles promulgated by Baranyi and co-workers are presented and resulting growth functions evaluated based upon their ability to mimic bacterial growth of the fish pathogen Flavobacterium psychrophilum. These growth functions make use of a dampening function to suppress potential growth, represented by a logistic, and are derived from rate:state differential equations. Dampening effects are represented by a rectangular hyperbola or a simple exponential, incorporated into a logistic differential equation and solved analytically resulting in two newly derived growth equations, viz. logistic × hyperbola (log × hyp) and logistic × exponential (log × exp). These characteristics result in flexible and robust growth functions that can be expressed as equations with biologically meaningful parameters. The newly derived functions (log × hyp and log × exp), along with the Baranyi (BAR), simple logistic (LOG) and its modified form (MLOG) were evaluated based upon examination of residuals and measures of goodness-of-fit and cross-validation. Using these criteria, log × hyp, log × exp and BAR performed better than, or at least equally well as, LOG and MLOG. In contrast with log × exp and BAR, log × hyp can be easily manipulated mathematically allowing for simple algebraic expressions for time and microbial biomass at inflexion point, in addition to maximum and scaled maximum growth rates.

Two models are proposed to describe atypical biphasic gas production profiles obtained from in vitro digestibility studies. The models are extensions of the standard Mitscherlich equation, comprising either two Mitscherlich terms or one Mitscherlich and one linear term. Two models that describe typical monophasic gas production curves, the standard Mitscherlich and the France model [a generalised Mitscherlich (root-t) equation], were assessed for comparison. Models were fitted to 25 gas production profiles resulting from incubating feedstuffs with faecal inocula from equines. Seventeen profiles displayed atypical biphasic patterns while the other eight displayed typical monophasic patterns. Models were evaluated using statistical measures of goodness-of-fit and by analysis of residuals. Good agreement was found between observed atypical profiles values and fitted values obtained with the two biphasic models, and both can revert to a simple Mitscherlich allowing them to describe typical monophasic profiles. The models contain kinetic fermentation parameters that can be used in conjunction with substrate degradability information and digesta passage rate to calculate extent of substrate degradation in the rumen or hindgut. Thus, models link the in vitro gas production technique to nutrient supply in the animal by providing information relating to digestion and nutritive value of feedstuffs.

Periodontal disease is of established etiology in which polymicrobial synergistic ecology has become dysbiotic under the influence of Porphyromonas gingivalis. Following breakdown of the host's protective oral tissue barriers, P. gingivalis migrates to developing inflammatory pathologies that associate with Alzheimer's disease (AD). Periodontal disease is a risk factor for cardiovascular disorders (CVD), type II diabetes mellitus (T2DM), AD and other chronic diseases, whilst T2DM exacerbates periodontitis. This study analyzed the relationship between the P. gingivalis/host interactome and the genes identified in genome-wide association studies (GWAS) for the aforementioned conditions using data from GWASdb (P < 1E-03) and, in some cases, from the NCBI/EBI GWAS database (P < 1E-05). Gene expression data from periodontitis or P. gingivalis microarray was compared to microarray datasets from the AD hippocampus and/or from carotid artery plaques. The results demonstrated that the host genes of the P. gingivalis interactome were significantly enriched in genes deposited in GWASdb genes related to cognitive disorders, AD and dementia, and its co-morbid conditions T2DM, obesity, and CVD. The P. gingivalis/host interactome was also enriched in GWAS genes from the more stringent NCBI-EBI database for AD, atherosclerosis and T2DM. The misregulated genes in periodontitis tissue or P. gingivalis infected macrophages also matched those in the AD hippocampus or atherosclerotic plaques. Together, these data suggest important gene/environment interactions between P. gingivalis and susceptibility genes or gene expression changes in conditions where periodontal disease is a contributory factor.

The nutritive value of 26 agro-industrial by-products was assessed from their chemical composition, in vitro digestibility and rumen fermentation kinetics. By-products from sugar beet, grape, olive tree, almond, broccoli, lettuce, asparagus, green bean, artichoke, peas, broad beans, tomato, pepper, apple pomace and citrus were evaluated. Chemical composition, in vitro digestibility and fermentation kinetics varied largely across the by-products. Data were subjected to multivariate and principal component analyses (PCA). According to a multivariate cluster analysis chart, samples formed four distinctive groups (A−D). Less degradable by-products were olive tree leaves, pepper skins and grape seeds (group A); whereas the more degradable ones were sugar beet, orange, lemon and clementine pulps (group D). In the PCA plot, component 1 segregated samples of groups A and B from those of groups C and D. Considering the large variability among by-products, most of them can be regarded as potential ingredients in ruminant rations. Depending on the characteristic nutritive value of each by-product, these feedstuffs can provide alternative sources of energy (e.g., citrus pulps), protein (e.g., asparagus rinds), soluble fibre (e.g., sugar beet pulp) or less digestible roughage (e.g., grape seeds or pepper skin).

All rumen models cover the main drivers of variation in rumen function, which are feed intake, the differences between feedstuffs and feeds in their intrinsic rumen degradation characteristics, and fractional outflow rate of fluid and particulate matter. Dynamic modeling approaches are best suited to the prediction of more nuanced responses in rumen metabolism, and represent the dynamics of the interaction between substrates and micro-organisms and inter-microbial interactions. The concepts of dynamics are discussed for the case of rumen starch digestion as influenced by starch intake rate and frequency of feed intake, and for the case of fermentation of fiber in the large intestine. Adding representations of new functional classes of micro-organisms (i.e. with new characteristics from the perspective of whole rumen function) in rumen models only delivers new insights if complemented by the dynamics of their interactions with other functional classes. Rumen fermentation conditions have to be represented due to their profound impact on the dynamics of substrate degradation and microbial metabolism. Although the importance of rumen acidity is generally acknowledged, more emphasis is needed on predicting its variation as well as variation in the processes that underlie rumen fluid dynamics. The rumen wall has an important role in adapting to rapid changes in the rumen environment, clearing of volatile fatty acids (VFA), and maintaining rumen pH within limits. Dynamics of rumen wall epithelia and its role in VFA absorption needs to be better represented in models which aim to predict rumen responses across nutritional or physiological states. For a detailed prediction of rumen N balance there is merit in a dynamic modeling approach compared to the static approaches adopted in current protein evaluation systems. Improvement is needed on previous attempts to predict rumen VFA profiles, and this should be pursued by introducing factors that relate more to microbial metabolism. For rumen model construction, data on rumen microbiomes are preferably coupled with knowledge consolidated in rumen models instead of relying on correlations with rather general aspects of treatment or animal. This helps to prevent disregard of basic principles and underlying mechanisms of whole rumen function.

The growing global demand for meat and particularly in countries such as Brazil is expected to increase intensive animal production. Consequently the main pollutants of interest are nitrogen (N), phosphorus (P) and greenhouse gases (GHG). Nitrogen can be a problem through nitrate leaching to water bodies, ammonia, and nitrous oxide emissions to air. Phosphorus loading in soil from manure application can be the main issue due to the tendency of P to accumulate in soil and leach to groundwater and rivers. The sources of agricultural GHG emissions include methane from enteric fermentation, manure storage and spreading, and nitrous oxide mainly from application of manure on land. Dietary manipulation has proven to be an effective tool to reduce nutrient/mineral pollution and GHG emissions. Several studies have shown that decreasing crude protein in the diet could reduce N excretion and ammonia volatilization substantially without compromising productivity. Similarly, reducing P intake in dairy cattle has been shown to reduce P excretion by up to 10%. Changing the type of N and P consumed and energy level of diet has also been reported to affect the amount and type of N and P excreted. Dietary manipulation also has an impact on the amount of GHG emissions, particularly, from enteric fermentation. Feeding cattle with a high starch and low fiber diet, for example, reduces acetate production in the rumen, and leads to lower methane production. Emissions from stored manure from high fiber fed animals tend to be higher. Evidence is also available that diet affects emissions from manure applied soil. As level of production is increased to meet global demand for ruminant meat and milk products, dietary manipulation will be useful in addressing environmental concerns.A crescente demanda global por carne, em particular no Brasil, deverá aumentar a produção animal intensiva. Por conseguinte, os principais poluentes são o nitrogênio (N), fósforo (P) e gases de efeito de estufa (GEE). O nitrogênio pode ser um problema devido à lixiviação de nitrato para os corpos d'água, amônia e emissões de óxido nitroso para o ar. Carga de fósforo no solo decorrente da aplicação de esterco pode ser o problema principal, devido à tendência de o P acumular no solo e atingir os lençóis freáticos e rios. As fontes de emissões de GEE agrícolas incluem metano proveniente da fermentação entérica, armazenagem e e distribuição de estrume e óxido nitroso principalmente da aplicação de adubo na terra. A manipulação dietética tem demomstrado eficiência na redução da poluição de nutrientes/minerais e das emissões de GEE. Vários estudos têm demonstrado que a diminuição de proteína bruta na dieta podem reduzir substancialmente a excreção de N e a volatilização de amônia, sem comprometer a produtividade. Da mesma forma,a redução da ingestão de P em bovinos leiteiros pode reduzir a excreção de P em até 10%. Alterações do tipo de N e P consumidos e do nível energético da dieta sobre a quantidade e o tipo de N e P excretado também foram relatadas. A manipulação da dieta também tem impacto sobre a quantidade de emissões de gases de efeito estufa, em particular, a proveniente de fermentação entérica. A alimentação do gado com dietas contendo elevado teor de trigo baixo de fibra, por exemplo, reduz a produção de acetato no rúmen e acarreta menor produção de metano. Emissões decorrentes de estrume de animais alimentados com alto teor de fibra na dieta tendem a ser maiores. Evidenciou-se também que a dieta afeta as emissões de esterco no solo. Em decorrência do aumento do nível de produção para atender à demanda global de carne de ruminantes e produtos lácteos, a manipulação dietética será útil para atender às preocupações ambientais.

Mannoheptulose (MH), a sugar found in avocados that inhibits glycolysis in vitro, has been preliminarily investigated as a novel food ingredient for dogs. This study aimed to determine the effects of dietary MH, delivered as an extract of un-ripened avocado, on energy expenditure (EE) in healthy adult Labrador Retriever dogs (total of 12 dogs, 26.99 ± 0.634 kg, 4.9 ± 0.2 y). The study was a double-blind, cross-over with each dog receiving both dietary treatments, control (CON) and MH (400 mg/kg of diet; 6 mg/kg BW), in random order. Resting and post-prandial (10 h) EE and respiratory quotient (RQ) were determined by indirect calorimetry (d 42). The following day, body composition was assessed using dual X-ray absorptiometry. Continuous activity monitoring was conducted using an Atical® accelerometer (d 43-47). A vastus lateralis muscle biopsy was obtained prior to the morning meal (d 49) and 4 h after consumption of their meal (d 56) to determine the protein content and phosphorylation of 5' adenosine monophosphate-activated protein kinase (AMPK). Diet did not affect body weight, resting EE or skeletal muscle AMPK phosphorylation. Dogs fed MH had significantly lower post-prandial RQ (p = 0.02) and ratio of fat to lean body mass (p = 0.02). Physical activity during light time periods (but not dark) was lower in dogs fed MH (p < 0.05) during weekends, but not on weekdays. These results suggest that MH affects energy balance of adult dogs, but that these effects are not dose dependent and not due to physical activity.

This study presents the first gaseous formic acid (HC(O)OH) concentration measurements collected over the Fenno-Scandinavian wetlands (67.9–68.0° N, 22.1–27.8° E) as part of the MAMM (Methane and other greenhouse gases in the Arctic-Measurements, process studies and Modelling) aircraft campaigns conducted in August and September 2013. A boundary layer box model approach has been used to calculate a regionally representative (~240 km2) surface flux for HC(O)OH of 0.0098 (±0.0057) mg[HCOOH]·m−2·h−1. A surface-type classification map was used to estimate proportional source contributions to the observed HC(O)OH flux over the measurement region. The removal of expected source contributions (using available literature parameterisations) from the calculated surface flux identified that 75% remained unaccounted for. This may suggest that HC(O)OH emission from wetland within the Fenno-Scandinavian region could contribute up to 29 times higher per unit area than previous theoretical HC(O)OH globally-averaged wetland estimates, highlighting a need for further constrained wetland studies of HC(O)OH emission to better understand its potentially significant impact on the Arctic HC(O)OH budget and consequent impacts on oxidative capacity.

The present study aimed to determine the effects of mannoheptulose (MH) (8 mg/kg) on energy expenditure (EE), respiratory quotient (RQ) and glycaemic response in healthy adult Beagle dogs (n 8; 9·62 (sem 0·31) kg; body condition score 4·5). The study was designed as replicated 4 × 4 Latin squares with a 2 × 2 factorial treatment structure. The dietary treatments were low carbohydrate (CHO) relative to fat diet (LC; 31 % CHO, 28 % fat) with placebo (0 mg/kg) or MH supplement and high CHO relative to fat diet (HC; 54 % CHO, 11 % fat) with placebo (0 mg/kg) or MH supplement. Dogs were fed to maintain body weight (HC and HC+MH 3625 (sem 295) kJ and LC and LC+MH 3542 (sem 284) kJ). Resting and postprandial (0–4 h; 5–10 h; 11–17 h; 18–23 h) EE and RQ were determined by indirect calorimetry (days 12 or 14). Glycaemic response to a meal (24 h) and plasma MH concentrations were determined on days 12 or 14. Plasma MH followed first-order kinetics, confirming that MH is absorbed and available to the animal. In the presence of high dietary CHO, MH increased postprandial EE (5–10 h only), suggesting MH increased dietary induced thermogenesis. In contrast to earlier reports, MH did not affect serum glucose or insulin in the present study. Irrespective of MH, dogs adapted RQ to diet composition and dogs consuming the LC diet had a greater incremental AUC for glucose, but not insulin, than dogs consuming the HC diet.

ABSTRACT: A reliable determination of endogenous phosphorus (P) excretion is required to measure P utilization in chickens accurately. The objective of this study was to investigate phosphorus (P) retention in broilers fed diets formulated with different feed ingredients. Sixty-four 15-d old broiler chicks were fed diets in which part of the dietary P was provided from dicalcium phosphate, maize, barley or soybean. Level of supplementation of each ingredient was calculated to provide two levels of total P (4.5 or 5.0 g kg−1 feed). Birds received a single injection of 3 MBq of 32P to determine endogenous P excretion using the isotope dilution technique. Four days after injection, blood and excreta were collected for analysis of inorganic and radioactive P. There were no differences among diets in total (p = 0.37) or endogenous (p = 0.65) P excretion or in P retention (p = 0.37) regardless of the supplemental feed material used in each diet. Daily P retention was increased (p = 0.004) as P intake increased, but the proportion of P ingested that was retained was not affected (p = 0.23) by the level of dietary P. The use of an isotopic tracer allows for accurate estimation of endogenous P in excreta, ranging from 0.24 to 0.42 mg P g−1 dry matter intake. The retention of P in growing chickens was not changed when 10-20 % of total P was provided by maize, barley or soybean.