High natural antibody titers of indigenous chickens are related with increased hazard in confinement.

Natural antibody (NAb) levels and survival rates were evaluated in 4 breeds of laying hens in Ethiopia: indigenous, improved indigenous, exotic layer, and crossbred. Titers of NAb isotypes IgG and IgM binding keyhole limpet hemocyanin (KLH) in serum were measured at 20, 26, 35, and 45 wk age. Repeated-measure ANOVA showed that IgG and IgM levels vary with time within each breed (P < 0.05). Indigenous chickens had significantly (P < 0.05) higher NAb levels at all ages. The Cox proportional hazard analysis showed increased hazard with increased levels of NAbs in the exotic layers (P < 0.05). However, the reduced hazards with increased levels of NAbs were not significant in the improved indigenous and crossbred chickens. Indigenous chickens showed increased hazard with increasing levels of NAb (P > 0.05). We concluded that not only the NAb levels but also the effect of Nabs on survival vary between indigenous and improved breeds. The results indicate that NAb levels are associated with survival in elite (improved) breeds, but are associated with increased hazard in indigenous chickens.

Effect of gestating sow body condition, feed refusals, and group housing on growth and feed intake in grower-finishing pigs.

The main focus of this study was to identify sow gestation features that affect growth rate (GR) and feed intake (FI) of their offspring during grower-finishing stage. Because the sow provides a specific environment to her offspring during gestation, certain features (e.g., BW of the sow), feed refusals or gestation group, may affect her ability to deliver and feed a healthy litter. Data on 17,743 grower-finishing pigs, coming from 604 sires and 681 crossbred sows, were obtained from the Institute for Pigs Genetics. Sow gestation features were collected during multiple gestations and divided into 3 clusters describing i) sow body condition (i.e., BW, backfat, and gestation length), ii) sow feed refusals (FR), the difference between offered and eaten feed during 3 periods of gestation: 1 to 28, 25 to 50, 45 to 80 d, and iii) sow group features (i.e., number of sows, and average parity). Sow gestation features were added to the base model 1 at a time to study their effect on GR and FI. Significant gestation features (P < 0.1) were fitted simultaneously in animal model to investigate whether they could explain common litter and permanent sow effects. Gestation length had effect on GR [1.4 (g/d)/d; P = 0.04] and FI [6.8 (g/d)/d; P = 0.007]. Body weights of the sow at insemination [0.07 (g/d)/kg; P = 0.08], at farrowing [0.14 (g/d)/kg; P < 0.0001], and after lactation [0.1 (g/d)/kg; P = 0.003] had effect on GR. Sow parturition-lactation loss in backfat thickness and weight were not significant for GR and FI. Days with FR during 25 to 50 and 45 to 80 d of gestation and average FR during 45 to 80 d of gestation had negative effect on GR and when substantially increased had also a positive effect on FI. Sow FR from 1 to 28 d of gestation were not significant. Number of sows in gestation group had effect on FI [-9 (g/d)/group member; P = 0.04] and day sow entered group had an effect on GR [-0.9 (g/d)/day; P = 0.04]. Sow gestation features explained 1 to 3% of the total variance in grower-finishing pigs. Gestation features did explain phenotypic variance due to permanent sow and part of phenotypic variance due to common litter effects for FI but not for GR.

Effect of daily environmental temperature on farrowing rate and total born in dam line sows.

Heat stress is known to adversely affect reproductive performance of sows. However, it is important to know on which days or periods during the reproduction cycle heat stress has the greatest effects for designing appropriate genetic or management strategies. Therefore, this study was conducted to identify days and periods that have greatest effects on farrowing rate and total born of sows using 5 different measures of heat stress. The data consisted of 22,750 records on 5024 Dutch Yorkshire dam line sows from 16 farms in Spain and Portugal. Heat stress on a given day was measured in terms of maximum temperature, diurnal temperature range and heat load. The heat load was estimated using 3 definitions considering different upper critical temperatures. Identification of days during the reproduction cycle that had maximum effect was based on the Pearson correlation between the heat stress variable and the reproduction trait, estimated for each day during the reproduction cycle. Polynomial functions were fitted to describe the trends of these correlations and the days with greatest negative correlation were considered as days with maximum effect. Correlations were greatest for maximum temperature, followed by those for heat load and diurnal temperature range. Correlations for both farrowing rate and total born were stronger in gilts than in sows. This implies that heat stress has a stronger effect on reproductive performance of gilts than of sows. Heat stress during the third week (21 to 14 d) before first insemination had largest effect on farrowing rate. Heat stress during the period between 7 d before successful insemination until 12 d after that had largest effect on total born. Correlations between temperatures on consecutive days during these periods were extremely high ( > 0.9). Therefore, for farrowing rate the maximum temperature on 21 d before first insemination and for total born the maximum temperature at day of successful insemination can be used as predictive measures of heat stress in commercial sow farms. Additionally, differences between daughter groups of sires were identified in response to high temperatures. This might indicate possibilities for genetic selection on heat tolerance.

Effect of match or mismatch of maternal-offspring nutritional environment on the development of offspring in broiler chickens.

In mammals, maternal food restriction around conception and during pregnancy results in low birth weight and an adjusted growth trajectory of offspring. If, subsequently, the offspring are born into a food-abundant environment, they are at increased risk of developing obesity, type 2 diabetes, hypertension and renal dysfunction. Here, we show similar effects of maternal undernutrition on hatch weight, growth and fat deposition in offspring of birds (domestic chicken). Both mothers and offspring were fed either ad libitum or restricted in a two-by-two factorial design, resulting in two matched and two mismatched maternal-offspring nutritional environments. Offspring of ad libitum mothers grew heavier than those of restricted mothers, possibly due to the larger muscle mass. Ad libitum-fed offspring, especially females, of restricted mothers were lighter at hatch, and were heavier and had more abdominal fat at 6 weeks of age than daughters of ad libitum-fed mothers. These results suggest a common mechanism in mammals and birds in response to a mismatch in the maternal-offspring nutritional environment. They also indicate that the common practice of restrictive feeding of the broiler breeders and subsequent ad libitum feeding of the broilers may result in reduced growth and increased abdominal fat as compared to broilers of less restricted broiler breeders.

Effect of excessive, hormonally induced intrauterine crowding in the gilt on fetal development on day 40 of pregnancy.

Selection for litter size may result in an increase in uterine crowding due to a faster increase in ovulation rate than in litter size. Increased ovulation rate does not result in a proportionally increased number of piglets born alive. In this study, the effect of ovulation rate on vitality characteristics of fetal-placental units at d 40 of pregnancy was investigated. For this, 43 Large White gilts were treated with hormones to induce superovulation. Average ovulation rate was 45.16 +/- 13.22; average number of vital fetuses at d 40 of pregnancy was 17.09 +/- 3.61 that weighed 11.26 +/- 1.99 g; their placenta weighed 31.88 +/- 14.79 g; and they occupied 11.69 +/- 4.90 cm of the uterus. Loss in oocytes (i.e., that did not result in a vital fetus at d 40) increased with increasing ovulation rate and occurred before (early mortality; P = 0.0003) and after implantation (late mortality, i.e., traces visible at d 40; P < 0.0001). With respect to the vital fetuses, increased ovulation rate resulted in decreased fetal (P = 0.0008) and placental weight (P = 0.0008) and decreased length of the area in the uterus that was occupied by the placenta (P = 0.0011). Strong correlations existed between placental and fetal weight [0.68; 95% confidence interval (CI) = 0.64 to 0.72], and placental weight and length (0.78; 95% CI = 0.74 to 0.82). Fetal-placental characteristics were weakly correlated to distance to the implantation sites of neighboring fetuses, a measure of crowdedness [-0.002 (95% CI = -0.042 to 0.038) with fetal weight to 0.16 (95% CI = 0.12 to 0.20) with placental length]. Increased ovulation rates, but more specifically increased late mortality rates, have negative effects on the remaining vital fetuses with respect to the fetal (P = 0.0085) and placental weight (P < 0.0001) and length of the implantation site (P = 0.0016). The most extreme effect was on placental weight, in which a uterus with <10 cases of late mortality was on average 25% greater than placental weight in a uterus with >18 cases of late mortality (P < 0.0001). Furthermore, increased ovulation rates resulted in decreased within litter variation for fetal (P = 0.0018) and placental weight (P = 0.0084). At increased ovulation rates, the number of live fetuses remained similar, but placental development is impaired and the growth of the fetus is retarded compared with reduced ovulation rate, with effects likely lasting into adult life.

Sow line differences in heat stress tolerance expressed in reproductive performance traits.

The objectives of this study were 1) to investigate if there were differences in the relation between temperature and reproductive performance traits in 2 different sow lines, a Yorkshire line producing mainly in temperate climates and a Large White line producing mainly in warm climates, and 2) to determine the upper critical temperature (UCT) for the reproductive performance of these 2 lines. Sows are exposed to heat stress when temperature exceeds the UCT of the thermo-neutral zone. Data included 32,631 observations on reproductive performance from 11,935 sows on 20 farms in Spain, collected from 2003 to 2005. Sows belonged to 2 different purebred sow lines, named D (Yorkshire sow line, producing mainly in temperate climates) and I (Large White sow line, producing mainly in warm climates). Only first insemination records per parity were used and were combined with the maximum outside temperature at day of insemination. Upper critical temperatures were studied for 3 reproduction traits: farrowing rate (0 or 1), litter size (range from 1 to 25), and total number of piglets born per first insemination (combination of farrowing rate and litter size, range from 0 to 25). Data were corrected for fixed effects, which included parity, service sire, and an interaction between farm and year. Corrected data were used as observations in the models to study the effect of outside temperature on reproductive performance. Two models were compared for goodness of fit: a linear regression model and a plateau-linear model with the plateau representing the thermo-neutral zone and a linear decrease above that zone. Farrowing rate of I-line sows was not affected by temperature. For litter size and total number born per first insemination of I-line sows no UCT could be estimated. These traits were linearly affected by temperature. For all 3 reproduction traits of the D-line the best model was the plateau-linear model; the UCT for the D-line sows was estimated to be 19.2 degrees C for farrowing rate, 21.7 degrees C for litter size, and 19.6 degrees C for total number born per first insemination. The decrease in reproductive performance of I-line sows with increasing outside temperature was less than in D-line sows. From this study it can be concluded that there are differences in heat stress tolerance between sow lines as measured by the differences in reproductive performance. These differences are an indication of genetic differences in heat stress tolerance in sow lines.

Modeling variability in immunocompetence and immunoresponsiveness.

The purposes of this paper were to 1) develop a stochastic model that would reflect observed variation between animals and across ages in immunocompetence and responsiveness; and 2) illustrate consequences of this variability for the statistical power of genotype comparisons and selection. A stochastic model of immunocompetence development and responsiveness kinetics was developed. This model enabled variability in immunological variables to be taken into account in the evaluation of challenge and measurement strategies for selection. The characteristics of the variation in model output reflect those observed in the literature, to the extent that variation in the literature shows a consistent pattern; knowledge of true variation and patterns of variation in immunological variables is limited. The model created correlations between immunocompetence and immunoresponsiveness components, as well as correlations within each component across time. These correlations were generally in agreement with literature estimates, where available. The model enabled predictions of the effectiveness of selection for improved health through immunocompetence or immunoresponsiveness. It was predicted that effective selection for increased general immunocompetence to improve health should be done only when baseline immunity has matured. Further, the model implied that selection is unlikely to be successful if it is based only on a single measurement. Problems with low statistical power to detect differences between genotypes can be reduced by increasing challenge age in the experimental design, and one should ensure that the effects of maternal immunity are minimal when the challenge is done. The ability to detect differences between different groups of animals differs substantially with measurement timing because of low repeatabilities of immunocompetence and responsiveness across time. In general, the probability of detecting differences becomes higher when the challenge age is increased. Consequently, both the age at selection and the age at which information is gathered for selection must be considered carefully when designing genetic evaluations.

Genetic analysis of results of a Swedish behavior test on German Shepherd Dogs and Labrador Retrievers.

The objectives of this study were to estimate genetic parameters and the influence of systematic effects on behavior test results in dogs. Behavior test results on 1,813 Labrador Retrievers (LR) and 2,757 German Shepherd Dogs (GSD) were analyzed. The behavior test included observations on courage, defense drive, prey drive, nerve stability, temperament, cooperation, affability, and gun shyness. Sex and age influenced most of the traits, and seasons of birth and testing and litter size and composition influenced some of the traits. Apart from defense drive in GSD, and courage, nerve stability, hardness, and affability in LR, all traits were heritable, with heritabilities ranging from 0.14 for hardness to 0.38 for affability in GSD, and from 0.03 for affability to 0.56 for gun shyness in LR. Genetic correlations ranged from 1.00 (LR) and 0.95 (GSD) between courage and hardness to -0.01 (LR) and -0.03 (GSD) between gun shyness and defense drive. Most genetic correlations were positive. Correlations with cooperation were mainly negative, especially in GSD. Genetic correlations between courage and defense drive in LR (0.26) and GSD (0.80), between courage and prey drive in LR (0.27) and GSD (0.65), between affability and nerve stability in LR (0.09) and GSD (0.64), between affability and temperament in LR (-0.24) and GSD (0.39), and between cooperation and hardness in LR (0.28) and GSD (-0.67) were significantly different between the breeds. Genetic parameters for defense drive and cooperation in GSD and hardness and gun shyness in LR were genetically different between the sexes. Results of this study indicate that correction for systematic effects is essential when making selection decisions. Estimating breeding values would be a good solution, incorporating both correction for systematic effects and using all genetic links. Genetic parameters need to be estimated for each breed separately.

Modeling immunocompetence development and immunoresponsiveness to challenge in chicks.

The purpose of this study was 2-fold: 1) to develop a deterministic model that describes the development of immunocompetence and the kinetics of immunoresponsiveness to a pathogenic challenge in chicks and 2) to use this model to illustrate the importance of factors in experimental design, such as type of variable measured, measurement timing, and challenge age. Difficulties in evaluating immunological variables hinder attempts to improve animal health through selection on immunological variables. In young chicks, evaluating immunological variables is additionally complicated by immune system development and maternal immunity. The evaluation of immunocompetence and immunoresponsiveness and the definition of appropriate challenge and measurement strategies may be enabled through a mathematical model that captures the key components of the immune system and its development. Therefore, a model was developed that describes the development of immunocompetence as well as the kinetics of immunoresponsiveness to a pathogenic extracellular bacterial challenge in an individual chick from 0 to 56 d of age. The model consisted of 4 components describing immunocompetence (maternal and baseline immunity) and immunoresponsiveness (acute phase and antibody response). Individual component equations generally fit published data adequately. Four scenarios that represented combinations of challenge age and measurement timing were simulated. In each scenario, the immunoresponsiveness to a particular challenge was compared for 3 different levels of baseline immunity, representing 3 broiler genotypes. It was illustrated that experimental design (type of immunoresponsiveness measured, measurement timing, and challenge age) can have an important effect on the ranking of genotypes, groups, or individuals and on the reliability of extrapolations based on this ranking. It is concluded that this model is a potentially useful tool in the definition of appropriate challenge and measurement strategies when evaluating immunocompetence and immunoresponsiveness. Further, it may be used as a generator of hypotheses on global immunological relationships to be tested experimentally.

Role of thyroid hormones, maternal antibodies, and antibody response in the susceptibility to colibacillosis of broiler genotypes.

The purpose of this study was to investigate whether differences in susceptibility to colibacillosis are associated with maternal antibodies, antibody response, and alterations in thyroid hormones [triiodothyronine (T3) and thyroxine (T4)] and to investigate the effect of genotype on the changes in T3 and T4 during challenge and antibody response. A challenge experiment was executed in 2 trials. Per trial, 24 chicks per genotype were challenged, and 20 chicks per genotype were controls. At 7 d of age, challenged chicks were intratracheally inoculated with 0.3 mL of Escherichia coli O78K80 and controls with 0.3 mL of PBS. All chicks were euthanized at 14 or 15 d. Thyroid hormone plasma concentrations and E. coli-specific antibody titers (AB) were measured at 7 d (T(3 d7), T(4 d7), and AB(d7)) and 14 or 15 d (change from 7 to 14 or 15 d was analyzed: DeltaT(3), DeltaT(4), and DeltaAB). Susceptibility was defined based on mortality, lesions, growth retardation, and eating behavior. There was a significant effect of challenge on T(3 d7); probably due to eating pattern in association with circadian rhythm. The challenge group was suggested to have functional hypothyroidism relative to the control group, indicating metabolic changes due to the challenge, and it was indicated that an antibody response was elicited. Differences in susceptibility were not significantly related to differences in T(3 d7), T(4 d7), DeltaT(3), or DeltaT(4) or to maternal antibodies (AB(d7)), but the antibody response tended to increase (decreasing DeltaAB) with increasing susceptibility. There were indications of genetic variation in T(4 d7), DeltaT(4), AB(d7), and DeltaAB, but there was no observed effect of genotype on DeltaT(3) and DeltaT(4) during challenge or on the antibody response. Further, there were indications that selection for growth traits has resulted in alterations in DeltaT(4) due to challenge, as indicated by a lower DeltaT(4) in the challenge group relative to the control group for more intensively selected genotypes as opposed to a higher DeltaT(4) for less intensively selected genotypes.

Defining susceptibility of broiler chicks to colibacillosis.

This study aimed to define the susceptibility of broilers to colibacillosis through quantification of clinical responses and to examine the relationship between susceptibility and growth retardation. A challenge experiment was carried out twice. In each trial, 192 chicks were challenged intratracheally with Escherichia coli (E. coli) at 7 days of age and 160 chicks served as controls. Surviving chicks were euthanized at 14 or 15 days. Parameters measured were: daily mortality, lesion scores, body weight at 1, 4, 7, 10, 12 and 14 or 15 days and feeding behaviour at 6, 11 and 13 days. The results were reproducible, and increasing susceptibility to colibacillosis was defined by four categories: chicks without lesions, chicks with airsacculitis but no systemic lesions, chicks with systemic lesions, and chicks that die. Increasing susceptibility was associated with increasing growth retardation, but growth retardation was not inevitably linked to challenge with E. coli.

Genetic variation among broiler genotypes in susceptibility to colibacillosis.

Selection for reduced susceptibility to colibacillosis in broilers may contribute to the prevention of colibacillosis. Such selection should focus on the responses to Escherichia coli rather than the associated primary agent(s). The purpose of the current study was to examine whether genetic variation is present in the susceptibility to colibacillosis. This was achieved through an evaluation of the susceptibility to primary colibacillosis in 5 pure broiler lines, a slow-growing line, and two 2-way crosses of the pure lines (altogether referred to as genotypes). A challenge experiment was executed in 2 trials. Per trial, 24 chicks per genotype were challenged and 20 chicks per genotype were controls. At 7 d of age, challenged chicks were intratracheally inoculated with 0.3 mL of E. coli O78K80 solution, and controls with 0.3 mL of PBS. All chicks were euthanized at 14 or 15 d. Traits measured were mortality, lesion scores (airsacculitis, pericarditis, and perihepatitis) at 14 or 15 d, and BW at 1, 4, 7, 10, 12, and 14 or 15 d. An effect of genotype on mortality, lesion prevalence, and growth retardation was found, indicating the presence of genetic variation in susceptibility to colibacillosis, and suggesting that selection for reduced susceptibility is possible. There were large between-genotype differences in mortality (up to 46%) and in lesion prevalence (up to 41%). Growth retardation was not observed for any genotype in chicks without lesions, whereas genotypes differed from none to 20% growth retardation for chicks with airsacculitis but no systemic lesions, and up to 13% for chicks with systemic lesions. The heterosis in susceptibility and growth retardation was found to be either negative or absent, indicating that crossbreeding would not be an advantage for the selection for reduced susceptibility, and that test crossing is essential.

A bivariate mixture model analysis of body weight and ascites traits in broilers.

The objective of the present study was to use bivariate mixture models to study the relationships between body weight (BW) and ascites indicator traits. Existing data were used from an experiment in which birds were housed in 2 groups under different climate conditions. In the first group, BW, the ratio of right ventricular weight to total ventricular weight (RV:TV), and hematocrit value (HCT) were measured in 4,202 broilers under cold conditions; in the second group, the same traits were measured in 795 birds under normal temperature conditions. Cold-stress conditions were applied to identify individuals that were susceptible to ascites. The RV:TV and HCT were approximately normally distributed under normal temperature conditions, whereas the distributions of these traits were skewed under cold temperature conditions, suggesting different underlying distributions. Fitting a bivariate mixture model to the observations showed that there was only one homogeneous population for ascites traits under normal temperature conditions, whereas there was a mixture of (2) distributions under cold conditions. One distribution contained nonascitic birds and the other distribution contained ascitic birds. In the distribution of nonascitic birds, the inferred phenotypic correlations (phenotypic correlations with 2 distinguishing underlying distributions) of BW with RV:TV and HCT were close to zero (0.10 and -0.07, respectively), whereas in the distribution of ascitic birds, the inferred phenotypic correlations of BW with RV:TV and HCT were negative (-0.39 and -0.4, respectively). The negative inferred correlations of BW with RV:TV and HCT in the distribution of ascitic birds resulted in negative overall correlations (correlations without 2 distinguishing distributions) of BW with RV:TV (-0.30) and HCT (-0.37) under cold conditions. The present results indicate that the overall correlations between BW and ascites traits are dependent on the relative frequency of ascitic and nonascitic birds in the population.

Possible negative effects of inbreeding on semen quality in Shetland pony stallions.

Inbreeding is widely believed to negatively affect reproductive performance. Indeed, in some species, high levels of inbreeding are thought to be the major cause of poor semen quality. It is, however, not clear whether inbreeding affects fertility in horses. In this study, the relationship between inbreeding and semen quality was examined in 285 immature Shetland pony stallions submitted for breeding soundness examination in March-April of the years 1992-1997. The majority of stallions examined were 3 years old (85%) and their coefficients of inbreeding ranged from 0 to 25% (mean+/-S.D.: 3+/-4.6%). For the purpose of analysis, stallions were divided into six inbreeding classes (0-1, 1-2, 2-5, 5-8, 8-12 and >12%) containing 132, 40, 42, 27, 25 and 19 animals, respectively. The degree of inbreeding significantly affected many aspects of sperm production and quality, based on a standard examination of two ejaculates collected at a 1.5-3h interval. In particular, coefficients of inbreeding above 2% were associated with lower percentages of motile (p<0.01) and morphologically normal sperm (p<0.001). When the data set was used to estimate heritability of semen characteristics, the high values calculated for sperm progressive motility (0.46) and concentration (0.24) suggested that these traits could be improved by phenotypic selection. These findings support the hypothesis that inbreeding has a detrimental effect on semen quality in Shetland ponies, although examination of multiple ejaculates after repeated semen collection to bring the animals to daily sperm output is needed to confirm this conclusion. Nevertheless, the results support previous suggestions that inbreeding is an important cause of reduced semen quality.

Broiler breeding strategies using indirect carcass measurements.

The objective of the current study was to determine the consequences of using indirect carcass measurements on the genetic response and rate of inbreeding in broiler breeding programs. In the base breeding scheme, selection candidates were evaluated based on direct carcass measurements on relatives. The possibilities of using indirect carcass measurements were investigated in alternative breeding schemes. Three alternative schemes, including indirect and own performance information for carcass traits on selection candidates, were evaluated by deterministic simulation. In the first scheme, indirect carcass traits were measured on male selection candidates. In the second scheme, indirect carcass traits were measured on male selection candidates, and direct carcass traits were measured on relatives. In the third scheme, indirect carcass traits were measured on male and female selection candidates, and direct carcass traits were measured on relatives. In the base scheme, the genetic response for breast muscle percentage (BMP) was 0.3%, and the rate of inbreeding was 0.96% per generation. In the third alternative scheme, the response for BMP increased by 66.2% compared with the base scheme, and the rate of inbreeding decreased to 0.79% per generation. The improved genetic gain resulted from increased accuracy of selection. The use of own performance information for selection candidates reduced the rate of inbreeding in alternative schemes, which is desirable for long-term selection. The accuracy of the indirect carcass measurements had consequences on the response for BMP and the rate of inbreeding. In most cases, an accuracy of 30% was sufficient to result in a higher gain for BMP and a lower rate of inbreeding as compared with the base scheme.

Genetic parameters for claw disorders in Dutch dairy cattle and correlations with conformation traits.

Impaired claw health is one of the major problems causing production loss and reduced animal welfare in dairy cattle. In response, the Dutch Animal Health Service (GD) Ltd. initiated this study, in which claws of lactating and near-term cows and heifers in 430 herds were trimmed by hoof trimmers and the health status of the rear claws recorded. Only herds with >75% of the animals having feet trimmed were considered, resulting in records on 21,611 animals. Eight claw disorders were scored: digital dermatitis (DD), interdigital dermatitis/heel horn erosions (IDHE), sole hemorrhage (SH), chronic laminitis (CL), sole ulcer (SU), white line disease (WLD), interdigital hyperplasia (HYP), and interdigital phlegmona (IP). The prevalence varied from 0.6% (IP) to 39.9% (SH). More than 70% of the animals had at least one claw disorder. Conformation traits and locomotion were recorded once during the animal's first lactation by trained classifiers of the Royal Dutch Cattle Syndicate and completely independent of the moment of claw trimming. Heritabilities were estimated using a sire model, and ranged from <0.01 (IP) to 0.10 (DD and HYP). Genetic correlations of incidences of claw disorders with locomotion were variable, ranging from 0.13 (SH) to -0.91 (CL). Genetic correlations with the rear leg conformation traits were lower, ranging from 0.04 (ID with rear leg side view) to -0.69 (IP with rear leg rear view).

Effect of age and housing system on genetic parameters for broiler carcass traits.

The effect of age and housing system on genetic parameters for BW and carcass traits was investigated. Traits were measured on broilers of different ages (48, 63, and 70 d). Birds in the 48 and 70 d groups were raised in group housing, whereas birds in the 63 d group were raised in the same housing up to 22 d and in individual cages between 22 and 63 d. Each group consisted of approximately 2,000 individuals from a single group of parents. Carcass, breast meat, abdominal fat, and back half were expressed as percentage of BW. The heritabilities of BW at 48, 63, and 70 d were 0.31, 0.26, and 0.19, respectively, and the heritabilities of back half percentage at 48, 63, and 70 d were 0.42, 0.38, and 0.21, respectively. For other carcass traits, heritabilities were in the same range in different age groups. A positive genetic correlation was found between BW and valuable parts of carcass (breast meat and back half) at 48 d; these relationships were negative at 70 d. The genetic correlation between BW and abdominal fat percentage at 70 d was higher than at 48 d. The increase in growth at 48 d was accompanied by increase in valuable parts; at 70 d it was accompanied by an increase in abdominal fat percentage. The genetic correlation of BW at 48 d between individual cage and group housing demonstrated a genotype by environment interaction for performance of birds, which has consequences for design of breeding schemes.

A resource allocation model describing consequences of artificial selection under metabolic stress.

Long-term selection on production results in increased environmental sensitivity. This often is expressed through decreased fertility and increased health problems. The phenomenon has been described in all common farm animal species. One theory is that potential resource intake is insufficient to express production potential. Additional resources are drawn away from fitness-related traits, such as fertility and health, to further increase observed production. In addition, resources for maintaining fitness depend on the demands by the environment. In a harsh environment, more resources are required for fitness-related traits than in an optimal environment. Literature results show that selection in an optimal environment will increase sensitivity to less optimal environments. The objectives of this paper were to increase understanding of the underlying mechanism behind the development of environmental sensitivity and to gain insight into correlated response(s) when selection is on observed production. A resource allocation model was defined where observed production depended on production potential, resource intake potential, and the allocation of resources to production or fitness, including maintenance, health, and reproduction. Penalties for reproductive performance and probability of survival were included when the proportion of resources assigned to fitness dropped below a certain, environment-related, threshold. Mass selection was practiced on observed production during 40 generations using stochastic simulation. Depending on the heritabilities of the underlying components and on the environment, selection on observed production resulted in a decrease in reproductive rate and in the development of environmental sensitivity when resource intake becomes limiting. Correlations of observed production with underlying components changed across generations, following a nonlinear pattern. The proposed model is simple, but increases the understanding of underlying mechanisms and consequences of selection for production when resources are limiting.

Using genetic markers for disease resistance to improve production under constant infection pressure.

Animals will show reduced production when exposed to a constant infection pressure unless they are fully resistant, the size of the reduction depending on the degree of resistance and the severity of infection. In this article, the use of QTL for disease resistance for improving productivity under constant infection pressure is investigated using stochastic simulation. A previously published model was used with two thresholds for resistance: a threshold below which production is not possible and a threshold above which production is not affected by the infection. Between thresholds, observed production under constant infection is a multiplicative function of underlying potential production and level of resistance. Some simplifications of reality were adopted in the model, such as no genetic correlation between potential production and resistance, the absence of influence of lack of resistance on reproductive capacity, and the availability of phenotypes in both sexes. Marker-assisted selection was incorporated by assuming a proportion of the genetic variance to be explained by the QTL, which thus is defined as a continuous trait. Phenotypes were available for production, not for resistance. The infection pressure may vary across time. Results were compared to mass selection on production under constant as well as intermittent infection pressure, where the infection pressure varied between but not within years. Selection started in a population with a very poor level of resistance. Incorporation of QTL information is valuable (i.e., the increase in observed production relative to mass selection) when a large proportion of the additive genetic variance is explained by the QTL (50% genetic variance explained) and when the heritability for resistance is low (h2R = 0.1). Under constant infection pressure, incorporating QTL information does not increase selection responses in observed production when the QTL effect explains less than 25% of the genetic variance. Under intermittent selection pressure, the use of QTL information gives a slightly greater increase in observed production in early generations, relative to mass selection on observed production, but still only when the QTL effect is large or the heritability for resistance is low. The additional advantage of incorporating QTL information is that use of (preventive) medical treatment is possible, or animals may be evaluated in uninfected environments.

Modeling selection for production traits under constant infection pressure.

This article presents a model describing the relationship between level of disease resistance and production under constant infection pressure. The model assumes that given a certain infection pressure, there is a threshold for resistance below which animals will stop producing, and that there is also a threshold for resistance above which animals produce at production potential. In between both thresholds animals will show a decrease in production, the size of decrease depending on the severity of infection and the level of resistance. The dynamic relationship between production and resistance when level of resistance changes, such as due to infection, is modeled both stochastically and deterministically. Selection started in a population with very poor level of resistance introduced in an environment with constant infection pressure. Mass selection on observed production was applied, which resulted in a nonlinear selection response for all three traits considered. When resistance is poor, selection for observed production results in increased level of resistance. With increasing level of resistance, selection response shifts to production potential and eventually selection for observed production is equivalent to selection for production potential. The rate at which resistance is improved depends on its heritability, the difference between both thresholds, and selection intensity. The model also revealed that when a zero correlation between resistance and production potential is assumed, the phenotypic correlation between resistance and observed production level increases for low levels of resistance and subsequently asymptotes to zero, whereas the phenotypic correlation between production potential and observed production asymptotes to 1.0. For most breeding schemes investigated, the deterministic model performed well in relation to the stochastic simulation results. Experimental results reported in literature support the model predictions.