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Direct and Maternal Genetic Effects for Ascites-Related Traits in Broilers

by 5m Editor
4 September 2006, at 12:00am

By A. Pakdel, J. A. M. Van Arendonk and H. Bovenhuis, Wageningen Institute of Animal Sciences and A. L. J. Vereijken, Nutreco Breeding Research Center - The objective of the present study was to estimate heritabilities for ascites-related traits in broilers and to assess the importance of maternal genetic effects for these traits. Several traits related to ascites were measured on more than 4,000 broilers kept under cold conditions.

Hybro

Heritabilities were estimated using an animal model with a direct genetic effect and a model with direct and maternal genetic effects. Estimated heritabilities from the direct genetic effects model were 0.46 for hematocrit value, 0.42 for BW, 0.47 for right ventricular weight, 0.46 for total ventricular weight, 0.45 for ratio of right ventricu-lar weight to the total ventricular weight, 0.32 for total mortality, and 0.18 for fluid accumulation in the heart sac.

Maternal effects significantly influenced the traits BW, total ventricular weight, and total mortality. Direct and maternal heritabilities, respectively, forBWwere 0.21 and 0.04, for total ventricular weights were 0.29 and 0.03, and for total mortality were 0.16 and 0.05. The heritability estimates for ascites-related traits and the significance of maternal genetic effects for most of these traits indicate that direct and maternal genetic effects play an important role in the development of the ascites syndrome.

Introduction

In all major poultry-producing countries, cases of ascites in broilers have been reported. Ascites syndrome, or water belly, is an increase in the amount of lymph normally found in the peritoneal space (Julian, 1993). This syndrome is a serious economic concern because it results in a loss of broilers (Shapiro, 1993) and has a negative impact on animal welfare. In a survey (Maxwell and Robertson, 1997), information on 18 countries from four continents showed that ascites affects 4.7% of all live broilers worldwide.

Genetically, the modern broiler seems to be more prone to develop ascites, which is probably due to selection for growth rate or feed conversion ratio, which puts high demands on metabolic processes and on the oxygen demand (Decuypere et al., 2000). To alter the metabolic load placed on the birds, the producer can reduce the growth rate, change the diet, or change other aspects of the management system, e.g., by raising the temperature (e.g., Bendheim et al., 1992).

Alternatively it might be possible to select for birds that can maintain a high rate of growth without succumbing to the penalties imposed on their health and, consequently, on their welfare. This requires alternative selection strategies, traits that indicate the sus- ceptibility of birds to ascites, and genetic parameters for these traits. Several studies have suggested traits that can be used as indicator traits for ascites. Lubritz et al. (1995) clearly demonstrated that ascites is strongly related to the ratio of right ventricular weight (RV) to the total ventricular weight (TV).

Shlosberg et al. (1996) showed that broilers with high hematocrit values have an increased chance of developing ascites when exposed to cold temperatures and suggested that hematocrit values might be a useful selection tool. Maxwell et al. (1998) indicated that in the presence of ascites, plasma troponin T, an indicator of heart muscle damage, is heritable. Moghadam et al. (2001) showed that heart defects, e.g., pulmonary hypertension, right ventricular failure, and fluid accumulation in the peritoneal cavity, are heritable and have a positive genetic correlation with BW. De Greef et al. (2001) estimated genetic parameters for a number of ascites-related traits. However, they also demonstrated that genetic parameters varied considerably with the severity of the disease.


The normal temperature schedule (----) and the coldstressed temperature schedule (——).
Koerhuis and Tompson (1997) reported maternal genetic effects on juvenile broiler BW. Other studies have shown positive phenotypic effects of egg weight on juve- nile broiler BW (e.g., Chambers, 1990). Further, Dewil et al. (1996) demonstrated that selection for ascites resistance is linked to several physiological variables at the embryonic stage. These results suggest that maternal genetic effects might play a role in the susceptibility of birds to ascites. To our knowledge no studies have actually established the significance of maternal genetic effects on ascites-related traits. The objective of the current study was to estimate heritabilities for ascites-related traits and to assess the importance of maternal genetic effects for these traits in broilers.

Materials and Methods

Birds and Traits
Birds. Ascites-related traits were recorded for 4,202 chickens, 1,736 females and 2,466 males. The experimental population was the result of a cross between two genetically different outcross broiler dam lines (Hybro) originating from the White Plymouth Rock breed. The maternal line had a relatively high reproductive performance and was fast-feathering, and the paternal line had a relatively high growth performance and was slow-feathering. The total pedigree file consisted of 5,096 birds of which 36 were pure-line birds (F0), 29 were F1 birds, 829 were F2 birds, and 4,202 were F3 birds. Observations were on F3 birds. A more detailed description of the experimental set up was given by Van Kaam et al. (1998).

The experimental birds were hatched at 6 different wk in 1994 and 1995. Broilers were kept in four different pens; however, most were kept in one pen. Nine batches were allotted by hatching day and pen number. In order to identify individuals that were susceptible to ascites, a cold stress temperature schedule was applied (Figure 1). At the time of hatching, the temperature was 30 C and then was gradually decreased to 10 C by 22 d of age. The temperature remained at 10 C until the end of the experiment. A normal temperature schedule starts at 33 to 34 C and then gradually decreases to 17 to 18 C by 35 d of age. Except for the adjusted temperature schedule, birds were kept under circumstances that closely resemble commercial practice, i.e., a standard commercial feed, artificially lighted housing for 23 h/d, and group housing with 20 birds/m2.

Traits. The hematocrit value (HCT) and BW of birds were measured 1 d before slaughtering at 5 wk of age. After birds were slaughtered, several ascites-related traits were measured. Livers and hearts were removed and visually inspected. Liver abnormalities (LIVER) were scored as follows: 0 represented no abnormalities observed, 1 represented an abnormal liver, and 2 represented serious liver abnormalities. Liver abnormalities were a lighter color, an irregular liver surface, or both.

Accumulation of fluid in the heart sac (HEART) was scored with 0 if no fluid had accumulated, 1 if fluid accumulation was observed, and 2 if there was serious accumulation of fluid in the heart sac. Further, the weights of RV and TV were measured. From these measurements, the RV:TV, RV as a percentage of total BW (% RV), and TV as percentage of total BW (%TV) were derived. The accumulation of fluid in the abdomen (ABDOMEN) was scored as 0, 1, or 2. A score of 0 indicated no fluid, 1 indicated the presence of fluid in the abdomen, and 2 indicated a serious accumulation of fluid in this section. For color of the breast (BREAST), a score of 0 represented normal color, 1 represented a color deviation, and 2 represented serious color deviation. In general, a deviation of breast color meant that the color was more red.

Also, the total mortality (MORT-TOT) of the birds was recorded as 0 or 1. A score of 0 represented a bird that was alive at the end of the experiment, and a score of 1 indicated a bird that died before the end of the experiment. For the first three batches of birds, mortality was not recorded. No observations were recorded for other traits of birds died before the end of experiment. In addition, information from 795 birds kept under a normal temperature schedule (Figure 1) was available.

These birds originated from the same F2 parents and, except for temperature, were kept under similar conditions as the birds kept under cold conditions. These birds were slaughtered between 6 and 7 wk, and the measurements for BW, HCT, RV, TV, and RV:TV were available. These birds were previously used in an experiment by Van Kaam et al. (1998). The data was only used to describe the mean and the distribution of traits under a normal temperature schedule and was not included in genetic analyses.

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August 2006