Effects of Three Twice-A-Year Breeding Schedules in Four Breeds of Sheep

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Title: Effects of Three Twice-A-Year Breeding Schedules in Four Breeds of Sheep
Author: Pala, Akin
Advisors: Q.W. Robison, Co-Chair
R.L. McCraw, Co-Chair
C.S.Whisnant, Member
R.E. McDowell, Member
Abstract: Straightbred populations of Dorset, Finnsheep, Composite I (50% Finnsheep, 25% Dorset, and 25% Rambouillet), and Composite II (50% Finnsheep, 25% Suffolk, and 25% Targhee) sheep were evaluated under three different twice-a-year breeding schedules. Ewes were exposed for 32 d starting on August 13 and February 5 for schedule I, on September 15 and March 10 for schedule II, and on October 22 and April 11 for schedule III. Approximately 100 ewes of each breed were exposed to rams during each breeding season. The experiment was conducted for five complete cycles starting with matings in the fall of 1984 and ending with matings in the spring of 1989. Data were obtained on 9419 lambs produced from 2334 ewes and 257 rams. Traits of primary interest were conception rate, litter size at birth, weaning weight, weaning weight adjusted for conception rate, litter weaning weight and litter weaning weight per ewe exposed. Lactation status had a significant effect on conception rate and litter size while number of lambs suckling did not have a significant effect on conception rate. Composite I ewes had the highest conception rates and litter size (P 0.10), both had larger litters (P < 0.01) than the other two breeds. Ewes giving birth in spring had the highest conception rate and heaviest lambs (P < 0.01). Animals bred in schedule I had the heaviest and largest litters, highest conception rate, heaviest lambs and highest litter weaning weight per ewe exposed (P < 0.01). Efficiency of twice-a-year lambing systems is greatly affected by breeds, breeding schedules and seasons. Twice-a-year lambing programs must use the correct breeding schedule. Further, using composite breeds can be of great benefit.Genetic parameters for conception rate, litter size at birth, weaning weight, weaning weight adjusted for conception rate, litter weaning weight and litter weaning weight per ewe exposed were estimated using REML with animal models. Heritability estimates for conception rate were adjusted to a normal scale. Standard errors of heritabilities for conception rate were calculated using three methods, including bootstrapping. Heritabilities were estimated overall and within breed. Estimates of heritability for conception rate ranged from 0.17 &#177; 0.01 (Dorset) to 0.27 &#177; 0.01 (Composite I). Heritability estimates for litter size were 0.08 &#177; 0.01, 0.19 &#177; 0.01, 0.14 &#177; 0.01 and 0.13 &#177; 0.01 for Dorset, Finnsheep, Composite I and Composite II, respectively. Heritabilities for litter weaning weight and litter weaning weight per ewe exposed were similar across breeds and ranged from 0.31 &#177; 0.01 to 0.36 &#177; 0.01. Heritability for weaning weight was higher for Dorset (0.65 &#177; 0.01) than for Composite I (0.57 &#177; 0.01). Finnsheep and Composite II had similar heritabilities (0.41 &#177; 0.01). Overall heritabilities for litter weaning weight, litter weaning weight per ewe exposed, weaning weight, weaning weight adjusted for conception rate, conception rate and litter size were 0.33 &#177; 0.02, 0.35 &#177; 0.01, 0.64 &#177; 0.01, 0.64 &#177; 0.01, 0.24 &#177; 0.01 and 0.16 &#177; 0.01, respectively. Overall Spearman rank-order correlations of litter weight traits with conception rate or litter size ranged from 0.81 to 0.88. Correlations within breed were generally high and positive. Genetic correlations between dry and lactating ewes for conception rate and litter size were small (0.009 and 0.108, respectively), indicating that rank of sires was inconsistent under different environments (lactation status). Selection should be practiced among lactating animals for conception rate and litter size in twice-a-year lambing systems.Heritability estimates were moderate to high for weight traits and low to moderate for reproduction traits, indicating that selection in twice-a-year lambing programs is feasible. There was no substantial evidence that heritabilities and rank correlations were different among the four breeds. Litter weaning weight may be increased along with conception rate and litter size in a selection program based on twice-a-year lambing. Small genetic correlation between dry and lactating ewes indicated that grouping sires according to the lactation status of the ewes is necessary in a selection program.
Date: 2002-02-12
Degree: PhD
Discipline: Animal Science
URI: http://www.lib.ncsu.edu/resolver/1840.16/3382


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