Cassell B G
Virginia Polytechnic Institute, Blacksburg 24061-0315.
J Dairy Sci. 1994 Jul;77(7):2130-6. doi: 10.3168/jds.S0022-0302(94)77155-1.
Genetic evaluations for mean somatic cell scores for lactation enable producers to reduce increases in SCC that accompany genetic improvement for milk yield. Current progeny test designs will produce lower reliabilities for somatic cell score evaluations than for milk yield because of lower heritability. Genetic correlations of somatic cell scores between first and second lactation are lower than for milk yield. Thus, changes in evaluations of sires over time should be greater for somatic cell score than for milk yield. Absence of interaction of genotype and environment for somatic cell score indicates that the same bulls would be selected for use in herds with high and low somatic cell scores. Most evaluations will be between -.25 and .25, averaging .01 for Holsteins, which represents an increase of approximately 50% in geometric mean SCC and an 11% higher incidence of clinical mastitis for daughters of bulls with highest evaluations compared with daughters of bulls with lowest evaluations. Selection to reduce somatic cell score does not appear to be justified economically. Properly constructed indexes would reduce the projected increases in mastitis incidence by 20 to 25% with a 2% decline in genetic progress for milk yield. The greatest impact of genetic evaluations for somatic cell score would likely result from choice of sires of bulls to progeny test.
对泌乳期平均体细胞评分进行遗传评估，可使养殖者减少伴随产奶量遗传改良而来的体细胞数（SCC）增加。由于遗传力较低，当前的后裔测定设计对体细胞评分评估产生的可靠性将低于对产奶量的评估。头胎和二胎泌乳期间体细胞评分的遗传相关性低于产奶量的遗传相关性。因此，随着时间推移，种公牛体细胞评分评估的变化应大于产奶量评估的变化。体细胞评分不存在基因型与环境的互作，这表明在体细胞评分高和低的牛群中会选择相同的公牛来配种。大多数评估值将在-0.25至0.25之间，荷斯坦奶牛的平均值为0.01，这意味着与评估值最低的公牛的女儿相比，评估值最高的公牛的女儿的几何平均SCC增加约50%，临床乳腺炎发病率高出11%。从经济角度来看，选择降低体细胞评分似乎不合理。构建合理的指数将使预计的乳腺炎发病率增加降低20%至25%，同时产奶量的遗传进展下降2%。体细胞评分遗传评估的最大影响可能源于选择用于后裔测定的公牛的种公牛。
