Department of Veterinary Medicine, University of Sassari, via Vienna 2, 07100 Sassari, Italy.
Department of Agronomy, Food, Natural Resources, Animals and Environment (DAFNAE), University of Padova, viale dell'Università 16, 35020 Legnaro (PD), Italy; Department of Veterinary Science, University of Parma, Via del Taglio 10, 43126 Parma, Italy.
J Dairy Sci. 2018 Apr;101(4):3164-3175. doi: 10.3168/jds.2017-13975. Epub 2018 Feb 7.
The present study investigated the effect of somatic cell count, lactose, and pH on sheep milk composition, coagulation properties (MCP), and curd firming (CF) parameters. Individual milk samples were collected from 1,114 Sarda ewes reared in 23 farms. Milk composition, somatic cell count, single point MCP (rennet coagulation time, RCT; curd firming time, k; and curd firmness, a, a, and a), and CF model parameters were achieved. Phenotypic traits were statistically analyzed using a mixed model to estimate the effects of the different levels of milk somatic cell score (SCS), lactose, and pH, respectively. Additive genetic, herd, and residual correlations among these 3 traits, and with milk composition, MCP and CF parameters, were inferred using a Bayesian approach. From a phenotypic point of view, higher SCS levels caused a delayed gelification of milk. Lactose concentration and pH were significant for many milk quality traits, with a very intense effect on both coagulation times and curd firming. These traits (RCT, RCT estimated using the curd firming over time equation, and k) showed an unfavorable increase of about 20% from the highest to the lowest level of lactose. Milk samples with pH values lower than 6.56 versus higher than 6.78 were characterized by an increase of RCT (from 6.00 to 14.3 min) and k (from 1.65 to 2.65 min) and a decrease of all the 3 curd firmness traits. From a genetic point of view, the marginal posterior distribution of heritability estimates evidenced a large and exploitable variability for all 3 phenotypes. The mean intra-farm heritability estimates were 0.173 for SCS, 0.418 for lactose content, and 0.206 for pH. Lactose (favorably), and SCS and pH (unfavorably), at phenotypic and genetic levels, were correlated mainly with RCT and RCT estimated using the curd firming over time equation and scarcely with the other curd firming traits. The SCS, lactose, and pH were significantly correlated with each other's. In conclusion, results reported in the present study suggest that SCS, pH, and lactose affect, contemporarily and independently, milk quality and MCP. These phenotypes, easily available during milk recording schemes measured by infrared spectra prediction, could be used as potential indicators traits for improving cheese-making ability of ovine milk.
本研究调查了体细胞计数、乳糖和 pH 值对绵羊乳成分、凝乳特性（MCP）和凝乳固化（CF）参数的影响。从 23 个农场饲养的 1114 只萨杜羊中采集了个体牛奶样本。获得了牛奶成分、体细胞计数、单点 MCP（凝乳酶凝固时间 RCT；凝乳时间 k；和凝乳硬度 a、a、a）和 CF 模型参数。使用混合模型对表型性状进行统计分析，以分别估计不同水平的牛奶体细胞评分（SCS）、乳糖和 pH 值的影响。使用贝叶斯方法推断了这些 3 个性状以及与牛奶成分、MCP 和 CF 参数之间的加性遗传、畜群和剩余相关性。从表型的角度来看，较高的 SCS 水平导致牛奶的凝胶化延迟。乳糖浓度和 pH 值对许多牛奶质量性状有显著影响，对凝乳时间和凝乳固化都有非常强烈的影响。这些性状（RCT、使用凝乳固化时间方程估计的 RCT 和 k）从乳糖的最高水平到最低水平表现出约 20%的不利增加。pH 值低于 6.56 与高于 6.78 的牛奶样本表现出 RCT（从 6.00 增加到 14.3 分钟）和 k（从 1.65 增加到 2.65 分钟）的增加以及所有 3 个凝乳硬度性状的降低。从遗传的角度来看，3 个表型的边缘后验分布表明存在很大且可利用的变异性。农场内的平均遗传力估计值分别为 SCS 0.173、乳糖含量 0.418 和 pH 值 0.206。在表型和遗传水平上，乳糖（有利）以及 SCS 和 pH（不利）与 RCT 和使用凝乳固化时间方程估计的 RCT 以及其他凝乳固化性状相关主要与凝乳硬度性状相关。SCS、乳糖和 pH 值彼此显著相关。总之，本研究报告的结果表明，SCS、pH 值和乳糖同时且独立地影响牛奶质量和 MCP。这些表型在通过红外光谱预测的牛奶记录方案中很容易获得，可作为提高绵羊乳制能力的潜在指示性状。
