This study investigated the factors relating to pasture chemical and fatty acid (FA) composition that influence the milk fat percentage of spring calving, grazing dairy cows. The relationship between milk fat percentage and FA composition of the milk in these herds was also investigated. Milk protein percentage, milk casein percentage and cheddar cheese yield were increased in milk from HMF herds. Cows from LMF herds did not have negatively altered milk processability including rennet coagulation time (RCT), pH and ethanol stability. Crude protein, NDF, ADF, ether extract and total FA content of pasture was not different between LMF and HMF herds. Milk fat concentration of conjugated linoleic acid (CLA) t10, c12 was not different between HMF and LMF herds. Pre-grazing herbage mass and pasture content of crude protein, neutral detergent fibre (NDF) and total FA were similar between HMF and LMF herds. Pasture offered to LMF herds had a higher concentration of monounsaturated fatty acids (MUFA). A strong negative relationship (r = -0.40) was evident between milk fat percentage and pasture crude protein content for MMF herds (3.31–3.94% milk fat). This research reports improved milk protein percentage, milk casein percentage and cheddar cheese yield from HMF herds compared to LMF herds. Milk processability was not impacted by low milk fat percentage. Pasture NDF and total fatty acid content was similar in HMF herds and LMF herds. Milk fat percentage had a strong negative association (r = -0.40) with pasture crude protein content in MMF herds (MF 3.31–3.94%). Correlation values between pasture chemical and FA composition and milk fat percentage in LMF herds and HMF herds were low, indicating that diet is not the only causative factor for variation in milk fat of grazing dairy cows. Comparison of milk fatty acid composition from herds with and without milk fat depression suggests that there may be other fatty acids apart from CLA t10, c12 that contribute to the inhibition of milk fat synthesis during milk fat depression in grazing herds.

中文翻译：

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比较有和没有乳脂降低的爱尔兰放牧奶牛群的牛奶和草成分

本研究调查了影响春季产犊、放牧奶牛乳脂百分比的牧草化学品和脂肪酸 (FA) 组成相关因素。还研究了这些牛群中乳脂百分比与牛奶 FA 组成之间的关系。HMF 牛群的牛奶中的牛奶蛋白百分比、牛奶酪蛋白百分比和切达干酪产量有所增加。来自 LMF 牛群的奶牛没有对牛奶加工性能产生负面影响，包括凝乳酶凝固时间 (RCT)、pH 值和乙醇稳定性。粗蛋白、NDF、ADF、乙醚提取物和总 FA 含量在 LMF 和 HMF 畜群之间没有差异。共轭亚油酸 (CLA) t10、c12 的乳脂浓度在 HMF 和 LMF 畜群之间没有差异。放牧前牧草质量和粗蛋白牧草含量，HMF 和 LMF 畜群的中性洗涤纤维 (NDF) 和总 FA 相似。提供给 LMF 牛群的牧场含有更高浓度的单不饱和脂肪酸 (MUFA)。MMF 牛群（3.31–3.94% 乳脂）的乳脂百分比和牧场粗蛋白含量之间存在明显的负相关 (r = -0.40)。该研究报告称，与 LMF 畜群相比，HMF 畜群的牛奶蛋白百分比、牛奶酪蛋白百分比和切达干酪产量有所提高。牛奶加工性能不受低乳脂百分比的影响。HMF 畜群和 LMF 畜群的牧场 NDF 和总脂肪酸含量相似。乳脂百分比与 MMF 畜群中的牧草粗蛋白含量呈强烈负相关 (r = -0.40) (MF 3.31–3.94%)。LMF 牛群和 HMF 牛群的牧草化学品和 FA 组成与乳脂百分比之间的相关值较低，表明日粮不是放牧奶牛乳脂变化的唯一致病因素。比较有和没有乳脂抑制的牛群的乳脂肪酸组成表明，除了 CLA t10、c12 之外，可能还有其他脂肪酸有助于抑制放牧牛群乳脂抑制期间的乳脂合成。