Influence of the seasons on the quality and composition of milk from dairy farms in the northern region of Rio Grande do Sul, Brazil

Abstract This study aimed to verify the relationship between parameters of physicochemical composition, microbiological quality, and volume of milk delivered to a dairy in the northern region of Rio Grande do Sul, Brazil, at four different times in 2020. For this purpose, the parameters volume, protein, fat, lactose, total solids, somatic cell count (SCC), and standard plate count (SPC) were evaluated. The data were collected in January, April, July, and October from 1634 dairy farms located in the northern region of Rio Grande do Sul. The data were evaluated using Pearson’s correlation, while the means were compared using Tukey’s test, both at a 5% significance. Milk fat and protein contents were higher during fall (4.02 and 3.35%) and winter (3.90 and 3.36%, respectively), contributing to a higher concentration of solids. The microbiological quality of milk, especially SCC, is compromised during the warmer months due to the thermal stress suffered by the animals, challenging their immune system and increasing their susceptibility to diseases. The highest milk SPC in the winter (247.12 CFU/mL) refers to the transfer of contamination from the teat to the tank due to the inefficiency of pre-milking procedures. Therefore, the microbiological quality of milk was variable between the studied periods, and fat and protein contents suffered reductions during the summer, reflecting lower remuneration for quality given the high SCC in the same season.摘要 本研究旨在验证巴西南里奥格兰德州北部地区送往乳品厂的牛乳的理化组成参数、微生物质量与牛乳送运量之间的关联，研究于2020年的四个不同时段开展。为此，本研究对牛乳送运量、蛋白质、脂肪、乳糖、总固体含量、体细胞计数（Somatic Cell Count, SCC）以及标准平板计数（Standard Plate Count, SPC）等参数进行了评估。数据采集于2020年1月、4月、7月和10月，采样覆盖南里奥格兰德州北部地区的1634家奶牛场。本研究采用皮尔逊相关分析对采集的数据进行统计评估，并采用Tukey检验进行组间均值比较，显著性水平设定为5%。结果显示，牛乳脂肪与蛋白质含量在秋季（分别为4.02%与3.35%）和冬季（分别为3.90%与3.36%）时更高，进而使得牛乳总固体浓度也相应提升。牛乳的微生物质量，尤其是体细胞计数，在温暖月份会受到负面影响：这是由于奶牛遭受热应激，其免疫系统受到挑战，疾病易感性升高所致。冬季牛乳的标准平板计数最高（247.12 CFU/mL），该现象与挤奶前操作效率低下导致的污染物从乳头转移至储奶罐有关。综上，牛乳的微生物质量在各研究时段间存在显著差异，且夏季牛乳的脂肪与蛋白质含量有所降低，加之同期体细胞计数偏高，最终导致该季度的优质牛乳对应报酬降低。