Abstract：
Our aims were to evaluate changes in body characteristics, milk yield and milk constituents as well as to determine the relationship between the thermal environment and production characteristics during the first lactation of dairy Gyr cows managed on pasture. Between 2013 and 2015, forty-five primiparous dairy Gyr cows were evaluated from prepartum to 10 months of lactation in Southeast of Brazil. Body weight, body condition score (BCS), subcutaneous fat thickness (SFT), milk yield (305 d), and milk constituents were collected monthly and progesterone was collected weekly. Additionally, we determined the temperature humidity index (THI) based on microclimate data. Overall, the cows lost body weight until six months of lactation and there was a progressive decrease in BCS, SFT, milk yield and milk lactose as the months in lactation progressed. In contrast, there was an increase in milk fat, milk protein and milk solids. The thermal environment did not pose a consistent heat challenge, nevertheless, we found a positive correlation between the average THI two days before milk collection with milk yield, fat and lactose contents, but in contrast a negative correlation was found with total solids and protein. In conclusion, the THI and months of lactation affected the yield and constituents of milk. However, more studies are necessary to understand the impacts of body characteristics and thermal environment on yield and milk constituents throughout the productive life of Gyr dairy cows.
摘要：
我们的目标是评估身体特征的变化，产奶量和乳成分，以及确定在牧场上管理的奶牛的首次泌乳期间热环境与生产特性之间的关系。在2013年至2015年之间，在巴西东南部对45头初产奶牛进行了从产前到泌乳10个月的评估。体重,身体状况评分（BCS），皮下脂肪厚度（SFT），产奶量（305天），每月收集牛奶成分，每周收集孕酮。此外,我们根据小气候数据确定了温度湿度指数(THI)。总的来说,奶牛在哺乳期六个月前体重下降，BCS逐渐下降，SFT,随着泌乳月份的进展，产奶量和牛奶乳糖。相比之下,牛奶脂肪增加了，牛奶蛋白和牛奶固体。热环境并没有带来一致的热挑战，然而，我们发现牛奶收集前两天的平均THI与牛奶产量之间呈正相关，脂肪和乳糖含量，但相比之下，发现与总固体和蛋白质呈负相关。总之,THI和哺乳期影响牛奶的产量和成分。然而,需要更多的研究来了解身体特征和热环境对Gyr奶牛整个生产寿命中产量和乳成分的影响。
