Oseni S, Misztal I, Tsuruta S, Rekaya R
Department of Animal and Dairy Science, University of Georgia, Athens 30602, USA.
J Dairy Sci. 2003 Nov;86(11):3718-25. doi: 10.3168/jds.S0022-0302(03)73977-0.
The objectives of this study were to establish a pattern for the seasonality of days open (DO) by state and region within the United States and to present statistics on regional trends for DO. Data included 8,676,915 records on DO for Holsteins from 1997 to 2002 covering all regions of the United States. Fixed effects in the model included herd, parity, milk-class, state x month of calving (MOC), year of calving x MOC, and parity x MOC. Least squares means of DO were highest for calvings in March and lowest for calvings in September. The highest mean DO of 155 d was recorded in the Southeast, while the mean DO for the Midwest, Northeast, Northwest, and Southwest were 142, 141, 140, and 137 d, respectively. Variation in monthly averages of DO was highest in Southeast with a range of 51 d, and less than 25 d in all the other regions. Seasonality of calving was defined as the ratio of the fewest to the most calvings in months. The SOC was > or = 60% in Southeast and < or = 23% in the other regions. Selected states: Texas, Oklahoma, and Arizona in the Southwest and Missouri, Kansas, and Kentucky in the Midwest showed patterns of variation in monthly averages and seasonality of calving similar to those of Southeast. Distributions of DO were bimodal for some months of calving due to postponed breeding during the hot season or depressed fertility as a result of thermal stress; the second mode at > 200 d was highest in the Southeast but also could be observed in Texas, Wisconsin, and California. High level of heat stress for DO exists in the Southeast and in selected states of the Midwest and the Southwest; these regions contribute less than 10% of national records. A methodology for analyzing DO especially under heat stress needs to consider effects of intentionally delayed breeding--by using a model that accounts for bimodality, for example.
本研究的目的是确定美国各州和各地区奶牛场开放天数（DO）的季节性模式，并呈现DO的区域趋势统计数据。数据包括1997年至2002年美国所有地区8676915条荷斯坦奶牛的DO记录。模型中的固定效应包括牛群、胎次、牛奶等级、产犊月份（MOC）的州、产犊年份×MOC以及胎次×MOC。DO的最小二乘均值在3月份产犊时最高，在9月份产犊时最低。东南部记录的最高平均DO为155天，而中西部、东北部、西北部和西南部的平均DO分别为142天、141天、140天和137天。东南部DO月平均值的变化最大，范围为51天，其他所有地区均小于25天。产犊季节性定义为各月份产犊最少数量与最多数量的比值。东南部的产犊季节性系数（SOC）≥60%，其他地区≤23%。选定的州：西南部的得克萨斯州、俄克拉何马州和亚利桑那州以及中西部的密苏里州、堪萨斯州和肯塔基州，其产犊月平均值变化模式和季节性与东南部相似。由于炎热季节繁殖推迟或热应激导致繁殖力下降，某些产犊月份的DO分布呈双峰；大于200天的第二个峰值在东南部最高，但在得克萨斯州、威斯康星州和加利福尼亚州也能观察到。东南部以及中西部和西南部的选定州存在高水平的DO热应激；这些地区的记录占全国记录的比例不到10%。分析DO的方法，尤其是在热应激情况下，需要考虑故意延迟配种的影响——例如使用一个考虑双峰性的模型。
