Sustainable Livestock Systems Group, Scottish Agricultural College, Roslin Institute Building, Easter Bush, Penicuik, Midlothian EH25 9RG, UK.
Animal. 2012 Nov;6(11):1857-67. doi: 10.1017/S1751731112000936.
Many governments have signed up to greenhouse gas emission (GHGE) reduction programmes under their national climate change obligations. Recently, it has been suggested that the use of extended lactations in dairy herds could result in reduced GHGE. Dairy GHGE were modelled on a national basis and the model was used to compare emissions from lactations of three different lengths (305, 370 and 440 days), and a current 'base' scenario on the basis of maintaining current milk production levels. In addition to comparing GHGE from the average 'National Herd' under these scenarios, results were used to investigate how accounting for lactations of different lengths might alter the estimation of emissions calculated from the National Inventory methodology currently recommended by Intergovernmental Panel on Climate Change. Data for the three lactation length scenarios were derived from nationally recorded dairy performance information and used in the GHGE model. Long lactations required fewer milking cows and replacements to maintain current milk yield levels than short ones, but GHGEs were found to rise from 1214 t of CO2 equivalent (CE)/farm per year for lactations of 305 days to 1371 t CE/farm per year for 440-day lactations. This apparent anomaly can be explained by the less efficient milk production (kg milk produced per kg cow weight) found in later lactation, a more pronounced effect in longer lactations. The sensitivity of the model to changes in replacement rate, persistency and level of milk yield was investigated. Changes in the replacement rate from 25% to 20% and in persistency by −10% to +20% resulted in very small changes in GHGE. Differences in GHGE due to the level of milk yield were much more dramatic with animals in the top 10% for yield, producing about 25% less GHGE/year than the average animal. National Inventory results were investigated using a more realistic spread of lactation lengths than recommended for such calculations using emissions calculated in the first part of the study. Current UK emission calculations based on the National Inventory were 329 Gg of methane per year from the dairy herd. Using the national distribution of lactation lengths, this was found to be an underestimate by about 10%. This work showed that the current rise in lactation length or a move towards calving every 18 months would increase GHGE by 7% to 14% compared with the current scenario, assuming the same milk yield in all models. Increased milk yield would have a much greater effect on reducing GHGE than changes to lactation length, replacement rate or persistency. National Inventory methodology appears to underestimate GHGE when the distribution of lactation lengths is considered and may need revising to provide more realistic figures.
许多政府根据国家气候变化义务签署了温室气体排放（GHGE）减排计划。最近，有人提出，在奶牛群中延长泌乳期可能会减少 GHGE。在国家基础上对奶牛 GHGE 进行了建模，并使用该模型比较了三种不同长度（305、370 和 440 天）的泌乳期和当前基于维持当前牛奶产量水平的“基础”情景的排放量。除了比较这些情景下平均“国家牛群”的 GHGE 外，结果还用于研究在不同长度的泌乳期的核算如何改变目前政府间气候变化专门委员会推荐的国家清单方法计算的排放量的估计。这三种泌乳期长度情景的数据来自全国记录的奶牛性能信息，并用于 GHGE 模型。与短期泌乳相比，长泌乳期需要更少的奶牛和替换品来维持当前的牛奶产量水平，但 GHGE 从 305 天泌乳期的每年 1214 吨二氧化碳当量（CE）/农场上升到 440 天泌乳期的每年 1371 吨 CE/农场。这种明显的反常现象可以用后期泌乳中发现的效率较低的牛奶产量（每公斤奶牛体重生产的牛奶量）来解释，在较长的泌乳期中这种影响更为明显。还研究了模型对替换率、持久性和牛奶产量水平变化的敏感性。替换率从 25%降至 20%，持久性从-10%变为+20%，GHGE 仅略有变化。由于产量水平的不同，GHGE 的差异要大得多，产量最高的 10%的动物每年产生的 GHGE 比平均动物少约 25%。使用本研究第一部分计算的排放量计算中推荐的更现实的泌乳期长度分布，研究了国家清单结果。基于国家清单，目前英国对奶牛群的甲烷排放量计算为每年 329 克。结果发现，这一数字比使用全国泌乳期长度分布的估计值低了约 10%。这项研究表明，与目前的情况相比，当前泌乳期长度的增加或转向每 18 个月产犊一次，将使 GHGE 增加 7%至 14%，假设所有模型中的牛奶产量相同。与泌乳期长度、替换率或持久性的变化相比，增加牛奶产量将对减少 GHGE 产生更大的影响。当考虑泌乳期长度分布时，国家清单方法似乎低估了 GHGE，可能需要修订以提供更现实的数字。
