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.

Milk production and reproductive performance of pluriparous dairy cows was compared for cows whose lactations were induced by hormonal treatment or followed natural calving. The study was conducted on 179 high-yielding dairy cows in a large dairy operation in a hot arid environment of northern Mexico, where bovine somatotropin (bST) was routinely used in all cows. To induce lactation, 98 cows that had previously failed to become pregnant were treated with 500 mg bST on days 1, 8 and 21. From days 2 to 8, cows were treated with oestradiol cypionate (0·30 mg/kg live weight (LW) per day) and progesterone (0·28 mg/kg LW per day). From days 9 to 15, oestradiol cypionate alone was given. PGF2α was given on day 16. Nothing was administered on days 17 and 18, and 15 mg flumetasone was administered from days 19 to 21. Lactation was induced successfully in all 98 cows subjected to the hormone treatment. All cows received 500 mg bST every 14 days throughout lactation from 63±7 days after lactation had started. Cows with induced lactations produced less (P<0·01) milk per 305-day lactation (9599±1387 kg) than controls (12 302±1245 kg). Proportion of cows pregnant was similar in induced cows and non-treated cows (71 v. 75%). Induction of lactation was associated with increased numbers of services per pregnancy (5·8±4·0 v. 4·22±2·98; P<0·01) compared with controls; however, impaired reproduction might not necessarily have been related to the hormonal treatment to induce lactation, but due to the nature of the cows utilized for this treatment group. This protocol can be used on dairy farms where bST treatment and prolonged steroid administration of dairy cows is legally permitted.