Implications of changing birth and slaughter dates of finishing sheep and beef cattle on greenhouse gas emissions

Significant variables in the calculation of greenhouse gas (GHG) emissions are estimates of birth date and slaughter date, as these alter the amount of time on-farm and hence feed used for animals destined for slaughter. Analysis of Beef + Lamb New Zealand Economic Service farm survey data calculated average birth and slaughter dates for both finishing sheep and beef cattle at a regional scale, from 1990-2019. Data were then used to calculate the potential GHG emissions related to lambs and slaughter cattle, and changes over time, and were compared to current national inventory calculations. There was no significant change in sheep mating date over the 30-year period, with a median lambing date of 10 September. Beef cattle mating date became later over the 30-year period. These resulted in calving dates of 20 September for the 1990-2000 period, and 25 September for the 2010-2019 period.
The proportions of lambs slaughtered by February (early) or October (late), categories used by the National Inventory, have shifted from 84 and 16% respectively in 1990 to 78 and 22% recently. The ages at slaughter of 1–2-year-old heifers and steers were approximately 24 and 35 days younger in 2019 than in 1993 (528 vs 552 days of age at slaughter for heifers and 520 vs 555 days of age at slaughter for steers). The changing proportion of lambs slaughtered early and late had a small effect on total emissions. Later birth dates and earlier slaughter dates have reduced the individual emissions from 1–2-year-old cattle, but individual emissions from older cattle and bulls have changed little over the 27-year period examined.