Biophysical implications and economic considerations when changing from spring to autumn calving in pasture-based dairy systems : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Agricultural Science at Massey University, Manawatū, New Zealand

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For a farmer considering changing their herd’s calving season from spring to autumn, one approach is to extend the herd’s calving interval (CI) and undertake an extended lactation. The objective of this thesis was to quantify the biophysical and economic implications of this approach. Two randomised and balanced farmlets were established on 1 October 2017 on an existing spring-calving research farm in South Taranaki, New Zealand. One farmlet (SPR; 301 cows, 104.0 ha) maintained a 12-month CI, spring-calving pattern, while the other farmlet (AUT; 301 cows, 104.8 ha) changed to autumn calving by delaying mating and extending the CI to 20 months. This resulted in an extended lactation, followed by a 12-month CI autumn-calving pattern for the remainder of the experiment. The experimental period was 1 June 2017 to 31 January 2020, and included two and a half lactations for the SPR farmlet and two lactations for the AUT farmlet. Weather, animal and pasture performance, supplementary feed use, and economic data were analysed for differences between farmlets, and used to model potential outcomes over a 10-year investment horizon, where the investment was to either remain spring calving or to use this approach to change to autumn calving. Throughout the experimental period, total milksolids (MS) production was similar between the AUT and SPR farmlets (1,194 vs. 1,174 kg MS/cow), but AUT farmlet cows were offered more supplementary feed (2,371 vs. 1,936 kg dry matter/cow). Cows in the AUT farmlet varied in their ability to sustain an extended lactation [488 mean days in milk (DIM,) 577 max DIM]. Implications from extending the CI in the AUT farmlet were: a greater winter feed deficit, which increased supplementary feed requirements; a greater summer feed surplus, which required more pasture conservation; greater BCS gain, above an optimal pre-calving level, for cows during the later stages of their extended lactation; an improvement in reproductive performance during the extended lactation; and greater grazing costs for replacement heifers, although this also meant that heifers were older when they entered the milking herd and produced more MS. There were also carry-over effects from the extended lactation on the first autumn-calving lactation. Cows in the AUT farmlet had more DIM and produced more MS during their first complete autumn-calving lactation compared with either of the two complete lactations from SPR farmlet cows. Extending the CI resulted in greater net costs during the first three years that had to be recouped by a relatively more profitable, steady-state autumn-calving system in subsequent years for the change to autumn calving to be a better investment than remaining spring calving. During the transition period AUT farmlet profit was $289–1,452/ha less compared with the SPR farmlet. Less annual MS production due to the extended lactation and required capital expenditure were major drivers of this relative difference in profitability in the AUT farmlet. Conversely, the availability of the winter milk premium, and greater MS production in the AUT farmlet, with good cost control, once a steady-state autumn-calving pattern had been achieved, were critical to the AUT farmlet recouping the relative profitability difference. Greater MS production and the availability of the winter milk premium meant profit was greater in the steady-state AUT farmlet compared with the SPR farmlet after the transition period. With the current winter milk premium available, the NPV of the AUT farmlet was $38,212/ha compared with $26,040/ha for the SPR farmlet, however the NPV of the AUT farmlet reduced to $25,723/ha with no winter milk premium, and this was less than the NPV of the SPR farmlet. This led to the recommendation that only farmers who can operate an economically-sustainable autumn calving system without a winter milk premium, should consider changing calving season.
The following Figures were removed for copyright reasons: Figures 2.1 (=Dillon at al., 2008 Fig 4), 2.2 (=Newton, 2016 Fig 2), 2.4 (=Roberts & Thomson, 1984 Fig 4), 2.5 (=Kay, 2017 Fig 2) & 2.11 (=Chapman et al., 2013 Fig 1). Other Figures were adapted from the source article.