Contents lists available at ScienceDirect Agricultural Systems journal homepage: www.elsevier.com/locate/agsy Quantifying sheep enterprise proftability with varying fock replacement rates, lambing rates, and breeding strategies in New Zealand L.J. Farrell ⁎ , P.R. Kenyon, P.R. Tozer, T. Ramilan, L.M. Cranston Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand ARTICLEINFO Keywords: Flock dynamics Terminal breeding Ewe wastage Ewe age Flock structure ABSTRACT Sheep sales are the major source of income for most New Zealand sheep enterprises. Terminal (meat breed) sires are used to increase lamb growth rates enabling lambs to be sold earlier at higher prices on a per kg basis. Their use in self-replacing focks is generally constrained by requirements for purebred ewe lambs from which to choose ewe replacements. This study used a bio-economic system-dynamics model of a sheep enterprise to investigate use of terminal sires for a ewe fock with a range of ewe replacement and lambing rates. Values from New Zealand industry survey data were used to simulate a representative New Zealand North Island East Coast Hill Country sheep enterprise (self-replacing fock of 2182 mature breeding ewes with a stable fock size on 549 ha of pasture where 60.8% of feed was consumed by sheep). Three ewe replacement rates (20%, 25%, and 30% per annum), lambing rates (110, 130, and 150%; lambs weaned per ewe presented for breeding), and breeding strategies (no terminal sires; breeding the maximum possible proportion of mature ewes with terminal sires; and breeding half of the maximum with terminal sires) were modelled. Income from lamb sales increased with use of terminal sires, due to increased production of lamb liveweight for sale and lambs sold earlier for higher prices, resulting in greater sheep enterprise cash operating surplus (COS). Sheep enterprise COS increases, from no terminal sire use compared with maximum use of terminal sires, ranged from $3/ha to $101/ha. Larger increases in COS occurred with a higher fock lambing rate and lower ewe replacement rate, which allowed for terminal sire use over a greater proportion of the fock. Up to 65% of the fock could be bred with terminal sires while producing sufcient numbers of purebred ewe lambs from which to choose replacements. For an average North Island East Coast Hill Country sheep enterprise with a replacement rate of 25% and lambing rate of 130%, sheep enterprise COS increased by $15,021 with maximum use of terminal sires (45% of the mature ewe fock), compared with use of only maternal sires. Use of terminal sires increased annual sheep energy demand by up to 6% and changed the sheep feed demand profle, with increases in energy demand post-weaning in December mostly compensated for by subsequent reductions in demand following crossbred lamb sales in January. 1. Introduction Revenue from sheep sales makes up the majority of the income for most New Zealand sheep farming enterprises (Beef + Lamb New Zealand Economic Service, 2016). During the last 25 years New Zealand farmers have focused on increasing the total weight of lamb carcass produced per ewe, through increasing both the number of lambs weaned per ewe and individual lamb carcass weight (MacKay et al., 2012; Morris and Kenyon, 2014). The majority of breeding ewes in New Zealand are dual-purpose breeds, with a Romney base, producing strong wool (fbre diameter > 30 μm) and lambs for meat production (Beef + Lamb New Zealand Economic Service, 2018a). Terminal breed sheep, such as Sufolk, Poll Dorset, and Texel, have a focus on meat production and are associated with greater lamb growth rates and heavier liveweights (Clarke and Meyer, 1982; McEwan et al., 1995). Sires from such breeds can be bred with dual-purpose ewes to produce a faster growing crossbred lamb destined for slaughter (Carter and Kirton, 1975; Clarke and Meyer, 1982). A frst cross lamb's growth superiority over the average performance of the two parental breeds, is referred to as heterosis or hybrid vigour (Donald et al., 1963). Advantages in lamb growth of up to 30% have been observed in crossbred lambs from terminal sires compared to purebred lambs in New Zealand focks (Clarke and Meyer, 1982; McEwan et al., 1995). Improvements in birth weight, lamb survival, and carcass dressing out % have also been https://doi.org/10.1016/j.agsy.2020.102888 Received 7 October 2019; Received in revised form 4 June 2020; Accepted 5 June 2020 ⁎ Corresponding author. E-mail addresses: L.Farrell@massey.ac.nz (L.J. Farrell), P.R.Kenyon@massey.ac.nz (P.R. Kenyon), P.Tozer@massey.ac.nz (P.R. Tozer), T.Ramilan@massey.ac.nz (T. Ramilan), L.Cranston@massey.ac.nz (L.M. Cranston). Agricultural Systems 184 (2020) 102888 0308-521X/ © 2020 Elsevier Ltd. All rights reserved. T