INTEGRATED SYSTEMS-ANALYSIS OF SOW REPLACEMENT RATES IN A HIERARCHICAL SWINE BREEDING STRUCTURE

Sow replacement rates in a three-tiered breeding structure were invest igated for a 10-yr planning horizon using a stochastic life-cycle swin e production model. Market hogs were produced in a three-breed static crossing program and marketed on a liveweight basis. Growth and reprod uctive traits of individual pigs were simulated using genetic, environ mental, and economic parameters. Sows were culled after a maximum of 1 , 5, or 10 parities. Systems were defined by maximum sow age at cullin g and included combinations of 1- and 5-parity nucleus and 1-, 5-, and 10-parity multiplier and commercial tiers. Economic response to index selection was considerable for all culling alternatives with yearly i ncreases in system profits ranging from $1.06 to 1.44 for each commerc ial hog marketed. When sows were culled after one parity in nucleus, m ultiplier, and commercial tiers, respectively (1,1,1), annual changes in net returns and all cost measures were 40 to 50% larger than respon ses in systems with lower sow replacement rates. Based on 10-yr averag es for net returns, systems with low multiplier- and commercial-level replacement rates were more profitable than systems with higher replac ement rates. The most profitable system (5,10,10) differed from the le ast profitable system (1,1,1) by more than $10 per pig, but when the ( 1,1,1) system was excluded, the range was only $3 per pig. The system with lowest replacement rates supported 3,388 more multiplier and 34,1 51 more commercial sows from a 750-sow nucleus level than the (1,1,1) system. Output from the two extremes differed by > 664,000 commercial market hogs sold. Greater benefit and larger tier sizes resulted from increasing maximum parity level from 1 to 5 than from increasing parit y from 5 to 10.