An economic simulation model to assess the effect of the 21-day pregnancy rate, semen type, and heifer survival rate on the optimal insemination eligibility period for lactating dairy cows

Our objective was to develop a discrete, monthly Markov-chain model to assess the economic performance and herd dynamics of herd scenarios with varying 21-d pregnancy rates (21-d PR), semen types (conventional vs. sexed and beef semen), and insemination eligibility periods (IEP). We classified the estimated herd 21-d PR as low (20%), below average (25%), average (30%), above average (35%), or high (40%). The semen type scenarios included (1) conventional semen for all inseminations regardless of parity (conventional) and (2) sexed semen inseminations for nulliparous heifers (75%) and for primiparous and secondiparous cows for first and second inseminations, with all remaining inseminations across parities to beef semen (sexed+beef). The IEP of cows varied from 50 to 260 d after the voluntary waiting period (70 DIM). We defined the optimal IEP as the period with the maximum net return (NR; $/cow per year) and sufficient replacements to maintain herd size. Treatments included varying market values of beef × dairy crossbred calves ($385-1,125, $185 increments), heifer survival rates (HSR; 75% to 90%, 5-percentage-point increments), and 21-d PR (20% to 40%, 1-percentage-point increments). The optimal IEP for conventional semen herd scenarios ranged from 80 to 170 d for high to low 21-d PR herds, respectively, but was 200 d for sexed+beef semen herd scenarios regardless of the 21-d PR. The herd turnover rate for the optimal IEP was lower for herd scenarios using sexed+beef semen compared with conventional semen because of differences in the month of lactation for replacement production and sex-skew. Increasing beef calf market values increased NR as expected. For conventional semen herd scenarios, the optimal IEP increased as the HSR decreased, but low 21-d PR herds with a 75% HSR had insufficient replacements regardless of the IEP. The optimal IEP for sexed+beef semen herd scenarios remained 200 d regardless of HSR except for low and below average 21-d PR herds with 75% to 80% or 75% HSR, respectively, due to insufficient replacements. For a 170-d IEP, which was viable based on sufficient replacements for both semen types, the base NR (20% 21-d PR) was $51 greater per cow/yr for sexed+beef semen than for conventional semen herd scenarios. The increase in NR ranged from $3 to $6 and from $2 to $7 per 1-percentage-point increase in the 21-d PR for conventional and sexed+beef semen herd scenarios with a 170 d IEP, respectively, depending on the initial 21-d PR. In conclusion, the optimal IEP depends on the 21-d PR, semen type, and HSR, making it herd specific. Furthermore, we concluded that opportunities still exist for herds to increase NR by increasing their 21-d PR and by using sexed and beef semen.