# Sow Management: How Many Litters/Sow Should I be Targeting?

By Glynn T. Tonsor (Department of Agricultural, Food and Resource Economics, Michigan State University) and Kevin C. Dhuyvetter (Department of Agricultural Economics, Kansas State University) and published in the Michigan State University Pork Quarterly.
4 August 2008

### Purpose Statement

As producers in the swine industry continue to specialize their production efforts and enhance their managerial focus on finer and finer details, an appropriate issue to address is the impact sow attrition rates have on costs and returns. The primary purpose of this short article is to examine the impacts of alternative sow culling strategies on costs and returns, as well as examine sensitivity of optimal strategy selection to factors including costs of replacement gilts, feed prices, conception rates of the breeding herd and weaned pig values.

### Analysis

To address the issue of identifying optimal sow attrition rates, we have updated the previous work of Dhuyvetter (2000). Dhuyvetter (2000) developed projected budgets for sow operations culling sows after their first through their tenth parities to determine if an optimal culling strategy exists. For clarity, note that a strategy of culling after the first parity results in an operation always having 100% gilts while a tenth parity strategy culls a smaller segment of sows across each of ten litters such that the on-going (or steady state) sow herd is comprised of 18% gilts (table 1).

Table 1. Parity Distribution and Production from Sow Herd

a Represents the sow culling strategy. For example, “3” would indicate sows are kept for three parities at which time they are culled. Sows that do not breed back prior to their final parity are culled at the time they are open.
b Average parity is simply the weighted average parity. For example, the average parity for sows culled after their third parity is calculated in the following manner: (38.2% x 1 + 33.2% x 2 + 28.6% x 3) = 1.90.
c Based on farrowing 220 sows every four weeks.

The economic budgeting analysis conducted by Dhuyvetter (2000) is updated in this article to reflect current feed prices, sow and gilt prices and building expenses. The specific adjustments made are:

Description/Unit Dhuyvetter 2000
Value
Updated
Value
Grain Price (\$/lb) \$2.48 \$4.50
Protein (\$/ton) \$200.00 \$300.00
Labor (\$/year) \$33,000.00 \$40,000.00
Utilities, fuel, and oil (\$/
month)
\$2,500.00 \$3,500.00
Buildings/Gestation Crate
(\$/sq ft.)
\$20.00 \$21.00
Buildings/Farrowing Crate
(\$/sq ft.)
\$30.30 \$31.82
Equipment/Gestation (\$/crate) \$180.00 \$189.00
Equipment/Farrowing (\$/crate) \$800.00 \$840.00
Price of cull sows (\$/cwt) \$33.37 \$21.76
Price of cull gilts (\$/cwt) \$42.03 \$40.92
Price of cull boars (\$/cwt) \$30.00 \$16.76
Purchase price for gilts (\$/
\$200.00 \$225.00

As shown in table 2, returns over total costs are negative (\$/head) for each of the ten evaluated culling strategies. This reflects the current situation of higher input expenses that are not currently offset by equivalent increases in revenue. While the existence of negative returns is certainly not desirable, it does not preclude us from using table 2 to identify the optimal culling strategy. Given the base assumptions underlying the cost-return budget (details are excluded from this paper, for finer details of budget assumptions see Dhuyvetter, 2000), returns over total costs are maximized (i.e., losses minimized) when sows are kept for 9 parities. However, the difference in returns between parities 6 and 10 is rather small (\$0.44/head). Referring back to table 1, we see that the optimal 9-parity strategy implies an optimal herd comprised of 18% gilts, 16% parity two sows, 14% parity three sows, etc.

Table 2. Cost-Return Budget for a Farrow-to-Weaned Pig Operation

Given the assortment of assumptions that are necessary in any cost-return budgeting exercise, it is important to analyze sensitivity to some key assumptions that may impact returns over total costs and hence the optimal culling strategy. Table 3 shows the sensitivity of returns over total costs to the cost of replacement gilts and feed ration diets. (Note that base case results are the same as those shown on line F in table 2). When replacement gilts are valued at 25% less (\$169/head) than the base assumed value (\$225/head) returns over total costs increase by \$1.77/head and are maximized when sows are culled after eight parities. However, when replacement gilts costs 25% more than the base of \$225/head, returns over total costs are further reduced by \$1.70 but the optimal culling strategy remains at nine parities. Table 3 also shows that while the assumed feed ration impacts returns, it does not impact the optimal culling strategy. That is, selection of a sow culling strategy is not influenced by feed costs.

Table 3. Analysis of Sensitivity to Replacement Gilt (\$/head) and Feed Diet (\$/ton) Assumptions

A third sensitivity analysis considered the impact of alternative conception rates. In particular, following Dhuyvetter (2000) we considered scenarios where conception rates are ±10 percent of the base rates as well as scenarios where the rate improves or declines more rapidly (rather than evenly by 10%) such that conception rates for the tenth parity are ±40% of the base rates. Results of these four scenarios, and the base case, are presented in table 4. While returns are impacted by conception rates, the optimal culling strategy remains an eighth or ninth parity approach. Of more interest is to note the asymmetrical impact that these scenarios reveal. In particular, the increase in returns associated with improved conception rates is notably less (in absolute terms) than the decrease in returns accompanying a setback in conception rates, providing strong evidence for the need to closely monitor actual conception rates.

Table 4. Analysis of Sensitivity to Conception Rate Assumptions

The final sensitivity analysis considered the impact of alternative weaned pig values (\$/head). The value (\$34.01/head) assumed in table 2’s base model was identified using a Kansas State University formula based upon grain, soybean meal, and market hog prices. Given the negative returns suggested by table 2, a likely question to be raised by readers of this article is what the impact of alternative output prices (weaned pigs) would be on culling strategies. While sensitivity analysis to wean pig prices, independent of changes in market hog prices may not be strongly advisable, table 5 helps answer this question by showing the results of considering situations where weaned pig values are ±25% of the base price. Results show that while weaned pig values certainly impact return levels, they do not alter the optimal culling strategy.

Table 5. Analysis of Sensitivity to Weaned Pig Value (\$/head) Assumptions

### Implications

Identification of optimal sow culling is an important component of a sound management plan by modern swine producers. Providing information to improve these managerial decisions was the focus of this article. The cost-return budgeting exercise summarized in this report reveals that the most economical time to cull a sow is after her eighth or ninth parity. It was also found that small differences in returns exist for any strategy keeping sows between six and ten parities. Consistent with expectations, sows should be culled sooner when the cost of replacement gilts decline. However, feed ration prices, conception rates of the breeding herd and weaned pig values, have relatively little impact on optimal culling strategies.

### Reference

Dhuyvetter, K.C. 2000. “What Does Attrition Cost and What is it Worth to Reduce?” Paper presented at the Allen D. Leman Swine Conference, College of Veterinary Medicine, University of Minnesota, August 11- 15, 2000, Minneapolis, MN. The article is also available by clicking here.