Time to critical overcrowding of Manitoba swine barns in the event of restriction on animal movement

Abstract

Canada maintains a stamping-out policy for control of foreign animal disease. Integral to stamping out, there are obligatory animal movement restrictions that, over time, can result in severe animal welfare problems. The purpose of this study was to estimate the time to critical overcrowding in a sample of Manitoba swine farms, if they were placed under movement restriction. Time to critical overcrowding in both static and dynamic populations was estimated by modeling swine growth and projecting anticipated on-farm stocking density through time and comparing it with a standard maximal stocking density. The time to critical overcrowding of 15 isowean piglet production units was 0.66 +/- 0.88 weeks. On average, 7 isolated nursery facilities reached critical overcrowding in 52 +/- 14 days and 5 farrow-to-finish operations reached critical overcrowding in 43 +/- 18 days. Animal welfare concerns should be included in contingency plans to control foreign animal disease.

Figure 1. Visual representation of the model applied to predict the time to critical overcrowding for a single age group of feeder pigs. In this example, each pen has a floor area of 17.84 m², placing 24, 59-day-old pigs in each pen on day 0. Pigs are quarantined on the day of placement. The line pig weight is derived from the standard growth curve based on animal age in days, (right hand scale). The pen pressure is calculated by dividing the total body weight of pigs in the pen by pen area, and the maximum allowable pressure is converted from the approximation of the European minimum space allowance standard (12), (left hand scale). Critical overcrowding was when the pen pressure equaled the maximal allowed pressure on day 127 of quarantine, when the pigs are 186 days old and have an average body weight of 120 kg.

Figure 2. Changing floor pressure over time as a measure of maximal allowable stocking density in an isolated nursery holding 7 age cohorts of pigs, 6 cohorts shown (Nursery 1 in Table 2). The maximal pen density allowed based on the weight of the pigs in Barn 1 is represented by S1; B1 is the actual pen density in Barn # 1. Intervention strategy; on day 27 of quarantine, Barn 7 pigs are at critical crowding. Three rooms in the least critically crowded barn, Barn 1, are reallocated to pigs in the B7 cohort. The B7 cohort would then occupy 15 rooms, 12 in Barn 7 and 3 in Barn 1. The B1 cohort would be restricted to 9 rooms in Barn 1. Pigs in both cohorts would be redistributed to even out the number of pigs per pen. Similarly on day 35, Barn 6 is critically crowded, and 2 rooms in Barn 2 are taken over by pigs from the B6 cohort. Pigs in both cohorts would be redistributed into pens within their cohort. On day 39, Barn 5 is critically crowded, and 2 rooms in Barn 3 and 1 in Barn 1 are reallocated to hold B5 pigs. One week subsequent to this intervention, that is on day 46 of quarantine, 4 of the 7 barns are critically crowded, the other 3 are near critically crowded, and no relocation of pigs will result in significant improvement of the overcrowding. Barn 4 (not shown) holding the median age cohort of pigs becomes critical on day 45 without introduction of other pigs.

Figure 3. Change in percentage of actual capacity utilized over time for 5 farrow-to-finish farms placed under quarantine on day 0. The line 100% describes the situation where every pen of pigs in the nursery and grow-finish areas of the farm are critically overcrowded. The slope of lines describes the rate of consumption of available remaining space over time in the units of percent of overall capacity per day (0.829 ± 0.0675%). Farm D, the only farm model where the data did not highly conform to a straight line, was a replacement gilt production unit with the majority of female pigs being sold off the farm at less than market weight. With the placement of quarantine, this farm became critically overcrowded after a similar passage of time as for other farms.

Figure 4. Graphical comparison of time to critical overcrowding in an idealized 800-sow farrow-to-finish unit with standard growth of pigs reaching market weight at 175 d (thin line) and the same unit with slow growth of pigs reaching market weight at 189 d (thick line). Lines graphed describe the consumption of space over time in the units placed under quarantine at time zero. Critical overcrowding occurs when the situation specific change in actual capacity utilized line intersects the 100% ACU line on the y-axis. The % ACU is different for the 2 performance situations at time 0, because slow growing pigs swell the inventory of animals on the farm at the time of quarantine in comparison to the same farm with standard pig growth. The slope of the lines, which reflects the rate of utilization of available space, is essentially identical under both growth rate assumptions.