The relationship between growth and sow longevity is difficult to resolve.

Gilt development programs are often complex and conflicting because the research does not conclusively reveal the impact of gilt growth on lifetime sow productivity.

Genetics play a big role in growth rate, body composition, age and age at first mating.

But University of Minnesota-Morris Extension swine researcher Lee Johnston perplexes over the specific role of growth rate during the grow-finish period on subsequent reproductive performance of sows.

Some researchers report that small increases in growth rate (under 1 lb. up to just over 1 lb.) in gilts from birth to 220 lb. can add between 0.3 and 0.4 pigs born live in Parities 2-5, but the data does not report effects of growth rate on lifetime performance of females. Other studies report no effect or a negative impact of growth rate on sow performance.

One point is clear: Very slow growth will likely delay age at puberty and reduce body weight at first mating, Johnston says. Slow growth can result from poor genetics, marginal nutrition, crowding and inferior health.

A trial by Australia's QAF Meat Industries altered an all-in, all-out, production system using deep-litter housing to provide more space and improve health of gilts from 17 to 29 weeks of age. Results over two parities showed a 15% improvement in pigs weaned/100 females entering the system and an 18% improvement in females weaning their second litter.

Johnston says without prospective studies for modern genotypes, the current recommendations that gilts gain at least 1.2 lb./day from birth to about 160 days of age for optimal expression of puberty seem warranted for lifetime sow productivity. Similarly, daily growth rates above 1.8 lb. should be avoided to optimize length and productivity of a sow's breeding life.

Relationship of body weight and/or backfat depth at 22 weeks of age to lifetime sow productivity is shown in Table 1.

Evidence is conflicting as to the effects of body fat on sow longevity, which may be partially explained by differences in structural correctness, Johnston explains. Increased body fat in retrospective studies has shown a positive correlation to structural soundness and improved lifetime sow productivity.

But overall, Johnston says, body fatness does not appear to play a central role in gilt reproductive performance compared to body weight and age at first estrus.

Body Condition at First Mating

Gilts should weigh at least 300 lb. but no more than 350 lb. at first mating, to enhance lifetime productivity, Johnston says.

PIC research in 2005 found that the number of pigs born over three parities was depressed when females weighed less than 300 lb. at first mating.

“This target range in body weight at first mating provides a reasonable balance between enough body mass and maturity for adequate skeletal development and tissue reserves without allowing the female to get too big,” Johnston says.

Weights are best captured by weighing gilts, but most commercial units aren't equipped with scales, he notes.

Some researchers use a flank-to-flank measurement to estimate sow body weight. “While this approach seems to have some utility in estimating weight of breeding sows, it is not clear if the prediction equations will be accurate for gilts,” he says.

Be sure to also visually evaluate potential breeding gilts at 250-275 lb. for structural and underline soundness and to determine their fitness for the rigors of breeding.

“Evaluation at this weight will allow culled gilts to be marketed without penalty for being too heavy, reducing gilt wastage rates and helping ensure a long productive life once the female enters the breeding herd,” Johnston says.

Table 1. Relationship of Body Weight and Backfat Depth at 22 Weeks of Age to Lifetime Sow Productivity1
Body weight, lb. Mean body weight, lb. Productive Days Live Pigs Weaned Pigs
147-196 187.7 285 17.9 15.9
196-205 200.4 298 18.7 16.5
205-213 210.0 288 17.8 15.8
213-222 218.5 301 18.6 16.4
222-235 228.4 284 17.5 15.4
235-304 247.1 269 16.6 14.4
Backfat depth, in. Mean backfat, in. Productive Days Live Pigs Weaned Pigs
0.24-0.35 0.32 260 15.8 14.1
0.35-0.39 0.37 277 17.1 15.2
0.39-0.43 0.40 286 17.8 15.5
0.43 0.43 302 19.0 16.7
0.43-0.51 0.48 295 18.4 16.4
0.51-0.94 0.56 304 19.1 16.7
1Data in the table include all females selected to enter the gilt pool even if they never conceived or farrowed a litter. This makes the level of performance quite a bit lower than other reports.