Effects of Energy Intake on Gilt Development

Performance of energy-restricted and ad libitum-fed gilts were tracked through four parities.

A common target weight before breeding replacement gilts is 300 lb., and it is generally believed that some minimum amount of backfat is needed for successful reproduction. Thus, gilts are often given ad libitum access to feed.

Experiments at the USDA Meat Animal Research Center have demonstrated that moderate feed restriction during prepubertal development of gilts may increase reproductive efficiency through Parity 1, but there is concern that this management strategy may be detrimental to long-term reproductive performance.

To address these concerns, an experiment was designed to examine whether restricting energy intake during gilt development will increase their longevity and lifetime productivity.

Because optimum gilt development regimens may vary depending on the genetic lines' prolificacy and rate of lean growth, gilts of two lines that differed in fertility, litter size and rate of lean growth were managed with either ad libitum access to feed or with 25% restriction of energy from 123 days of age to breeding age.

Reproduction through Parity 4 was evaluated. The experiment had four replications and included 661 gilts. The fourth replication, completed in summer 2009, concluded the experiment and a four-parity summary of the production parameters were reported.

Two populations of gilts were used. One was the Large White × Landrace crossbred female used in the University of Nebraska-Lincoln (UNL) swine nutrition program. The project gilts were the progeny of Large White-Landrace crossbred sows inseminated with semen of industry maternal line (LM) boars, which were designated as LW × LR cross. The other population was progeny of UNL selection Line 45 sows that had been inseminated with semen of the same LM boars. Those gilts are designated as Line 45X. Line 45 has now undergone 29 generations of selection for increased litter size with additional selection for increased growth and decreased backfat in the last seven generations. The Line 45 dams in this study were from generations 25, 26 and 27.

Line 45X gilts were expected to be more prolific than LW × LR gilts, but typically would grow slower and have more backfat.

Project gilts were born in batches during December 2004 and January 2005 (Rep 1), May 2005 (Rep 2), November 2005 (Rep 3), and May and June 2007 (Rep 4). Of the 661 gilts that began the experiment at 60 days of age, 639 gilts completed the growth phase that ended at 226 days of age.

Dams of project gilts were managed alike during the farrowing/lactation period. After weaning, all gilts were managed alike in the nursery until approximately 60 days of age (46 lb.), when they were moved to grow-finish and placed 10/pen by line-treatment designation. All gilts were allowed ad libitum access to a corn-soybean meal-based diet and managed alike until 123 days of age.

A three-phase, grow-finish diet was used: Phase 1 with 1.15% lysine fed 60 days of age to 80 lb., Phase 2 with 1.0% lysine fed 80 to 130 lb. and Phase 3 with 0.90% lysine fed 130 lb. to 123 days of age.

At 123 days of age, pens of gilts on the ad libitum regimen (A) were allowed full access to a corn-soybean meal-based diet (0.70% lysine, 0.70% Ca, 0.60% P) until they were moved into the breeding barn. Gilts on the restricted intake regimen (R) received a corn-soybean meal-based diet at approximately 75% of the energy intake as A-fed gilts until being moved to the breeding barn.

Energy restriction was achieved by predicting intake with a quadratic equation of average daily feed intake on body weight of A-fed gilts. The predicted ad libitum intake (based on the projected body weight for the upcoming two-week period) was multiplied by 0.75 to determine the daily feed intake for R gilts. The diet contained 0.93% lysine, 1.0% Ca, and 0.80% P. All vitamins and minerals, except selenium, were increased so that daily intake of these nutrients per unit of body weight was expected to be equal for gilts on both diets.

Gilts were weighed and backfat and longissimus muscle area recorded every 14 days until final measurements were recorded at an average age of 226 days. Beginning at approximately 140 days of age, gilts were moved by pen to an adjacent building where boar exposure and estrous detection occurred. Date of first observed estrus and subsequent estrous periods were recorded.

Gilts in good health that could be mated at second or later estrus during a predetermined breeding period were identified as breeders (Brds) and moved to the breeding barn at approximately 230 days of age. Breeding commenced approximately 10 days later.

A breeding period of 25 days (Rep 1), 24 days (Rep 2), 26 days (Rep 3) and 28 days (Rep 4) was used to match the unit's production schedule. Gilts were checked twice daily for estrus and inseminated each day they were observed in estrus. Semen from commercial terminal sire line boars was used.

Gilts were housed approximately eight per pen until inseminated, then moved to gestation stalls. Gilts that did not express estrus, were mated but diagnosed open with an ultrasound pregnancy test 50 days post-breeding, were diagnosed pregnant but did not farrow a litter, or were lame or were in poor health, were culled.

During breeding and gestation, all gilts were fed 4 lb./day of a standard corn-soybean meal-based diet (13.8% protein, 0.66% lysine) until 90 days of gestation, then fed 5 lb. daily. At approximately 110 days of gestation, females were weighed, scanned for 10th rib backfat thickness and placed in farrowing crates (12 crates/room), where they were fed 6 lb./day of a corn-soybean meal-based lactation diet (18.5% protein, 1.0% lysine) until farrowing.

Females received a small amount of feed on the day they farrowed, 6 lb. on the second day and 10 lb. the third day, then were given ad libitum access to feed.

Total pigs born and number born live in each litter were recorded. Pigs were fostered among litters without regard to line or gilt developmental regimen to reduce variation in number nursed per sow. Average age at weaning was 17 days, with number weaned and total litter weight recorded. Weight and ultrasonic backfat depth was recorded on each sow at weaning. Sows were moved to breeding pens and housed approximately eight per pen.

Breeding continued until 10 days after the last sow in the replication was weaned. The breeding period for sows within replications and parities ranged from 24 to 32 days. Feeding, estrous detection, insemination, and management during gestation and subsequent lactations were as described above for gilts.

Sows that did not express estrus, those found open by ultrasonic pregnancy test and those diagnosed pregnant but did not farrow, were culled, as were lame and unhealthy sows.

Reproductive success is an all-or-nothing outcome; gilts and sows either produced a litter or did not. This outcome is a binomial trait that can be coded as 1 (success) or 0 (failure). Gilts completing the growth test were coded as 0 if they did not express a pubertal estrus and 1 if they did. Then, based on females designated for breeding, they were coded as 1 or 0 if they did or did not produce litters at each of four parities. Each female received four scores.

A gilt designated for breeding that did not produce a Parity 1 litter received scores of 0, 0, 0, 0 for reproductive success at each parity. A gilt that produced a Parity 1 litter, but not a Parity 2 litter, received scores of 1, 0, 0, 0; gilts producing two litters received scores of 1, 1, 0, 0, and so on.

The scores measure reproductive success rates. They were fitted with general linear models designed for binomial data to determine the importance of line, gilt treatment, and interaction of line with treatment on reproductive rate through four parities. The effects of replication, sire, and litter of gilt were fitted in models as random effects to account for those sources of variation.

Number of live pigs per litter and total litter weight at weaning, by sow, were analyzed with models that included line, treatment, interaction, random effects of replication, sire and litter of gilt. Number of pigs the sows was given to raise (number after crossfostering) and age at weaning were included as covariates to adjust all sows to a common number nursed and lactation length.

Lifetime productivity of each female designated for breeding was calculated as the total number of live pigs, total number of weaned pigs and total weight of weaned pigs through Parity 4. These measures of lifetime production were fitted to the same model as described above.

Trial Results

Table 1 (page 21) shows the numbers of gilts at each stage of development. Of the 661 gilts starting the experiment, 22 died or were removed for structural or health reasons. The losses were approximately equal across lines and treatments.

Of the 639 gilts completing the developmental period, 568 expressed a pubertal estrus by 230 days of age, but 15 were culled because they expressed estrus very late in the period and could not be mated at second or greater post-pubertal estrus.

Line and treatment affected both age at puberty and the proportion of gilts that expressed pubertal estrus. Eleven gilts that expressed estrus either died or were culled for structural or health reasons before breeding, and an additional 33 gilts that qualified as breeders were culled at random to reduce numbers to available breeding and farrowing spaces. A total of 509 gilts were designated as breeders, on which lifetime production scores and productivity were recorded and analyzed.

Table 2(page 22) illustrates the effects of line and treatment on growth, average final weight, backfat, longissimus muscle area (LMA), and age at puberty. Lines responded similarly to treatment as interaction of line by treatment was not significant for any trait. LW × LR gilts were 8.8 lb. heavier with 0.05 in. less backfat and 2.2 sq. in. greater LMA than L45X gilts. L45X gilts were 5.6 days younger at puberty than LW × LR gilts. Treatment effects were significant for all traits.

Gilts on the 25% energy restriction regime weighed 49.3 lb. less, had 0.39 in. less backfat, 0.82 sq. in. less LMA, and were 3.4 days older at puberty than gilts fed ad libitum.

Causes for Culling

Table 3 (page 22) contains the number of females that produced litters at each parity and numbers removed due to deaths, reproductive failure, health and/or structural conditions. Most losses were mated sows that subsequently returned to estrus, which accounted for 13, 17, 11, and 16% of Parities 1 to 4, respectively. Culling for return to estrus after insemination did not differ significantly between lines or treatments, although it was somewhat greater for LW × LR than L45X sows (16% vs. 13%), and for gilts developed with restricted energy intake vs. those fed ad libitum (13.5% vs. 10.3%).

The second most common cause for culling was failure to express estrus during the breeding period (including those that the technicians may have missed). Failure to express estrus was low in gilts (3.5%); quite high for Parity 1 sows (16%); and low again for Parity 2 and 3 sows (4.3% and 2.4%, respectively).

In many herds, recycles are inseminated again. Some, but not all, eventually conceive. To ensure a uniform culling policy for all replications, parities, lines and treatments, those sows were culled in this experiment.

Of the 509 gilts designated for breeding, 27 (5.3%) died during one of the four gestation periods and nine (1.8%) died during farrowing.

Likelihood of Success

Table 4 contains the probability of reproductive success. The first column indicates the probability that a gilt that finished the development period expressed estrus by 230 days of age. Genetic line and treatment significantly affected this probability; 95% of L45X gilts reached puberty, vs. 88% of LW × LR gilts; 96% of gilts developed with ad libitum intake reached puberty, compared with 86% of gilts on the energy-restriction diet.

Probabilities of females producing Parity 1 to 4 litters are all based on gilts designated as breeders at 230 days of age. No effect, line, treatment, nor interaction was significant for any of these probabilities. The greatest difference was between lines for the probability that gilts designated as breeders produced a Parity 1 litter, which was 84% for L45X gilts and 78% for LW × LR gilts. The probability that females produced Parity 2, 3, and 4 litters was greater for those developed with restricted energy intake, but the differences were not statistically significant.

Still, line differences in this experiment are consistent with those observed in previous comparisons. Line 45X females had 0.4 more live pigs/litter than LW x LR females, but their maternal abilities were not as good. When given an opportunity to raise the same number of pigs, LW × LR females weaned 0.25 more pigs/litter and total litter weights were 5.5 lb. heavier than the L45X females. Gilt development regimen had almost no effect on subsequent litter size or maternal ability in either line.

All measures of lifetime production were greater for L45X females than LW × LR females and for females developed with restricted energy intake (Table 5). However, none of these measures — total pigs born alive, total number weaned, total weight of litter weaned — were significantly affected by line, treatment or interaction.

Lifetime sow productivity is a difficult trait to evaluate experimentally. The observed differences were relatively large, ranging from 8 to 11%, and if real, are economically important. Yet in this experiment, where 509 females produced 1,049 litters and in which variation was controlled and culling criteria strictly followed, natural variation was still large enough that observed differences could be explained by chance.

Thus, we conclude from this project that prolific gilts that differ in rate of lean growth respond similarly to a developmental regimen in which energy intake from 123 days of age to breeding was restricted to 75% of that of gilts developed with ad libitum intake. Further, this energy restriction decreased the proportion of gilts that had expressed estrus by 230 days of age and increased the age at puberty for those that did express pubertal estrus. Thereafter, females developed with both regimens had similar reproductive performance at each parity and similar lifetime production.

Researchers: Rodger Johnson, Phillip Miller, Roman Moreno, Matt Anderson, Jeff Perkins, Donald McClure, Tom McGargill, Arlan Kettelhake and James Spath, University of Nebraska-Lincoln. Contact Johnson by phone (402) 472-6404 or e-mail [email protected].