The costs of achieving 30 pigs/sow/year (p/s/y) may be too high to justify, agree North American reproductive physiologists.

To accomplish the 30 p/s/y goal, we must gain a better understanding of reproductive biology and the “largely untapped reproductive capacity of contemporary dam-line sows,” said George Foxcroft, leader of the swine research and technology center at the University of Alberta, in his opening comments at a day-long reproduction workshop held at the Leman Swine Conference in St. Paul last fall.

Joined by other reproductive specialists, the day was spent challenging what Foxcroft describes as “the comfortable assumption that a lack of reproductive performance in today's industry is due to inherent fertility problems within existing dam-line populations.”

Parity 1 Sow Condition

At the outset, Foxcroft doesn't buy the argument that super-thin sows cannot maintain their condition and perform well. This is a complex issue, reflected in changing wean-to-estrus intervals (WEI), ovulation rates and embryonic survival.

He turned to an ongoing study, in which groups of Parity 1 sows were either fed to appetite (controls) or severely restricted (fed 50% less) during the last week of their 21-day lactation periods. The restricted feeding represented typical sows with low appetite and/or subjected to poor management, he explained.

The control sows generally lost about 22 lb. during lactation, while restricted sows lost nearly twice as much weight.

After weaning, nearly 200 control sows and restricted sows were fed the same diet from weaning to 30 days into gestation during a 2-½-year stretch. Results showed all sows cycled and 92-100% became pregnant, but restricted sows were actually heavier at the 30-day gestation mark. “This represents classic compensatory growth of weaned sows,” he pointed out. Embryo survival was essentially the same in both groups.

Foxcroft's take-home message from the study was twofold:

  • At weaning, Parity 1 sows still have massive requirements for lean growth potential. “If you get them the ingredients to recapture that lean growth potential, they will respond. These are amazing animals in terms of the ability to grow,” he said.

  • From the experimental model, examining the negative effects of catabolism (metabolic breakdown of tissues and excessive weight loss) and the Parity 1 sows' breeding performance (measured by embryo survival), Foxcroft said the experiments reinforced his belief: “This totally removes any excuse at all that there's something wrong with these sows.

“One of the great failures of the pork production industry has been the inability to capture the true production potential of the excellent dam lines already available,” he continued. “To correct these deficiencies, appropriate and achievable key performance indicators (KPIs) should relate closely to known characteristics of commercial dam-line sows and be both measurable and economically meaningful.”

Thus, the stage was set to identify these key drivers, including the physiological challenges and limitations that may prevent producers from achieving the 30 weaned pigs/sow/year benchmark.

“Our producers are trying to make money, and should not be encouraged to see a simplistic measure of productivity, like maximal numbers of pigs produced, at any cost, as a worthwhile goal,” Foxcroft stated.

“A consistent flow of good quality weaned pigs should be the principal goal, and is not necessarily best served by developing breeding herd strategies that assume the hyper-prolific sow will necessarily meet this need,” he continued. “Indeed, increased prolificacy may well be associated with increased variability in weaned and finished pig quality.”

Biological Changes

“The need to develop management techniques that reflect the changing biology of contemporary dam-line females seems to be an urgent issue,” Foxcroft continued. Much of this effort should be aimed at improving the performance of second-parity sows.

“But, producers must also study the growing evidence that prenatal programming affects post-natal performance. In the end, the most profitable production systems may need to consider segregated parity and litter management as a more profitable strategy.

“Once you get to an acceptable gilt litter size — from 10 up to 15 pigs — we are definitely having an impact on the birth weight of the pigs. This leads to a whole list of questions about whether offspring (of gilts) are appropriate to place alongside the offspring of mature sows.

“I think there is more and more acceptance that the growth potential of these pigs (from P1 sows) is limited. Also, it is likely that their survivability and health status are also compromised, because we are pushing these gilts very hard, and there is some sort of a programming effect that we need to consider,” Foxcroft said.

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Sow Performance Risk Factors

“We've got to get the management of the gilt as a Parity 1 female right — on that, there is no debate,” Foxcroft said. Nearly two-thirds of reproductive problems are associated with failure to observe estrus and failure to conceive in gilts and Parity 1 sows.

Producers will likely never be in the position of culling sows strictly on their reproductive performance. Table 1 on page 28 illustrates the reasons for sow removals from several commercial herds in North America between 1999 and 2004.

In North America, producers consistently turn over 30% or more of the sow herd, thereby limiting production to less than 1.5 litters from those sows, he added.

“If we can get a gilt into the herd and get her to produce her second and third litter, and keep our retention rates in the 70-80% range, we've pretty well solved most of the problems we have today,” he argued. “The reason we have average parity structure of about three litters/sow lifetime is that we have a whole subset of the herd that are continually taken out.”

Some blame inadequate body condition for this cull rate. Foxcroft doesn't. In most “reasonably managed systems,” it's pretty difficult to find any association between body weight and backfat depth and subsequent performance, he said.

If sows are selected for reasonable physical maturity, with some emphasis on reproductive merit (recorded estrus,100% bred by second estrus, etc.), much of the challenge to keep sows in the herd is met.

Reproductive merit is easily tracked, but to get individual body mass, sows must be weighed or measured with a weight tape.

“I don't see any way around that,” Foxcroft continued. “When you look at a population of gilts or sows, with their range of weights and fatness, there's no way you can guess accurately. You have to manage the individual females, and that means you have to have data on the individuals.”

And don't succumb to the temptation to make sows heavier and fatter than they need to be as insurance, he warned. “In most modern genotypes, that puts them at risk of being over-conditioned, and it just pours money down the drain.”

Is there any reason to pass over the second estrus and breed on the third or fourth? Foxcroft doesn't think so. “Any analysis I've seen says each estrus will cost you $50 in non-productive sow days (NPSD). In a 3,000-sow herd with a typical 50-60% replacement rate, those unnecessary cycles could cost you $90,000/year. That's big money!”

The challenge, then, is to better understand the metabolic and physiological drivers of body condition and reproductive performance.

Foxcroft defined body condition as a combination of live weight, backfat and lean body (protein) mass. Typically, sows gain condition in gestation and lose condition in lactation. The latter tends to dictate the body reserves needed to prevent poor performance. Therefore, protein mass and protein turnover are the key drivers.

“It's the extremes that will kill you. If you get them too fat or too heavy, you're equally going to run into problems. At the other extreme, too thin means too light, and this will cause welfare problems, probably reduce longevity and extend wean-to-service intervals,” he explained.

“Because of the selection pressures that we put on our dam lines, they are remarkably good milking machines. I think we underestimate the amount of milk a sow can produce. If they have small reserves of lean and fat, they won't be able to meet the demands of milk production, and that's when we end up with the extremes of catabolism.

“A decline in subsequent reproductive performance starts to occur when sows lose about 10-12% of the body protein present at farrowing,” Foxcroft said.

“The lighter the animal, the smaller the protein reserves to draw upon. To help ensure good herd productivity and high longevity, sow weight after first farrowing should be 385-410 lb. with an assumed backfat target of 0.66-0.78 in. (17-20 mm.) at farrowing,” he said. Targets will vary with management systems, genetics, diet and facilities.

“Changes in protein mass appear to be more closely linked to changes in sow lifetime productivity than do changes in fat reserves,” he commented. “From a practical perspective, recent research suggests that it is simpler to achieve designated target weights at breeding, and incremental increases in sow weight (lean tissue mass), than it is to manipulate backfat.”

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Some Things Have Changed

Wageningen University researcher Bas Kemp, from The Netherlands, said today's sows respond differently to low feed intake during lactation.

Citing studies conducted over a 35-year time frame, he noted: “During the '70s and early '80s, feed restriction resulted in quite dramatic effects on wean-to-estrus intervals (about 10 days) and hardly had any effects on ovulation rates and embryo survival.”

The trend has been reversed in modern-day sows. “Recent papers show the effects of wean-to-estrus interval are quite small (less than a day), while effects on ovulation rate and embryo survival are far more pronounced. Feed restriction seems to decrease ovulation rate by 2-4 (eggs) and embryo survival by 10-20%,” Kemp observed.

Kemp said this slippage is a result of strong selection against variation in wean-to-service interval the past 20 years. “This implies that in the modern sow, the effects of low feed intake during lactation on wean-to-estrus intervals is less pronounced, but the effects on litter size are more pronounced — especially in first-litter sows,” he said.

And selection for higher ovulation rates has impacted the physiological components of litter size. In a review of 78 research papers from 1954 to 1986, embryo survival at 28 days of gestation held at 74-79%, and fetal survival was recorded at 69-74%. More recent data has signaled substantial changes in ovulation rate (increased to about 25 eggs), but embryo survival has declined to about 60%, while fetal survival has slipped to only 50%, he stated.

Therefore, selection for litter size appears to have substantially increased ovulation rate and, with high fertilization rates (90% or more), high numbers of embryos can be found early in gestation.

However, uterine capacity has not expanded with higher ovulation and fertilization rates; therefore, embryo and fetal mortality rates are proportionately higher, Kemp said.

Foxcroft concurred: “We seem to have ended up with a situation where we have more embryos available than we have uterine space to put them. The whole dynamic of pig development is changing because we are pushing these hyper-prolific traits in some of our sow lines.

“We've found in mature sows, rather than 12-18 eggs ovulated, we're now dealing with populations of sows that ovulated 20-30 eggs,” he added.

With very high ovulation rates, many of the ova are impregnated and the embryos survive. But, when the uterus gets very crowded during the fetal development period, up to half the litter does not survive. “That's not in the textbooks,” Foxcroft assured.

Even with the massive losses, modern, prolific sows will farrow 12-20 pigs. “So, in these mature sows, we're losing 40-60% of the potential litter during gestation,” he added.

Costs of Uterine Crowding

“Selection for litter size without concurrent selection for uterine capacity, piglet viability and survival will inadvertently intensify the adverse impacts of uterine crowding, particularly in early gestation and in highly prolific sows,” added John Harding, DVM, the University of Saskatchewan Western College of Veterinary Medicine. “More-over, uterine crowding has economically important consequences for the surviving progeny that potentially extend into adult life.”

In his work, Harding found embryonic survival rates exceeded 60% into the very critical period of fetal development in early gestation. “While the upper limit of uterine capacity for litter size is historically considered to be 14 embryos, the organ/tissue development of surviving fetuses was adversely affected when gestational litter sizes exceeded 9-10. Thus, uterine crowding is a common phenomenon. It limits fetal number and size, and also has a profound impact on the development and weight of the surviving embryos that ultimately impacts post-natal performance.”

“If you crowd the uterus, the first thing you do is reduce the potential size of the fetuses,” agreed Foxcroft. “If we have a uterus with twice the number of embryos, they're going to be competing for nutrients. With this limitation, we start to see intrauterine (IUGR) growth retardation effects, or runting.”

Under these circumstances, the brain is the priority, so other tissues and organs suffer, such as muscle fibers. The same holds true for gut development. This is called the “brain sparing” effect.

“We have a little pig that is severely compromised, and no amount of love, crossfostering, treatment or feeding will make him a normal pig. He is not,” Foxcroft emphasized. Most of these effects become a limiting factor after Day 25 and up to Days 30-40 of gestation.

Harding cited recent work by John Deen, DVM, University of Minnesota, which showed that with each additional 0.25 lb. of birth weight, the likelihood of preweaning mortality was reduced by 0.4%.

“Genetic selection against piglet mortality may not necessarily increase birth weight, but it will increase body fat deposition and proportional organ development, which is vital to piglet survival,” Harding said.

“While attaining 30 pigs/sow/year will be commendable, attaining 30 p/s/y without consideration for the hidden ramifications of uterine crowding, intrauterine growth retardation and pre-natal maternal stress will be reckless,” he added.

Foxcroft, drawing on French research on hyper-prolific sows, said in closing: “Yes, we are getting more total pigs born, but as a proportion of total pigs born, we're weaning an increasingly smaller percentage. So the question is: How far do we want to follow these trends before we end up with such a poor population of weaned pigs that it is really not a logical way to go?”

Table 1. Example of the Percentage of Sow Removals, by Parity and the Various Reasons for Removals
Parity
0 & 1 2 to 5 6 & 7 All
% of all sow removals 40 45 7
Removal reasons
Reproductive, %* 59 32 13 39
Locomotor disorder, %** 22 25 14 21
Farrow performance, %*** 3 15 15 11
Body condition, % 2 2 2 2
Parity/size, % 0.4 3 45 11
Sudden death, % 6 6 4 5
* Reproductive reason: no heat, not in pig, repeat, reproductive, aborted
** Locomotor disorder: lameness, splayed legs and issues with legs and feet
*** Farrow performance: farrowing productivity, high stillbirths, lactation-weaning productivity, rearing performance,farrow performance, farrowing complications, difficult farrowing and retained pigs
Source: Cowes (unpublished data): 34,802 animals (1999-2004)