When designing a feeding program for gestating sows, it is important to remember the overall goals for the nutrition program:

  • Prepare sows to be in proper body condition at farrowing;

  • Maximize reproductive performance (farrowing rate, litter size); and

  • Meet the daily nutrient requirements at the lowest cost possible (measured as cost/sow/day).

Overfeeding gestating sows creates unnecessary expenses and potential problems with impaired mammary development, and reduces feed intake in lactation.

Over-conditioned sows used to be the main problem on swine farms. In recent years, thin sows have become a more prevalent problem. Too little backfat reserves can reduce reproductive performance and increase sow mortality. Low backfat reserves also can be an animal welfare concern as thin sows have a greater chance of developing shoulder sores.

Accurate Feeding Levels

There is little disagreement on the importance of having sows in the correct body condition at farrowing. Although there is some disagreement on whether the ideal backfat level at farrowing should range between 16-18 mm (0.62-70 in.) and 18-21 mm (0.70-0.82 in), most people agree that the most important point is to have as few sows as possible under 15 mm (0.59 in.) or over 24 mm (0.94) at farrowing.

The big disagreement among nutritionists, veterinarians and barn managers is over the best way to set feeding levels to make sure sows are not under-or over-fed in gestation.

Backfat scanning on commercial farms has convinced us that a visual body condition score is a poor predictor of actual backfat levels. The best correlation we have found between backfat and condition score on any farm we have measured is an r2 of 0.23, suggesting that body condition score only explains about 23% of the variation in backfat levels.

If body condition score is used to set feeding levels, feed usage should be tracked and backfat at farrowing should be monitored periodically using ultrasound measurement to try to reduce swings in backfat levels for the entire farm (see sidebar). If over 75% of the sows are between 15 and 24 mm at farrowing, you are doing a pretty good job of setting feeding levels during gestation.

Because of our frustration with condition scoring, we have tested and implemented a method to feed sows based on backfat and body weight estimates using the concepts proposed by the late swine nutritionist Frank Aherne, which are presented below.

Whether you feed sows based on body weight and backfat or on body condition score, it is useful to understand the energy requirements of the sows, and the energy level of your gestation diet to determine feeding range for your situation.

Gestating Sows Requirements

The maintenance requirement of the sow accounts for the majority of the feed requirement. Thus, an estimate of body weight is extremely important to accurately feed the sow.

Because weighing individual sows is not feasible on many farms, we have established weight categories that can be estimated by using a girth or flank measurement.

Girth is measured with a cloth tape directly behind the front legs and in front of the first mammary glands. The flank measurement is measured immediately in front of the back legs from the point of one flank over the back of the sow to the point of the other flank. The flank-to-flank measurement is much easier to obtain, especially when sows are housed in gestation stalls.

Because body weight is important in determining the daily feed allotment, it is essential that a high percentage of sows are measured for their body weight estimate.

The daily feed requirement for the sows to maintain body weight increases approximately 0.3 lb. for every 50 lb. increase in sow weight for sows fed a corn-soy diet (Table 1). The sows' maintenance requirements will increase as sows gain weight during gestation.

The next biggest component of the gestation feed requirement is the amount of weight or backfat that you want the sow to gain. If you only feed the sows the maintenance feeding level, they will maintain body weight, but will lose backfat. Sows require approximately ¼ lb. of feed daily for a low level of weight gain to maintain backfat. The daily feed requirement increases approximately 0.4 to 0.5 lb./day for every 3 mm (0.12 in.) increase in desired backfat gain during gestation (Table 2).

Backfat can be measured with one of several different ultrasound machines. The Renco machine is used on some farms because of the relatively low cost. Real-time ultrasound machines used for pregnancy diagnosis can also be used for backfat measurement.

It is critical to properly train the individuals gathering ultrasound measurements, particularly the point where backfat is measured. Sows are scanned at the last rib, approximately 2.5-3 in. off the midline. We recommend scanning the sow on both sides, then averaging the values to determine backfat.

The last component of the gestation feed requirement is fetal and uterine gain. Fetal gain in late gestation increases exponentially, thus, feeding levels should be increased by 1 to 2 lb./day during the last two weeks of gestation to meet this need.

The daily energy requirement for fetal and uterine gain (330 kcal) during the rest of gestation can be met by a relatively low feed level of 0.25 lb.

The requirements for maintenance, weight or backfat gain and fetal growth are added to determine the total daily feed requirement (Table 3).

Set Feeding Levels

Once each week, the person responsible for setting feeding levels scans sows for backfat and determines the weight category for all sows bred during the previous week. The backfat is written on the sow card, and the feeding level is adjusted using a table customized for the farm, based on the energy density of their diet and volume of their feed boxes.

At approximately seven weeks after mating, sows that are visibly very thin are marked and scanned to determine if backfat gains are on target. Approximately 10 to 15% of the sows will have to be scanned at this time.

If the sows are not reaching targets, feed intake is increased by 1 lb./day. Sows remain on their feeding level until Day 100 of gestation, when the feeding level is increased by 2 lb./day for the last two weeks before farrowing.

The procedure is relatively simple and easy to implement. The three critical issues for success are:

  1. A person must be trained to scan and estimate weight;

  2. Know the energy level of the gestation diet; and

  3. Know the volume being dropped at each feed box setting.

Full details on procedures and the spreadsheet can be found at: www.asi.ksu.edu/swine under the swine Extension sow feeding tools link.

Gestation Feeding Patterns

Feeding levels in particular stages of gestation have been shown to influence sow productivity and performance of their offspring. The periods when excessive feed intake are most detrimental are immediately after breeding (Day 0 to Day 2) and from Day 75 to Day 90 of gestation for gilts.

High levels of feed intake after breeding can reduce embryo survival in gilts. Providing gilts high levels of feed from Day 75 to Day 90 of gestation can increase fat deposition in the mammary gland and reduce milk production.

From a practical perspective, feeding pattern is less important than providing a total energy level over the entire gestation period that prevents excessive fat gain or inadequate body reserves at farrowing.

Feeding Once vs. Twice Daily

Although research on this subject is limited, feeding sows once vs. twice a day doesn't appear to change production parameters. Therefore, the choice of feeding frequency is a personal choice.

Some producers cite improved sow satiety and decreased ulcer potential as the reason for choosing twice-per-day feeding. Others will argue that feeding twice a day increases sow agitation and noise levels in the barn. A real advantage with twice-per-day feeding is that synthetic amino acids can be used without worrying about the reduced utilization that occurs when sows are fed once daily.

A practical concern with twice-per-day feeding is that most gestation boxes are relatively large and difficult to set accurately at the low feeding levels required.

Genetic, Parity Differences

Genotype of the sow doesn't have a major impact on the feed requirements for maintenance or fetal gain. However, genetics can vary in sows' milk production and lactation feed intake.

Sows with high levels of milk production and low lactation feed intake will require higher levels of feed during the subsequent gestation period to recover the weight and backfat lost during lactation. If the sows are fed with the weight and backfat procedure, feeding levels will automatically be adjusted to meet the requirement of different genetics.

Similarly, older sows require higher levels of feed intake to meet their maintenance requirement because they are heavier than younger sows. If feed levels are adjusted as sows become heavier, the higher feed requirements for older parities will automatically be met.

Amino Acid Levels

A corn-soybean meal diet containing 0.60 to 0.65% lysine will meet the requirements of most sows. Because threonine requirements are higher for maintenance than many other amino acids, L-threonine must be added with L-lysine HCl if synthetic lysine is used in the diet. Remember, synthetic amino acids are only used at about 50% efficiency when sows are fed once a day.

If parity segregation is being practiced, different gestation diets can be used. The gestation diet for first-parity sows can be formulated to a higher lysine level (0.65%), and the diet for older sows can be formulated to a lower lysine level (0.50 to 0.55%), depending on energy level of the diet.

Water is Critical

Like all areas of production, clean, fresh water should be made available at all times. While this suggestion seems simple, it can be a source of concern if water is not an area of focus.

Plugged nipple waterers, poorly designed water troughs, or incomplete filling of troughs because of feed blockage can all lead to inadequate water intake in gestation. A part of the daily schedule on sow farms should be to ensure that all sows have access to feed and water.

Other Nutrients

Sow gestation diets need to contain the normal minerals and vitamins added to growing pig diets. Besides higher levels of calcium and phosphorus, sow gestation diets also should contain higher levels of specialty vitamins required for fetal development, such as folic acid, choline and biotin. In recent years, research has demonstrated that sow productivity can be further improved with additions of other nutrients, such as chromium and carnitine.

Table 1. Energy and Feed (lb./day) Required to Maintain Body Weight

Dietary energy, kcal/lb.
Sow weight, lb. ME, kcal 1,400 1,500
350 4,741 3.4 3.2
400 5,240 3.7 3.5
450 5,724 4.1 3.8
500 6,195 4.4 4.1
550 6,654 4.8 4.4
600 7,103 5.1 4.7

Tracking Gestation Feed Usage

Regardless of whether backfat and weight or body condition scoring is used to set the daily feed allowance for each sow, it is useful to get a global picture of gestation feed usage for a swine farm to determine whether any long-term trends towards over- or under-feeding is occurring. This can be done simply by dividing the total feed delivery for the period by the number of gestation places in the farm and the number of days in the period.

Certainly, if the sow space is not fully utilized on the farm, this measure will need to be adjusted for actual inventory. However, for most farms, simply knowing the number of gestation spaces is adequate. This calculation is especially useful in production systems with multiple sow farms to determine if one sow farm routinely feeds 5.5 lb./day, while another farm routinely feeds 4.6 lb./day when provided the same gestation diet.

In reality, most farms should have gestation feed usage of 7.2 to 7.8 Mcal metabolizable energy (ME)/sow/day. This equates to 5.1 to 5.5 lb./day of a gestation diet containing 1.4 Mcal ME/lb. or 4.8 to 5.1 lb. of a diet containing 1.5 Mcal ME/lb. If feed usage for the farm is outside of these bounds, reasons for the discrepancy should be explored.

For this example, a 3,000-sow farm containing 2,800 gestation crates used 1,210 tons of feed in a six-month period. The calculations are as follows:

Total Feed/Crates × Days = 1,210 tons × 2,000 lb./2,800 crates × 182 days = 4.75 lb./day

A period of six months or longer should be used with this method to account for fluctuations in feed deliveries. A six-month rolling average is a good method for tracking gestation feed usage.

Table 2. Energy and Feed (lb./day) Required for Backfat Gain

Dietary energy, kcal
Backfat gain, mm ME, kcal 1,400 1,500
0 342 0.24 0.23
3 991 0.71 0.66
6 1,649 1.18 1.10
9 2,307 1.65 1.54

Table 3. Feeding Levels (lb./day) for Gestating Sows Based on Backfat and Weight Category at Breedinga

Backfat at breeding, mmb
Flank-to-flank, in. Estimated weight, lb. 9 to 11 12 to 14 15 to 17 >18
< 35.5 250 to 325 5.0 4.4 3.9 3.4
35.6 to 38.0 325 to 400 5.5 5.0 4.4 3.9
38.1 to 41.0 400 to 475 5.9 5.4 4.9 4.3
41.1 to 44.0 475 to 550 6.4 5.9 5.4 4.8
> 44.0 550 to 650 6.9 6.4 5.8 5.3
aBased on a diet containing 1,500 kcal ME/lb.
Feeding level should be increased by 2 lb./d on Day 101 of gestation.
bMost ultrasound machines measure backfat in millimeters (mm); to convert mm to inches, multiply mm by .039.

Future Research in Gestation Feeding

While optimizing gestation sow feeding to maximize performance and longevity will continue, more specialized research associated with fetal development will grow in popularity.

Specifically, research includes utilizing omega-3 fatty acids of eicosapentaenoic (EPA, C20:5) and docosahexaenoic (DHA, C22:6). These fatty acids are believed to assist in the development of brain function, which may lead to a more viable pig at birth.

Organic selenium sources are now available for use in sow diets. It has been shown that sows fed organic vs. inorganic selenium resulted in higher levels of selenium in sow colostrum and milk, and in piglet tissue. However, impacts on fetal development are currently less known.

Feeding gestation sows L-carnitine has been shown to enhance muscle fiber development in fetuses, which has ultimately led to a greater loin muscle depth and percentage of lean. The exact mode of action for L-carnitine's ability to improve fetal growth and development is unknown. However, it is most likely related to its ability to influence energy metabolism, increase maternal IGF-1 concentrations and influence maternal leptin concentrations.

These are examples of future research areas influencing fetal development, which will continue to improve gestating sow performance.