Pork producers in the United States shaved 9.5 days off the average age at weaning (from 28.8 days down to 19.3 days) between 1990 and 2000, according to figures from the U.S. Department of Agriculture.

This transition to early weaning produced healthier, more uniform pigs, improved lifetime sow productivity and allowed herds to market more pigs/year.

However, PigChamp summaries show average weaning age has been creeping up for the last five years (Figure 1) in an effort to optimize overall reproductive efficiency.

Maximum throughput in a pork production system can only be achieved when the use of farrowing facilities is maximized, and weaning age does not significantly reduce sow reproductive performance.

What, then, is the impact of weaning age on sow herd performance?

Uterine Involution

The sow remains anestrus during lactation because the suckling influence of the litter suppresses ovarian and pituitary hormonal activity, and stimulates the release of prolactin.

Once suckling is reduced or the litter is weaned, prolactin levels gradually decline, and increases in blood levels of luteinizing hormone (LH) and estradiol work to stimulate estrus. It appears that the hormonal suppression provides the sow time for what is called “uterine involution.”

For the sow to breed back satisfactorily, the uterus must undergo this involution process in which it rapidly loses length and weight during the first two to three weeks of lactation. Involution continues until after weaning (Figure 2).

Involution allows for uterine repair so that it can maintain future pregnancies. The rates of uterine involution and wean-to-estrus interval, and subsequent embryonic deaths, have been linked to lactation lengths of less than 19 days.

At the extreme, weaning sows within 24 hours of farrowing may also cause the formation of cystic follicles because both LH and follicle-stimulating hormone (FSH) have not been suppressed. It takes a minimum of two to three days to suppress LH and FSH.

Sows with cystic follicles will show one or more of the following: prolonged and unpredictable return to estrus, constant estrus, prolonged anestrus and irregular estrus.

Wean-to-Estrus Interval

Studies indicate that wean-to-estrus interval decreases as weaning age increases (Figure 3). Wean-to-estrus intervals are shortest for sows weaned between three and four weeks after farrowing. Sows weaned at less than 10 days of lactation demonstrate a much longer wean-to-estrus interval.

In contrast, a greater percentage of sows that lactate for more than 20 days return to estrus by Day 7 than do sows that lactate for 14-15 days.

Parity 1 sows are more prone to increased wean-to-estrus intervals associated with lactation lengths averaging less than 21 days than multiple-parity sows (Figure 4). This may be related to feed intake during lactation.

Minimizing weaning ages of less than 17 days, especially in gilts, will aid in balancing return to estrus with maximizing throughput.

Since wean-to-estrus interval typically has an inverse relationship with duration of estrus, sows that lactate for a short period vs. a long period tend to have shorter estrous periods.

Weaning age can potentially impact insemination protocols. However, a simple and consistent protocol that delivers two inseminations during estrus, separated by 24 hours, appears to consistently optimize sow reproductive performance.

Acceptable wean-to-estrus intervals for weaning litters between 10 and 19 days of age can be achieved when multiparous sows eat an average of 12.5 lb. or more each day.

Maximum feed intake needs to be achieved during lactation to maintain body condition. Extremely thin sows resulting from inadequate energy intake during lactation often experience reproductive failure regardless of weaning age.

Maximum feed intake by sows during lactation is very important and very difficult to achieve. Feed intake during lactation is maximized by increasing feeding frequency, ensuring that feed is fresh, increasing the nutrient density of the diet, and providing for a constant water supply that can deliver 0.25 gal./minute. (See related article on page 21).

Feed intake can also be maintained by appropriate environmental management to minimize the effects of heat stress.

Ovulation to Farrowing

Most research studies have concluded lactation length doesn't influence subsequent ovulation rate (Figure 5). However, the conception rate of the sow does generally decrease with shorter lactation lengths (Figure 6).

Research also shows that embryo survival decreases as lactation length decreases. It is possible that the reduced embryonic survival rate of sows weaned at less than 21 days is related to incomplete restoration of the uterine endometrium (the mucous membrane lining the uterus).

Farrowing rate also decreases as weaning age decreases. Farrowing rate is significantly lower when pigs are weaned at 11-19 days, compared to weaning at 23-25 days of age.

Furthermore, a substantial amount of variation occurs when sows are weaned at eight to 10 days of lactation.

Reduced conception and farrowing rates will also vary between farms due to other management factors.

Subsequent Litter Size

Studies have shown sows weaned across a range of lactation lengths have substantial variation in subsequent litter size (Figure 7). However, subsequent litter size will generally be lower for sows with litters weaned at less than 18 days. Subsequent litter size of early weaned sows may be influenced by time needed for uterine involution, ovulation rate, fertilization rate and embryo survival rate.

Weaning Weight

Obviously, sows that lactate longer will produce heavier litter weaning weights. Average litter weaning weight increases significantly between 15 (96 lb.), 18 (103 lb.), 21 (112 lb.) and 24 (123 lb.) days of lactation.

Sow Longevity and Condition

Sows with short lactation periods have a greater number of litters/female/year, which creates a greater metabolic demand on their reproductive systems. This demand appears to result in higher culling rates, as indicated by a lower average parity at removal for herds with shorter lactation lengths, compared to herds with longer lactation lengths. However, there is some compensation for sows with shorter lactation lengths, as they lose less body weight during lactation.

Genetics

All genetic lines tend to respond in the same curvilinear fashion with increasing wean-to-estrus interval and decreasing lactation length. However, some genetic lines are less responsive to change in weaning age than others. Therefore, producers should evaluate the effect of reducing weaning age in their own herd.

Pigs Weaned/Sow/Year

Pigs born alive, preweaning mortality and litters/female/year influence the number of pigs weaned/sow/year. While shorter lactation lengths negatively impact the number of live born pigs, it can improve preweaning mortality and litters/mated female/year.

Reductions in preweaning mortality at earlier weaning ages may reflect, in part, their greater risk of dying during the longer lactation periods.

Analysis of PigChamp data shows that pigs weaned/sow/year increased as lactation length decreased from 25 to 13 days (Figure 8).

However, there is a substantial amount of variation in pigs weaned/sow/year between farms, regardless of weaning age. Management skill, production environment, nutrition and genetics all influence each farm's reproductive performance at a given weaning age.

Summary

Weaning age has an important influence on litters/mated female/year. But to maximize overall sow herd performance, weaning age must be set at an appropriate level. Reducing lactation length will decrease subsequent fertility of the female by extending the wean-to-estrus interval, reducing conception rate and decreasing subsequent litter size.

Therefore, to maximize throughput in an operation, set the weaning age where it least affects sow reproductive performance. In most herds, the greatest impact on reproduction is observed in lactation periods of less than 17 days. If a sow is to be rebred, she needs a minimum of three days of nursing to suppress the secretion of LH, avoiding the formation of follicular cysts or remaining anestrus.

Careful monitoring and maximizing lactation feed intake to achieve more than 12 lb./day can minimize the impact of shorter lactations.

However, many of the effects of weaning age on reproductive performance are herd specific. Farms with the same genetics, nutrition, facilities, health status and standard operating procedures can have very different responses to short lactation lengths.

Regardless of weaning age, skilled sow farm personnel using good management practices can make a solid contribution to any breeding program. Each farm should independently evaluate the impact of weaning age on productivity and throughput before implementing changes.