Genetic Basis for Variation in Weaning to Estrus and Weaning to Farrowing Interval in Sows

Rodger Johnson

University of Nebraska

INTRODUCTION

Variation in weaning to estrus contributes to variation in traits such as weaning to successful service, non-productive sow days, and farrowing intervals. These traits are components of variation in litters per sow per year and number of live pigs per sow per year, traits often used to measure the reproductive efficiency of herds.

Many factors, including weaning age, nutrition, parity of sow, and type of housing, affect the interval from weaning to first observed estrus in sows. Variation is quite great for sows of all parities, but is most obvious in sows after weaning their first litter. Much of the variation is due to management and environmental effects; but genetic variation also exists because mean interval from weaning to first service differs among breeds and breed crosses.

The purpose of this report is to summarize research on genetics of rebreeding intervals. Distributions of weaning to estrus and weaning to farrowing intervals, and their heritability and relation to production traits are presented. Procedures to include weaning to first service interval in genetic evaluation programs are discussed.

DISTRIBUTION OF WEANING TO ESTRUS INTERVAL

ten Napel et al. (1995) determined that variation in the interval from weaning to first observed estrus is a mixture of two types of distributions. It is illustrated in Figure 1. Part of the distribution is normal and has the bell-shaped curve typical of most traits. But the distribution has a long right hand tail caused by sows with prolonged intervals. Some sows have a mildly prolonged interval and cannot be separated from those sows in the right hand side of the normal distribution. Other sows have a distinctly prolonged interval that extends far to the right of the normal curve. This tail is best described with an exponential distribution. The observed distribution is the sum of the two components, each weighted by their frequency.

The distribution in Figure 1 is for the control line in an experiment done in The Netherlands in which selection was for decreased weaning to estrus interval in gilts after weaning their first litter between 32 and 38 days of lactation. There were 31.4 % of the sows in the normal part of the distribution with a mean weaning to estrus interval of 5.6 days and a standard deviation of .97 days. Approximately 95 % of sows with normal intervals expressed estrus between 3.6 and 7.6 days after weaning. However, 68.6 % of the sows were in the exponential distribution which had a starting point of 5 days and a mean of 36.9 days. A property of exponential distributions is that the mean and variance are equal, thus the standard deviation of records in the exponential part was 6.1 days.

SELECTION FOR REDUCED WEANING TO ESTRUS INTERVAL

Eight generations of selection for decreased weaning to estrus interval were practiced in the experiment of ten Napel et al. (1995). Almost all selected dams in the select line had weaning to estrus intervals of 4 to 6 days. Control sows were selected randomly and mated when they expressed estrus. The response is summarized in Table 1. The decrease in weaning to estrus interval in the select line occurred entirely from a decrease in the proportion of sows with prolonged intervals. Selection did not change the parameters of the distribution. The average and variance for sows in the normal and exponential parts were nearly identical for both lines, as were the starting points of the exponential distribution. The parameters in Table 1 are averages over all 8 generations of selection, and thus reflect differences after 4 generations. Total response in the select line was predicted to be an increase to 47.2% of sows in the normal part of the distribution after 8 generations.

Table 1. Average distribution of weaning to estrus interval during 8 generation of selection (after ten Napel et al. (1995)


Selection Line

Control Line

Normal distribution



Fraction

.393

.314

Mean

5.57

5.63

Standard deviation

1.08

.971




Exponential distribution



Fraction

.607

.686

Starting day

36.8

36.9

Mean (= variance)

5

5

Variation explained

94.9

92.9

DIFFERENCES AMONG POPULATIONS IN DISTRIBUTIONS OF REBREEDING PERFORMANCE

Other studies confirm that the distributions of rebreeding intervals have the general shape of those found by ten Napel. ten Napel and Johnson (1997) studied the distribution of weaning to farrowing interval in Large White and Landrace sows in two farms. A total of 3,243 intervals were analyzed. Litters were weaned between 17 and 23 days of age.

The weaning to farrowing interval includes the weaning to first service interval, the interval between services for repeat matings, and variation in gestation length. Some truncation of data occurred as sows not expressing estrus within 25 days of weaning were usually culled. All populations had very similar distributions with nearly identical parameter estimates (Table 2). Populations differed because of the different proportion in each part of the distribution.

The proportion of sows with normal intervals was greater for Large White than Landrace, and was greater for second parity sows than first parity sows. Subtracting 114 days, a normal gestation length, from the mean of sows in the normal distribution, and from the starting point of the exponential distribution, gives values of approximately 8 days for the normal mean, and 6 days for the starting point of the exponential distribution. These values are very similar to those of the distribution of weaning to estrus interval found by ten Napel et al. (1995).

Table 2. Parameter estimates for distribution of weaning to farrowing intervals after ten Napel and Johnson (1997)



Normal

Prolonged



n


Proportion


Mean

St. Deviation

Mean (=var)

Start Day

Farm 1







Parity 1

612

.52

122.6

2.3

34.6

119

Parity 2

426

.65

121.8

2.1

32.8

120

LW

1077

.70

122.0

2.4

36.3

120

LR

507

.48

121.8

2.0

29.4

119

Farm 2







Parity 1

613

.60

122.6

2.7

26.9

119

Parity 2

442

.72

122.0

2.6

28.5

119

LW

1105

.76

121.8

2.5

31.0

119

LR

554

.52

122.2

2.4

24.7

120

Greater variation in weaning to farrowing interval than weaning to estrus interval is due to the addition of intervals between services due to conception rates being less than one, and the small amount of variation in gestation length. The distribution of intervals after first, second and third parities in Figure 2 illustrates that sows of different parities differ only because of different proportions in the normal and prolonged parts of the distribution. Means and standard deviations for sows in the normal part of the distribution were nearly equal for all parities.

Adamec and Johnson (1997) found distributions of weaning to service and weaning to conception intervals in sows in the Czech National herd were similar to those described above. Records of 2,896 Landrace and Large White sows in 7 farms were analyzed. Weaning age averaged 27.7 days, and varied from 15 to 49 days for individual sows. The mixture distribution explained a high percentage of the variation in both weaning to service and weaning to conception intervals. The proportion of sows in the normal part of the distribution of weaning to conception interval varied from .35 to .69 among farms. The mean of the normal distribution was 8.7 days with a standard deviation of 2.76 days. The starting day of the exponential distribution was 5 days.

HERITABILITIES AND CORRELATIONS OF REBREEDING INTERVALS

Studies done in the 1970's and 80's found heritability of weaning to estrus interval of approximately .2, but heritabilities of farrowing intervals were lower, approximately .1. Results of more recent studies are very similar (Table 3). The estimate from ten Napel et al. (1995) is a realized heritability from selection; the other estimates are maximum likelihood estimates calculated with relationships among relatives.

Table 3. Heritability estimates of rebreeding performance

Interval

Breed

h2

n

Sourcea

Weaning to estrus, parity 1

Dutch Landrace

.17

2133

1

Weaning to farrowing, parity 1, farm1

Large White & Landrace

.00

987

2

Weaning to farrowing, parity 2, farm1

Large White & Landrace

.05

658

2

Weaning to farrowing, parity 3, farm1

Large White & Landrace

.24

539

2

Weaning to farrowing, parity 1, farm2

Large White & Landrace

.12

935

2

Weaning to farrowing, parity 2, farm 2

Large White & Landrace

.16

635

2

Weaning to farrowing, parity 3, farm2

Large White & Landrace

.00

545

2

Weaning to service, across parities

Czech LW & LR

.14

2896

3

Weaning to conception, across parities

Czech LW & LR

.06

2896

3

Weaning to farrowing, across parities

Czech LW & LR

.05

2896

3

a 1 = ten Napel et al. (1995); 2 = ten Napel & Johnson (1997); 3 = Adamec & Johnson (1997).

The realized heritability of weaning to estrus interval was .17. Heritabilities of other intervals for first parity sows were generally less. Heritability of intervals after second and third parities were greater than intervals after first parity. Greater environmental variation affecting first parity intervals is likely the explanation.

Genetic correlations among weaning to farrowing intervals after first, second, and third parities were .78 in Farm 1 data, and .74 in Farm 2 data (ten Napel and Johnson , 1997). Genetic correlations between weaning to estrus intervals in different parities are expected to be at least this large, and probably greater. Phenotypic correlations were much lower, ranging from -.01 to .15. Phenotypic correlations are estimates of repeatability of traits, thus environmental effects common to intervals after different parities appear to be unimportant.

CORRELATIONS WITH OTHER REPRODUCTION AND PRODUCTION TRAITS

In a review, ten Napel et al. (1995) hypothesized that genetic selection for increased daily gain, decreased backfat, increased litter size, and increased litter weaning weight may impair normal resumption of cycling activity after weaning. ten Napel and Johnson (1997) studied these relationships in two farms. Genetic correlations of weaning to farrowing intervals with average daily gain ranged from .04 to -.21, and those with backfat were between -.07 to .05. Significant differences in estimated breeding values for daily gain and backfat were found only in parity 1 sows. In general results were consistent with the hypothesis that selection for faster growth and leaner pigs may increase the incidence of prolonged rebreeding intervals, although differences were small.

Adamec and Johnson also found low genetic correlations of production traits with rebreeding performancce. Genetic correlations of average daily gain and different intervals were between .06 and .14, those with backfat were between .06 and .17. Rebreeding performance seems not to be related to production traits or to have very low correlations with them.

Genetic correlations with number born were also low and inconsistent across studies, but correlations with litter weaning weight were consistently positive (approximately .15). Sows with genetic potential for above average milk production had some tendency for expressing prolonged weaning to estrus intervals, although the relationship is quite weak.

Genetic correlations of intervals from weaning to estrus or weaning to farrowing with production and other reproduction traits may not be useful in selection. It is likely that correlations are close to zero for sows with normal intervals, but may be significant and undesirable for sows with prolonged intervals. Calculated correlations are some pooled value of these two relationships and may differ across populations depending on the incidence of prolonged intervals.

IMPLEMENTATION

The expected result of selection to improve rebreeding performance is a reduction in the proportion of sows with prolonged rebreeding intervals, those in the exponential part of the distribution. Selection is not expected to decrease the mean of sows in the normal part. Therefore, whether to implement selection for improved rebreeding performance depends on the current distribution of intervals of sows in the population. If a high percentage are returning to estrus within normal intervals, selection will have little effect. If prolonged intervals are prevalent, selection for shorter intervals is expected to cause improvement.

Selection to improve rebreeding performance can be implemented in several ways. A very simple procedure is to retain all replacement boars and gilts from sows that rebreed within normal intervals and to cull sows with prolonged intervals. When sows were weaned at 17 days lactation or greater, intervals of 7 days or more to first observed estrus were nearly always in the exponential part of the distribution. Therefore, retaining replacements only from sows that expressed estrus within 7-8 days following weaning is expected to be effective.

A somewhat more effective procedure is to select on estimated breeding values for rebreeding performance. The first step is to define the trait to be recorded. Either weaning to first observed estrus interval, weaning to farrowing interval, or farrowing interval are recommended as they can be systematically recorded in databases. Repeat intervals can be treated as the same trait with repeatability equal to or slightly greater than heritability, as common environmental effects across intervals seem not to be important. Heritabilities of .17 for weaning to estrus interval, and .10 for other intervals are recommended. Alternatively, intervals after weaning each litter can be considered a different trait with genetic correlations among intervals of approximately .75. Single-trait analyses are appropriate as genetic correlations with other production traits seem to be very low.

Once breeding values are estimated, they are weighted along with those for other traits according to their relative economic value and combined into an index. Value of reduction in rebreeding intervals will vary across populations. Value may be low in populations with a high proportion of sows with normal intervals, but will be greater in populations with greater incidence of sows with prolonged intervals. Determining economic value of rebreeding intervals was not an objective of this paper. But obtaining estimates of these values is as important as estimating breeding values and should be done before selection is implemented.

A last word of caution. Most data in the literature are in sows with litters weaned at 17 days or greater. Earlier weaning is becoming more common. Rebreeding intervals when 10 to 14 day weaning is practiced have not been studied. Whether parameters reported here apply to that management practice is not known.

BIBLIOGRAPHY

ten Napel et al. 1995. A biological approach to examine genetic variation in weaning-to-oestrus interval in first-litter sows. Livest. Prod. Sci. 41:81.

ten Napel et al. 1995. Genetics of the interval from weaning to estrus in first parity sows: distribution of data, direct response of selection, and heritability. J. Anim. Sci. 73:2193.

ten Napel and Johnson. 1997. Genetic relationships among production traits and rebreeding performance. J. Anim. Sci. 75:51.

Adamec and Johnson. 1997. Genetic analysis of rebreeding intervals, litter traits, and production traits in sows of the National Czech nucleus. Livest. Prod. Sci. 48: 13.


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