By W.L. Flowers, North Carolina State University Department of Animal Science
Attainment of puberty in gilts is dependent upon the age at which boar exposure begins. In most production systems, if this occurs when females are at least 6-months-old, then a high proportion of females, 70% to 90%, show estrus over a short period of time, two to four weeks. In every population of gilts there also is a subset of animals which has the ability to exhibit estrus much earlier than this.
From a physiological perspective, their reproductive systems probably mature at a faster rate compared with their contemporaries but remain in a “holding pattern” until boar exposure is initiated. These gilts are commonly referred to as “early responders.” It has been suggested that these animals have increased sensitivity to estrogens which is why they are able to initiate estrus at young ages. Whether this is true remains to be established scientifically but, if it is, then it is also reasonable to speculate that these animals should also have increased productivity since production of estrogens is involved in establishing litter size, conception rates and rebreeding intervals due to its influence on ovulation rate, maternal recognition of pregnancy and gonadotropin secretion, respectively.
Studies that have examined the lifetime productivity of early responders indicate that this speculation probably is correct (Figure 1 and Table 1). In these studies, boar exposure was initiated at 150 days of age, but breeding was delayed until gilts were between 240 and 260 days of age. Females were classified as early or late responders if they exhibited their first estrus less than or greater than 180 days of age, respectively.
The frequency distribution for age at puberty (Figure 1) shows that there were two distinct populations: those that reached puberty around 170 days of age (early responders) and their counterparts that exhibited their first estrus around 200 days of age (late responders). It is interesting to note if boar exposure had begun at 180 rather than 150 days of age, then it would not be possible to distinguish between the early and late responders.
Figure 1: Distribution of first detected estrus when boar exposure was begun when gilts were 150 days of age. Gilts exhibiting estrus before and after 180 days of age (30 days from onset of boar exposure) were classified as early or late responders, respectively.
Table 1 contains selected lifetime productivity measures for each of these groups. It is important to remember that gilts in the study were bred between 240 and 260 days of age so all exhibited at least one, and probably two, post-pubertal estrous cycles prior to being inseminated. The proportion of females that were rebred after their sixth parity; average number born alive and total number of live pigs produced were all significantly higher for the early versus the late responders. It is not feasible in most commercial production systems to begin boar exposure at 150 days of age in order to identify the early responders.
However, an alternative approach would be to avoid breeding gilts that do not exhibit estrus within the first 30 days upon delivery to commercial farms, provided that all gilts are at least 160 days of age. This should have the same overall effect since it basically selects against the late responders and should reduce the proportion of these that enter production.