Reproductive Age, what does genetics have to do with it? Ronald O. Bates The pork industry is relearning that stimulating the onset of heat or estrus in gilts, breeding them and having them farrow is an important component to improved reproductive efficiency and profitability.
It has been shown in several studies that Age at First Farrowing is a favorable contributor to lifetime productivity and longevity. Typically gilts that achieve puberty (first heat) sooner than their counterparts have a reduced age at first farrowing if mated at a constant heat cycle (e.g. second or third estrous cycle).
Age at puberty is an interesting trait in that both environment and genetics play large roles in explaining the variation in expression. It is well known that nutrition, environmental temperature, stocking density, disease status, daylength and boar exposure can impact the onset of puberty.
Age at puberty has both a large heterosis component and also will respond easily to selection within breeds and lines. This is unusual since many traits that have a large heterosis component (e.g. littersize, litterweight, pig survival) are more difficult to improve through direct selection. It has been estimated that age of puberty will be reduced by as much as 6.5% when breeds are crossed in crossbreeding programs. For example, the average age at puberty for two pure breeds is 180 days of age. The age at puberty for F1 females produced by crossing the two lines should be 170 days.
Age of puberty also responds well to selection within lines. Heritability for age at puberty has been estimated at 0.40. This is moderate in size and similar to that of backfat thickness and average daily gain and contrasts to traits like number born alive which has a heritability of approximately 0.10. Thus if selection occurs for reduced age at puberty the genetic change could occur as rapidly as when selecting for reduced backfat.
Age at puberty is also genetically associated with several growth traits. It has been estimated to be favorably related to body weight gain but unfavorably related to backfat thickness and loin muscle area. This suggests that when selecting for increased growth rate, age at puberty may naturally decrease. However, if backfat thickness and loin muscle area are also included in the selection criteria age at puberty may increase, depending on the amount of emphasis placed on growth rate, backfat thickness and loin muscle area.
In addition it has been shown that age at puberty also has a moderate association with return to estrus after weaning, for the first litter. Thus females that are younger when they express their first estrus should also have a shorter wean to estrus interval after they wean their first litter, compared to their contemporaries.
The question that arises is that in modern pig production how has age at puberty changed due to increased selection pressure for lean growth rate which has caused a reduction in backfat thickness and increased muscle mass? This is a difficult question to answer since there have not been published results on how age at puberty has changed in maternal commercial lines selected for improved lean gain. However, research that has evaluated females representing populations that differed in backfat thickness did show that gilts from populations that were leaner at a constant weight (e.g. 250 lb) were older at puberty.
What all of this suggests is that selection for improved lean growth can shift physiological maturity and cause gilts to be older at puberty after they have reached their peak for lean gain. These females may farrow at an older age and may have a longer wean to estrus interval than desired.
For farms in which this is the case, can this situation be improved? First it is important that farms know approximate age at puberty or at least when most gilts are cycling. For this to occur, management and environmental conditions have to adequate. In other words, females have to be consuming an acceptable plane of nutrition, have acceptable pen space, not be suffering from health challenges, not be exposed to high environmental temperatures and have adequate boar exposure. If these types of environmental and management conditions are compromised then expression of puberty will be delayed.
If farms have adequate conditions that will not hamper expression of puberty then there are some generalizations that can be made. For example, if two-thirds of females are cycling by 190 days of age, females bred after 200 days of age should be mated on their second or third estrus cycle which is often suggested as a target when breeding gilts. However, if at 190 days of age less than two-thirds of newly entered gilts are not cycling then this would suggest that gilts are reaching puberty at a more advanced age and it would be recommended to wait additional time to allow those gilts to develop. They should be bred at a time when the majority has achieved their second or third estrous cycle. Furthermore, gilts that achieve puberty at an older age may have different dietary needs with specific gestation and lactation diets necessary to meet their nutritional needs to achieve optimum lactation and rebreeding performance.
There are other items to consider. As previously mentioned, F1 gilts achieve puberty at an earlier age due to a full complement of heterosis. Avoiding backcross gilts (e.g. Yorkshire x Yorkshire-Landrace F1) should improve the age at puberty experienced on commercial farms. In addition, seedstock suppliers that incorporate age at puberty in their selection program, or related traits such as wean to estrus interval or age at first farrowing, should have commercial females that express first estrus at a younger age and be able to be mated to farrow at a younger age as well.
Many factors do impact age at puberty and associated traits such as subsequent age at first farrowing and wean to estrus interval. Environmental factors can have desirable or deleterious effects on age at puberty and subsequently age at first farrowing. Farms must be managed so not to hamper the expression of these traits. In addition, commercial crossbreeding systems must exploit heterosis so as not to deter their onset. However, these traits can be measured and incorporated into a genetic improvement program. Their inclusion into a genetic improvement program will cause on-going improvement over time and facilitate continued improvement within the commercial pork industry
State Swine Specialist
Michigan State University