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Factors Influencing Litter Size and Birth Weight

by 5m Editor
13 January 2009, at 12:00am

By Hypor. The production of large litters of high quality piglets with good, even birth weights is an important aspect of maximizing Weaning Capacity because it results in the highest number and weight of piglets weaned. Attention to the areas of management discussed below will help to achieve this goal.

High litter size is an important contributor to maximizing Weaning Capacity because number of pigs weaned per litter is a key component in its calculation.

However, it is pointless having large numbers of piglets born if this results in an unacceptable level of losses due to stillbirths and pre-weaning mortality. The Hypor selection index avoids this by including factors such as numbers born alive, birth weight and pigs weaned per litter. In addition, rigorous selection for number and quality of teats helps to ensure excellent milking ability, resulting in maximum piglet survival rate and large, high-quality pigs at weaning.

Genetic Influences

Birth weight is an important trait that influences piglet quality and survival. Traits like average birth weight and total birth weights appear to have very high heritability; 0.25 and 0.15, respectively. As most producers know, small piglets (<800 grams) and non-uniform litters are the main factors that determine the pre-weaning losses of a litter. Heritability for these factors is also fairly high, showing that there is potential for genetic improvement. Traits like the number of small piglets and uniformity of litter at birth have a heritability of 0.10 and 0.07 respectively. A single focus on litter size can obviously result in lower birth weights and decreased uniformity because litter size and piglet quality traits appear to be negatively correlated. That means that a single focus on one of them indirectly promotes a selection in the reverse direction for the other traits. Overall, the focus in selection is to create more litter weight at birth made up of quality piglets. Having 15 quality piglets of 1.5 kg average birth weight results in a litter weight of 22.5kg. This can only be achieved as a target when piglet quality and birth weights do not get compromised too much while selecting for litter size.

The Do's: Environmental, Nutritional and Management Factors

While genetics is a major influence on litter size, there are also a large number of environmental, nutritional and management factors that are important at the farm level. By focusing on the most important of these, it is possible to improve not only litter size but also, more importantly, Weaning Capacity. Birth weight is notoriously difficult to influence on the farm but there are some things that can be done to improve it slightly and also to improve piglet viability at birth.

Weight at first mating

The weight of gilts when they are first bred, and oestrus number, strongly influence first litter size, which in turn determines subsequent litter size. This topic is dealt with in more detail in the leaflet Gilt management for maximum lifetime productivity.

Breeding technique and timing

An effective breeding, carried out at the correct time during the estrus period and which results in a high number of fertilized ova, will help to improve numbers born. Attention to heat detection procedures, semen quality, hygiene standards and insemination technique, especially good sow stimulation, will pay dividends. The use of a breeding protocol, with clearly defined times for breeding sows and gilts, which takes into account the impact of wean-to-oestrus interval on the length of standing oestrus, will also help to improve litter size.

Management for high embryo survival

In the early stages of gestation, the fertilized eggs implant into the wall of the uterus, a process that is not completed until about 28 days after breeding. During this period of their development, the embryos are very sensitive and may become detached from the uterine wall and die unless conditions are ideal. Therefore, sows should not be moved until at least day 28 of gestation and their environment should be quiet and undisturbed. Any stress during this time can lead to embryo losses and very high temperatures or low temperature combined with a draft may result in sows returning to oestrus. Research suggests that the presence of a boar in early to mid-gestation and good lighting both positively influence embryo survival.

Feeding in early gestation

Until recently, it was widely recommended to feed at a low level in the first 21-28 days of gestation as this was thought to maximize embryo survival. However, recent research and practical experience suggests that, for today’s extremely lean and highly productive sows, this practice may be counterproductive and lead to reduced litter size and farrowing rate. It appears that a feed intake in the range 2.4-2.6kg for gilts and 2.6-2.8kg for sows may be the best strategy to maximize litter size, although further evaluation of this practice is required. Further information can be found in the leaflet Feeding the sow and gilt in gestation.

Lactation length

The length of the lactation period has a significant effect on litter size. Over the range 14 to 28 days, each additional day suckling typically leads to an extra 0.1 piglets born in the subsequent litter. Litter size does not appear to increase with lactation lengths of more than 28 days. Balancing the cost of extending lactation length with the productivity benefits, suggests an optimum length of 24-25 days. Irrespective of lactation length, producers should avoid variation in weaning age resulting from variation in weekly farrowing numbers, because this will impact growth rate and pig flow in the nursery and finishing stages, leading to irregular production and reduced margins.

Weaning to breeding interval

The interval between weaning and breeding or, more correctly, weaning to standing oestrus, is probably the most important indicator of subsequent productivity, especially litter size. Litter size will be highest where the wean-to-breed interval averages 6 days or less and where 95 per cent of sows are bred by 7 days after weaning. Of the many factors that influence this, lactation feed intake is the most important and this is dealt with in detail in the leaflet Lactation feeding for maximum intake.

Weaning to mating interval as a trait has been in the Hypor breeding value estimation for quite some time. It has received continuous emphasis, which has never been large. However, through the years a significant genetic improvement has been realized in most maternal lines of around 1 to 2 days. Results from our benchmark data show that our breeding stock averages a relatively short weaning to oestrus interval of 5.8 days. A shorter duration of the weaning to oestrus interval results in a related longer duration of oestrus.

Young females, especially gilts after their first weaning, tend to take longer to show oestrus and will benefit from special attention. Using a higher lysine diet (1.2-1.3 per cent total lysine), where possible, or feeding 0.5kg/day of a concentrated top-dressing for the last 7 days of lactation and up to breeding, will help to reduce body weight loss and consequently hasten the onset of oestrus.

A high feed intake between weaning and breeding is also essential to maximize litter size. Feeding four times per day or ad libitum, provision of plentiful clean water and good trough hygiene will all help to achieve this. Housing sows in groups after weaning has been shown to result in a shorter wean to oestrus interval compared to sows housed in stalls. Provision of adequate lighting – a minimum of 100 lux and preferably 150 lux, for a period of 14-16 hours per day will help to stimulate a rapid onset of oestrus. Good boar contact is also an important factor, especially in the 2-3 days prior to expected oestrus.

Parity distribution

Litter size increases until the third parity and then tends to decline as sows get older. Also, the number of stillborn pigs increases with age and birth weight becomes more variable as sows get older. Therefore, in order to achieve consistent litter size and the highest quality piglets, herd parity structure should be stable. This requires a regular flow of gilts into the herd, a high number of females in the most productive 3 to 6 parity range and strict culling on age after 7-8 parities. Conversely, an unbalanced parity structure caused by uneven gilt introduction, high dropout rates in young females or poor culling policy will result in unwanted variation in average litter size and birth weight over time. This will have many negative implications for the growth and efficiency of pigs from weaning to market. More information on parity management can be found in the leaflet Achieving the correct parity structure.

Increasing piglet birth weight

Birth weight is the most important determinant of weaning weight, one of the key components of Weaning Capacity. Although it is difficult to influence, there are several things that may improve it. The most widely practised of these is increasing sow feed levels in the last 21-28 days of gestation, typically to 2.6-2.8 kg for gilts and 2.8-3.0 kg for sows. While the impact on birth weight is generally small, this technique has been shown to positively influence piglet viability. Recent French research suggests that the addition of oils to the diet (total 5 per cent oil) may lead to fewer stillborn pigs, improved survival of smaller piglets and higher weaning weights. Correct feed levels throughout the breeding cycle and attention to sow health will also contribute to improving birth weight.

January 2009