About 50 percent of all sows have a lower litter size in their second parity, compared with their first parity – and recent studies indicated that second parity litter size and litter size could be related to subsequent litter sizes and farrowing rates.

However, by altering management and sow nutrition pig producers may be able to reverse this trend and improve second – and subsequent – parity reproductive performance.


Sow body condition

Reduced reproductive production in second parity sows has been related to excessive weight loss during the first lactation. During the past decade, litter size and number of piglets weaned have increased, as shown in Figure 1 . As a result, this has increased the metabolic demands on first litter sows.

However, due to genetic selection on lean grower and finisher pig traits, sow feed intake has not increased. This discrepancy can easily result in a high sow weight loss during lactation which can reduce follicle development and oocyte quality. As a result, subsequent farrowing rate, litter size and litter quality can be affected. 


The effects of weight loss on sow reproductive performance seem to have shifted over the years. Studies from the 1980s’ and 1990s’ show that lactation weight loss had a big influence on weaning to oestrus interval, while more recent studies show that sow weight loss during the lactation affects ovulation rate and embryonic survival. 

The results of two recent studies on the effect of sow lactation weight loss on reproductive performance are shown in Table 1 .

Gilt feed intake studies 

In the first study, gilts feed intake was restricted to 60 percent and 90 percent of ad libitum feed intake during the last week of 20-day lactation. Feed allowance was not different in the weaning to oestrus interval and gestation.

In the second study, gilts were only mildly restricted, which is different from most experiments on the effects of weight loss on subsequent reproductive performance. Feed allowance (kg) was calculated based on 1 percent of body weight + 0.4kg per piglet, with a maximum of 7kg. Maximum feed intake was reached on Day 14 after farrowing. Sow lactation length was 26 days. Results from both studies showed that weight loss had a negative effect on either embryonic weight (Study 1), or embryonic survival (Study 2). 


The lack of effect on embryonic survival in Study 1 could be due to the relative low weight loss of sows, compared with Study 2. This indicates that weight losses of up to 11 percent probably do not influence embryonic survival and eventually litter size in second parity sows. 


Piglet birth weight can, however, be affected. The conclusion is that a weight loss of more than 11 percent negatively affected embryonic survival and possibly embryonic weight and should be prevented. 


Sow reproductive issues

Weight loss during lactation consists mainly of losses of body fat, body protein and body water. Of these three, body protein losses have been reported to have the largest effect on reproductive performance (Clowes et al. 2003ab; Willis et al., 2003).

Therefore, in Study 2, back fat depth, as a measure for fat loss was measured. In addition, loin muscle depth as a measure of protein loss was measured. Although high weight loss sows lost more weight than low weight loss sows, back fat loss was similar at 4.6mm and 4.8mm respectively, as shown in Figure 2 . 

 

Figure 2 also shows that loin muscle depth loss was 4.2mm higher for high weight loss than for low weight loss sows. These results indicate that weight loss was more related to loin muscle depth loss (correlation 0.6, P<0.05) than back fat loss (correlation 0.15, P<0.05).

As described above, loin muscle depth was used as a measure of protein (muscle) loss. Loin muscle depth loss, however, only showed a low correlation with protein mass loss (r=0.27), indicating that losses of other muscles might be more indicative of total body protein mass loss during lactation. Furthermore, loin muscle measures are very labor-intensive and loin muscle loss was not significantly related to embryonic survival or development. 


It can, therefore, be concluded that under practical conditions weight loss has the best correlation with subsequent reproductive performance. Used in combination with back fat measurements, a producer has a great tool to monitor sow body condition changes which means that appropriate management actions can then be taken in order to optimize sow reproductive performance. 

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Preventing weight loss

Several management factors can be applied to decrease sow weight loss during lactation. Increasing sow feed intake seems to be the most obvious solution. However, in many cases this is not as easy as it seems.

The first management step should be to optimize the temperature in the farrowing room. Room temperatures in farrowing rooms are often around 23 degrees C to 24 degrees C, which is optimum for piglet survival. Sow feed intake, however, is negatively affected by these temperatures. 


Sow feed intake decreases by 0.17kg with every degree increase in environmental temperature above 16 degrees C (Black et al., 1993). It is possible, by insulating piglet nests, to decrease the farrowing room temperature to between 18 degrees C and 20 degrees C which is close to the optimal temperature for sows. This will encourage sow feed intake to increase without affecting piglet survival. Furthermore, sow cooling systems, such as drip or snout cooling, can also increase sow feed intake. 


Secondly, plenty of good quality drinking water should be provided because water and feed intake have been shown to be correlated. The drinking nipple should provide water at a rate of 2.5 liters per minute. It is worth checking this regularly. In addition, water quality should also be checked especially on farms with their own water well. 

Thirdly, the sow ration could be more concentrated and could be formulated with compounds that produce the least amount of internal heat when digested.

Reducing lactation burdens 

Another way to decrease weight loss during lactation is to decrease the lactation burden of sows. Possible routes are to reduce the lactation length, but this may be limited to remain in accordance with animal welfare conditions. Alternatively, the number of piglets suckling can be decreased, or piglets can be creep fed.

Decreasing the number of piglets suckling can be done by split weaning, by weaning part of the litter in the last week of lactation, or intermittent suckling by separating the sow and piglets for a few hours each day. This reduces suckling and encourages creep feed intake by piglets. 


Both methods stimulate follicle development, but their effects on weaning to oestrus interval and weight loss have been found to be variable. (reviewed by Soede et al., 2009). 

Creep feeding piglets can be done by using dry or liquid feed. Dry creep feed is commonly used from four days after birth and prepares the piglet and its intestine for the shift from milk to dry feed after weaning (research has shown that feeding a milk replacer increases piglet growth and could reduce sow lactation weight loss).

Optimizing gilt development 

Lastly, gilt development should be optimized. Well-developed gilts are better able to withstand lactational losses and show a better performance throughout lactation, compared with less developed gilts. (Clowes et al, 2003; Hoving et al, 2010). Furthermore well-developed gilts are more likely to have better feed intake during the lactation than a less developed one (Lewis and Bunter, 2011).

The commonly used target is 140kg body weight at first insemination, which is at the second oestrus after puberty. The age at which this weight should be achieved varies between husbandry systems and countries, but is typically between 210 and 240 days. 


Increased litter size and number of piglets weaned has increased the metabolic demands on the lactating sow which can lead to increased weight losses. Sow weight losses above 11 percent should be prevented, because they have a negative effect on embryonic survival, litter size and farrowing rate. 

Gilt development should ideally be optimized as well developed gilts are better able to cope with lactation losses. Litter size and/or farrowing rate in second parity might therefore improve.