Sow Longevity

Effect of genotype

There is a lot of evidence that the genotype of sows affects their longevity. In the US six lines of crossbred gilts (about 550 per line) were compared side by side in two 1,600 sow units. Seventy percent of one line finished parity 4 compared with 48%, 50%, 50%, 52% and 52% of the other five lines (Moeller et al, 2004).

Pre-pubertal feeding, liveweight and backfat at first mating.

Rapid growth rate is sometimes associated with an increase in the incidence of leg problems e.g. Jorgensen et al. (1998).

According to Hughes and Varley (2003) the bulk of the evidence would suggest that sows with low fat reserves at first breeding have a shorter breeding life than those with greater reserves though there are many studies that do not agree. Table 5 shows one such study. Whether fat reserves or protein reserves are most important is not agreed.

Similar data from Canada examined survival to parity 4 of Landrace (n = 19,031) and Large White gilts (n = 17,414) varying in backfat at 100kg (Table 6).

Having good fat reserves at first mating may be more an insurance than an absolute requirement. Foxcroft et al. (2004) suggested that where weight and condition loss in lactation are minimised body reserves at first mating are not important.

Many gilts especially those home-reared will be fed a typical growing pig diet (or diets) designed to maximise growth rate and carcass lean. This is probably not the best strategy for a long breeding life. The MLC has published interim results of a trial examining high (1.0 to 1.2%) and low (0.5 to 0.6%) lysine diets from 30kg to mating, combined with high (0.75%) or low (0.5%) lysine diets in pregnancy and high (1.0%) or low (0.7%) lysine diets in lactation giving in total 8 treatments (Edge et al., 2003).

Gilts fed the high lysine diet in the rearing period were heavier and had less body fat at mating. Pigs weaned up to parity six (calculated per gilt mated at the beginning of the trial) was highest (40.9 pigs) on the Low-High-High lysine sequence and lowest (31.4 pigs) on the High-Low-Low sequence. Sows reared on the high protein diet were twice as likely to be culled for leg problems as those reared on the low protein diet.

Many pig units now feed finisher diets of low phosphorus content. This results in some reduction in bone strength and these diets may not be suitable for future breeding stock.

Age and weight at first mating

There is a lot of evidence that gilts mated at an early age produce fewer litters (Table 7). This data is from a survey of breeding herds in France practicing early, normal or late breeding strategies. The effect is complicated by the fact that gilts first mated at very high ages may contain a higher proportion of inherently infertile animals.

Mating at the second or third oestrus may increase first litter size but there is little evidence that it will improve lifetime performance (Hughes and Varley, 2003).

Foxcroft et al. (2004) recommended that puberty be induced at an early age by boar stimulation on the basis that early responders have better lifetime fertility. It is important to remember that these early responders are not necessarily bred at an early age or at a light weight.

Sow feeding

Most of our knowledge of sow feeding has come from trials conducted in the 1960s and 1970s when sows were fatter, productivity was lower and management systems were very different to present day conditions. Feeding these high producing sows has been discussed by Bruce Mullan in another paper at this conference.

Body condition

Thin sows have a reduced life expectancy, being more prone to cold and less fertile while the poor condition may indicate an underlying health problem. Hughes (2001) compared return to oestrus, wastage (failure to conceive, anoestrus, abortion, not in pig) and next litter size of heavy and light, thin and fat sows (parity 3 to parity 7) as shown in Table 8.

¹ failure to conceive, anoestrus, abortion, not in pig

Assessing backfat depth by condition score using a reference scorecard is a simple exercise but of limited value. Hughes and Smits (cited by Hughes and Varley, 2003) found that while most sows assigned a condition score of 3 on visual inspection were between 12 and 18mm in backfat depth, a small number of sows on score 3 were under 10 or over 20mm.

Lameness

Lameness and foot/leg problems are important causes of culling young sows. Keeping sows in stalls without exercise results in poorer muscle tone, weaker bones and thinner cartilage all of which can lead to lameness and inability or difficulty in walking and changing posture (Orth, 2003).

In Australia, a survey identified the following risk factors for lameness in sows (Cargill, 2001).

• Conformation at selection
• Flooring
• Overstocking
• Infections

Cargill suggested a simple protocol for assessing lameness. Make all sows stand e.g. after feeding. If over 30% lie down in the next 40 minutes examine the first 20 or 30 who lie for:

• Leg and toe lesions
• Swollen joints
• Shifting weight from one leg to the other more than 4 times in a minute
• Percentage time weight is borne on one leg.

The "downer sow "syndrome is caused by breaks in bones (especially in the pelvic area) weakened by poor mineral or vitamin nutrition. These breaks and lameness often occur after weaning but may happen in the farrowing house. In rare cases, lameness or complete rear leg paralysis is caused by injuries received in delivery of extremely large pigs. This lame condition may be short-term, but sometimes the sow will not recover. Many of the sows may not recover sufficiently to salvage any market value.

Osteochondrosis is one of a number of conditions of the cartilage in the joints where damage results in lameness. It appears to be very common in pigs and several contributory factors have been identified though there is little agreement on nutritional or management strategies to prevent or cure it (Nakano and Aherne, 1993; Nakano et al., 1993).

Flooring contributes to lameness and paralysis. Fighting is an important reason for foot damage in group-housed sows and loss of dewclaws is common A study conducted at Moorepark showed that covering the slatted floor with mats at mixing reduces the risk of sows being injured at mixing (Boyle and Lynch, 2003).

Lameness generally results in poor lactation performance, reduced litter size and weight and possibly culling of the affected sow. Rough flooring can damage foot pads or cause cuts and scrapes. Any of these injuries can be infection sites for lameness-causing bacteria. Slippery flooring causes injuries and may discourage sows from attempting to get up, eat and move about.

Diet and skeletal development

The recommended calcium and phosphorus concentrations in diets for growing replacement stock and for mature breeding stock are higher than those for slaughter pigs (Close and Cole, 2000). This means that a diet suitable for finishers may not be correct for gilts. However, daily feed intake will complicate any assessment of requirement.

There is still uncertainty over the optimum calcium and phosphorus levels in diets for breeding animals. Bone mineralisation is sometimes used as an indicator of bone strength but Crenshaw (2003) questioned whether maximum bone mineralisation is desirable and he suggested that over-supplementation of calcium and phosphorus might in fact contribute to lameness in later life. He also speculated that feeding a high level of dietary calcium in late pregnancy might precipitate hypocalcaemia or milk fever at parturition which could be a factor in a high stillbirth rate.

NRC (1998) suggested feeding a diet with about 0.1% more calcium and 0.1% more phosphorus than that fed to finishing pigs.

When sows are fed inadequate amounts of calcium and phosphorus the skeletal reserves may be used to maintain foetal growth and for milk production. This can contribute to the “downer sow syndrome” in which sows are unable to stand (OSU, 1998).

Staff

Staff supply and staff retention are concerns in the pig industry. The quality of stockmanship affects sow productivity and it seems reasonable to assume that the same holds true for sow longevity, culling and mortality. Attention to individual sows, prompt care for ill or injured animals will reduce culling and mortality. Having an adequate number of properly trained staff is important. In the USA Loula (2002) claimed that an inexperienced labour force with poor training was a common feature of units with poor sow longevity.

Bonus payments to staff have the potential to distort an effective culling/replacement programme e.g. breeding targets may be met by retaining unsuitable sows which are later culled or farrow in an unfit condition.

An example of how stockmanship can have an impact on later skeletal quality and how easy it is to cause such damage was shown in a trial where young pigs (29kg) were dropped from waist height or placed gently into a cart. Those dropped had a higher incidence of osteochondrosis at 90kg than those placed gently (Nakano and Aherne, 1988).

Sow deaths

There is a perception that sow mortality rates have increased and this may be due to more strict criteria for sows at slaughter resulting in more on-farm deaths or euthanasia.

In the USA sow mortality is higher in summer, higher in the first three weeks after farrowing and is higher in low parity and high parity animals (Deen and Xue, 1999). This survey used 3.6 million sow parity records. Interestingly, they found that where the gilt pool was too small at the time of mating there was an increased risk of sow death in that pregnancy. This was most likely due to sows being retained too long in the absence of a suitable replacement.

Abiven (1997) in a survey of 100 herds in France reported a higher risk of sow deaths where there was high incidence of urinary tract infection, metritis or lameness. There was a lower risk where pregnant sows were fed three times daily rather than twice and where sows had feed intake increased rapidly after farrowing.

There is no information on causes of sow mortality in Ireland. In the USA, the principal causes of sow deaths vary from farm to farm according to Geiger et al. (1999). They conducted post-mortem examination of dead or euthanased sows on six farms and found a high proportion of very thin sows on one farm, a high proportion of very fat sows on another. They found musculo-skeletal conditions as the most common cause of death of sows (38%) but this varied from 15% to 55% of deaths on different farms.

On some farms lesions of the gastro intestinal tract e.g. stomach ulcers were important reasons for mortality. Overall they found that only 28% of sow stomachs had “normal” epithelium with the others showing mild to severe keratinisation, erosion, ulcers or strictures.

Culling to sale

Sows are very inefficient converters of feed to weight gain. Feeding of sows to improve body condition prior to sale is seldom justified with a feed efficiency ratio as poor as 7:1. Holding these non-producing sows increases sow stock number and depresses the calculated output per sow per year. Watch the timing of injections and vaccinations to avoid long retention of sows marked for culling.