Pre-weaning piglet mortality varies to a large extent between farms and ranges from 5 to 35%.
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Piglet deaths occur mainly during the first 48 h after farrowing, crushing by the sow being the first cause.
Although several studies indicate that crushing by the sow is the ultimate cause of piglet death, crushing often results from the effects of perinatal hypothermia and starvation. Undernourished piglets spend more time close to the sow and are more likely to be crushed.
The immune system of the newborn piglet is immature and consequently the acquisition of colostrum - the source of dietary energy which contains immunoglobulins- for up to 36h after birth is essential.
Furthermore, starvation is often secondary to, and interacts with, the effects of perinatal hypothermia, making piglets more lethargic, less competitive at the udder and less likely to achieve a sufficient colostrum intake.
The lower critical temperature of newborn piglets is 34ºC and when ambient temperature is below 34 ºC piglets will attempt to gain heat shivering and huddling. One of the reasons why newborn piglets are so sensitive to cold is that they lack brown adipose tissue.
Piglets weighing less than 1 Kg at birth have a higher risk of dying before weaning.
Piglets suckle simultaneously every 30-70 minutes and up to 20 times a day. These regular suckling bouts include five phases.
The first three phases involve the communication and coordination between the sow and her litter to ensure that piglets are at the udder when milk flow begins.
The sow lies down, exposes the udder and initiates a rhythmic grunting that increases gradually. Piglets compete to get access to the udder (phase1), massage the udder rhythmically (phase2) and stimulate oxytocin release from the mother by performing slow sucking movements (phase3).
The fourth phase involves milk ejection. During this phase, the sow increases her rhythmic grunting and piglets perform rapid sucking movements.
The fifth phase includes both massage and slow sucking movements by the piglets. The sow decreases her rhythmic grunting rate. It is suggested that piglets prepare their future milk consumption by massaging the udder after milk ejection.
Both piglet vitality and body weight at birth determine the degree of udder stimulation and consequently the amount of milk consumed.
Sow restlessness affects piglet mortality because of piglet crushing and impedes early teat seeking and suckling attempts. Crushing results from the sow's movements to lie down from either a standing or sitting position, to sit up from a lying position or from rolling behaviour (changes between udder and side lying).
Sows that do not crush any of their piglets show a more protective mothering style, in terms of more nest building activity, responding faster to piglet distress calls and nosing their piglets more often during posture changes than sows that crush several piglets.
The percentage of savaging sows - sows that behave aggressively towards her piglets newborns - ranges from 1 to 15%. Savaging is considered to be more frequent in gilts than in multiparous sows and is thought to have a genetic component.
Insufficient milk production in sows might account for 6 to 17% of pre-weaning mortality. Lactation failures may be due to heat stress, metabolic disorders and endocrine imbalances in the sow and to health conditions such as metritis. Agalactia should be considered when more than 3 piglets from the same litter die. ■