Most producers are aware of the value of attaining high weaning weight.
In many studies, it has been shown that, on a herd basis, a 1kg improvement in weaning weights is "worth" a reduction of at least 9-10 days in age at slaughter (100+kg live weight). This has the effect of reducing the amount of feed needed for maintenance by 12-14kg, itself worth more than £2.50 per pig. (Close to slaughter weight, approximately half of the total daily intake is required for maintenance). Measured in a different way but with the same outcome, the difference in weight between 2 pigs will triple in each of the rearing and growing phases. Thus an 1kg difference at weaning becomes 3kg difference at 12 weeks old and 9kg difference at 22weeks.
However, we tend to neglect the effect of birth weight on ultimate weaning weight. Research studies have suggested that pigs of birth weight below 800g attain a weaning weight at 3 weeks of age 2kg lighter than litter mates above 800g "at birth". This would project to being equivalent of a 3kg penalty at 4 week weaning. Moreover, the mortality rates in such pigs are at least 50% greater than in the larger animals with an average birthweight above 1.25kg with pigs below 500g rarely surviving. The effect of a 2kg penalty at weaning is probably even greater than the 3 week extended weaning to sale period that the above figures would suggest, given the difficulties of establishing pigs of 4-5kg at weaning especially on diets where, in the UK, highly digestible protein of animal origin such as skimmed milk powder, meat and bone meal and porcine plasma are not available. Creep diets tend to rely on other less digestible milk based products, fishmeal and synthetic amino acids. Complex vegetable proteins such as those present in soya and rape are not suitable for the immature gut of the underdeveloped small piglet.
The limits that PMWS poses on the ability to cross foster pigs during lactation and back marking of small pigs at weaning creates even greater problems in the management of very diverse sized pigs post weaning.
Therefore, producers should look at the factors which influence birth weight of pigs, both as whole litters and individuals.
1) The size and age of the gilt at first serve which has an effect on mature body size. Gilts also tend to produce lighter pigs and the smaller/younger the gilt is, the greater will be the penalty. A minimum target of c135kg at 210 days from F1 hybrid gilt is valid to produce a farrowed gilt that will optimise piglet growth and milk production as well as ensuring a decent productive life.
2) The condition of the sow throughout life with low levels of backfat tending to limit the growth of the litter in utero.
3) The genetic base of the pig - both the sow and the boar. Some female lines produce larger pigs than others - producers should consult breeding companies for data. Hybrid terminal sires tend to produce a bigger and more vigorous offspring.
4) Health of the sow and the deleterious effect that e.g. parasitism (worms and or mange) will have an energy balance and nutrients available to the developing litter as well has limiting the sows' body condition and subsequent milk production. Reproductive pathogens such as Parvovirus & PRRS will limit growth.
5) Feeding regime. There are 2 significant stages to the gestational feeding regime with respect to the influence on the litter. Placental development occurs in the first half of pregnancy in advance of the developing piglet - the time when any shortfall in body condition resulting from the previous lactation needs to be made up especially in younger sows/weaned gilts. Any limit to intake at this stage will limit total placental growth, which is directly related to the ultimate size and weight of the piglet. If there is insufficient placenta, there will be insufficient oxygen and nutrient supply to the piglet, limiting growth. Performance and piglet birth weights can recover in subsequent litters if feeding is corrected.
Even assuming that placental growth is adequate in the first half of pregnancy, nutrient intake in the last 3 weeks of gestation must be adequate to take advantage of the placental supply. The foetus should double in size in these last 3 weeks. If inadequate feed is given in late pregnancy, there will be a tendency for piglets to be born with a well-developed skeleton but poorly fleshed i.e. skinny pigs are born. This can differentiate the pigs where placental growth is inadequate early on, which will be physically small.
6) Litter size will obviously affect individual piglet size at birth. Variation within a litter is to some extent normal and will increase as the litter size increases. However, it also tends to increase as the sow ages - possibly associated with repeated damage to the wall of the uterus by repeated pregnancies. A balance must be struck between achieving a reasonable lifetime production and the penalty that will be paid for keeping older sows in terms of stillbirth levels, genetic lag and the effects on subsequent growth of undersized pigs at birth. This problem may be increasing as hyper-prolific sows producing average litter sizes in excess of 15 pigs born alive are becoming more common.
With correct modern genotypes, nutrition and husbandry techniques new targets for litter production are starting to appear. A litter live birth weight of 20kg is readily achievable and this increasingly is being translated into weaning 100kg live weight of pig per litter at 28days of age, from a mature 240kg sow (e.g. 12 piglets reared at 8.33kg live weight).
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