Sows in mid parity are best for the pre- and post-weaning performance of both light and heavy piglets.

To improve the performance of lightweight piglets during suckling, producers are advised to create uniform litters using young sows. However, fostering piglets to primiparous sows may confer penalties due to their lower milk yield and milk immunoglobulin concentrations compared with multiparous sows. The objective was to determine the effect of foster sow parity (primiparous (F), second (S), and mid parity (M: parity 3 - 5)) on the performance from birth to d68 of piglets born light (L: ≤1.25 kg) or heavy (H: 1.50-2.00 kg) and on creep feed consumption. Piglets (n = 507) considered L or H were cross-fostered, creating litters of 13 similar-sized piglets/ litter and were randomly fostered to one of the foster parities. All litters were offered creep feed with a green dye to discern between consumers and non-consumers, and the medication administered was recorded. Medication administrated pre- and post-weaning did not differ (P > 0.05) across the different experimental groups. A significantly (P ≤ 0.025) lower number of H piglets were removed as a result of pre-weaning weight loss from F and S, rather than M litters. The interaction between birth weight and foster parity only affected piglet body weight (BW) at d10 (P = 0.020); foster parity did not influence BW of L piglets, but influenced that of H piglets. H piglets in F and M litters (3.82 and 3.80 kg) were significantly lighter (P ≤ 0.013) than H piglets in S litters (4.15 kg). As expected, L piglets performed worse pre- and post-weaning than H piglets; they were 4.50 kg lighter at d68. Foster parity significantly affected BW: F piglets were weaned lighter (P = 0.004) than S and M piglets (7.52 vs 8.02 kg). Post-weaning (d68) however, F piglets achieved similar BW as S piglets (29.7 vs. 29.9 kg), whereas M piglets performed best (31.2 kg, P ≤ 0.079). Almost none of the L piglets consumed creep feed than H piglets (P < 0.001); significantly (P = 0.007) more F and M piglets were considered consumers than S piglets. The results suggest that irrespectively of birth weight, piglets tend to perform better when in M litters, being weaned heavy and having a high creep feed intake; however, more piglets are removed from such litters pre-weaning. Although S litters were weaned heavy, they were unable to maintain this BW advantage post-weaning, due to their low creep feed intake and F litters remained small throughout. Long term performance monitoring to slaughter is recommended.