Effect of large litter size and within-litter differences in piglet weight on the use of milk replacer in litters from hyper-prolific sows under two housing conditions

Due to genetic selection for large litters in pig production, piglets are often supernumerary to the teats on the sow. Providing milk replacer as a second feed source is increasingly being used to rear large litters. This results in increased competition at the udder when all piglets remain at the sow. We hypothesise that larger litter size and being amongst the smaller piglets within-litter (irrespective of absolute weight) is associated with increased likelihood of drinking milk replacer as a supplement to suckling or to compensate for low suckling success. The study design was 2 x 2 factorial with two standardised litter sizes at day 1 (LSD1) of either 14 piglets (LS14) or 17 piglets (LS17) and two housing conditions in either crates (CRATE) or loose housing (LOOSE). If piglets died, they were not replaced. The piglets' suckling and drinking behaviour along with their growth rate was recorded weekly. The results showed a clear preference for suckling the sow over milk replacer. None of the piglets changed entirely to drink milk replacer. L517 compared to LS14 was associated with more piglets with low suckling success (P <= 0.05) and the risk of having low suckling success was higher for the smaller piglets within-litter (P <= 0.05). About half of the piglets with low suckling success were able to compensate and sustain growth by drinking milk replacer (P <= 0.05). The largest piglets within-litter were more likely to drink milk replacer as supplement to high suckling success compared to only suckling (P <= 0.05). When taking into account the absolute weight of the piglets in the start of the week, piglets drinking milk replacer in addition to suckling had a higher growth rate in the following week compared to piglets who did not supplement suckling with milk replacer (P <= 0.05). Thus, the present study could confirm that some of the piglets experiencing a competitive disadvantage at the udder could compensate with milk replacer. However, the milk replacer was mainly used by the larger piglets within-litter and as a supplement to increase growth of piglets who had access to a teat. Several factors may have affected the piglets' ability to use the milk replacer: high effort to drink from the milk cup due to inadequate design, poorer nutrient quality of milk replacer than of sow milk, and/or suckling the sow satisfies other behavioural and physiological needs than solely nourishment.