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The objective was to determine the effects of increasing energy in lactation diets fed during the summer period on sow and litter performance. A total of 356 sows (PIC Camborough) were randomly assigned at entry into the farrowing room to 1 of 4 treatments within parity in a randomized complete block design with body weight and breeding day blocks. Prefarrowing caliper scores were balanced across treatments. The trial was conducted from July to September 2021 in Carthage, Illinois, with external barn minimum and maximum temperatures averaging 21-32°C. Treatments consisted of increasing levels of added fat in the lactation diet: 0, 1.67, 3.34, and 5.00% of corn oil. Diets were corn-soybean meal-based with a fixed soybean meal inclusion rate of 28.90%. Net energy (NE) concentrations of the treatment diets consisted of 2.345, 2.427, 2.509, and 2.591 Mcal NE/kg. Sow was the experimental unit and data were analyzed using generalized linear mixed models with the lmer and glmer functions of R. Increasing corn oil inclusion rates marginally (linear, P=0.086) reduced lactation average daily feed intake but did not significantly (P>0.10) impact total or average daily NE intakes. There was no evidence (P>0.05) for treatment effects on sow body weight, sow caliper score or sow backfat depth changes during lactation, percentage of removed and fall-off piglets, pre-weaning mortality, number of weaned pigs, litter weaning weight, total litter weight gain, and piglet average daily gain. In summary, increasing energy in the lactation diet by adding up to 5% corn oil during the summer period resulted in a marginal linear reduction in lactation average daily feed intake, but did not influence daily NE intake. Under the conditions of this study, no evidence for treatment effects were observed for sow or for litter performance during the nursing period. [ABSTRACT FROM AUTHOR]