Student Competition: Physiology and Reproduction

After 50 years of genetic selection, the production level of commercial laying hens has increased by ~40% . Selection for early maturation and extended laying capacity has likely impacted the hypothalamic-pituitary gonadal (HPG) axis, which must be stimulated earlier in life, and for a longer period of time . The objective of this study was to identify potential mechanisms contributing to the differential activation of the HPG axis between strains of laying hens . Three strains representing different time points across genetic selection history were used with 120 Lohmann LSL-lite (L) as a commercial strain, 100 heritage Shaver white leghorn (S) as a mid-selection strain, and 68 Smoky Joe white leghornderivative (J) as a non-selected strain with a production comparable to a 1960s strain . Birds were group-housed from hatch up to 12 weeks of age (woa), then transferred to colony cages (2 birds per cage) and monitored until 29woa . They were fed ad lib from hatch . Photoperiod was set at 10h from 6woa until photostimulation at 18woa (12h light), increasing 1h per week to 16h (22woa) . J hens were lighter than L and S (P < 0 .001) from 21 to 29woa . From 20 to 28woa, L and S hens had significantly greater production than J (P < 0 .0001) . At 19 woa, a correlation (0 .32) between body weight (bw) and egg production (P = 0 .01) was observed only for L, while at 23woa, when photostimulation is expected to be the primary drive on reproduction, the correlation was −0.26 (P = 0 .05) . Egg analysis was conducted at 26woa and weight, width, and shell thickness were different between groups (P < 0 .0001) . Eggs from J were shorter than from S and L (P < 0 .001), eggs from L had a higher breaking strength than from S and J (P < 0 .0001), and the albumen height was higher in eggs from S (P = 0 .04) . Estradiol (E2) profiles determined that hens from L and S displayed increases in E2 at 19woa (P < 0 .0001) and 26woa (P < 0 .05) . J birds displayed a delayed increase at 21woa (P < 0 .0001) . Based on these results, a correlation between bw and production is present only in modern strains, leading to the hypothesis that selection for early lay may have increased the importance of metabolic factors in the activation of the HPG axis regardless of photoperiod. Conversely, as shown by E2 profiles, non-selected strain (J) primarily rely on photoperiodic cues .