How molting of laying hens influences body composition and blood parameters

The physiological and metabolic changes laying hens undergo during molt are poorly understood, but could aid in understanding why hens stop egg production during the first cycle of lay. We therefore induced a molt and studied how this influenced body composition, blood parameters and production performance. Additionally, four diets postmolt were fed in a 2 × 2 factorial design with two levels of metabolisable energy lay (ME Lay; low = 11.0 MJ and high = 11.9 MJ) and two apparent faecal digestible lysine levels (AFD; low = 0.58% and high = 0.72%). Data were subjected to mixed model analyses. A molt was successfully induced at 58 weeks of age, during which hens stopped consuming feed and producing eggs, and lost on average 21% BW. Most of this BW loss was due to body breast weight loss (-56 g, time effect P < 0.05) and ovary loss (-33.6 g, time effect P < 0.05) and to a lesser extent due to fat pad loss (-7.1 g, time effect P > 0.05). Early laying rate and egg mass production of hens fed the high AFD Lys diets postmolt were significantly higher compared to hens fed the low AFD Lys diet. Egg weights of hens fed high AFD Lys diets were lower. Both effects were only short-term in weeks 59-62 and indicated that high amino acid intake is important for early laying rate in the second cycle of lay, potentially related to feather growth and restoration of body protein. Hens fed low ME Lay diets increased average daily feed intake (ADFI) in weeks 62-65, compared to hens fed high ME Lay diets (P < 0.05). This resulted in higher ME Lay and AFD Lys intake (P < 0.05). Hens fed these low ME Lay diets had a higher egg mass production in weeks 62-65 (P < 0.05), due to higher egg weights (P < 0.05), without a difference in laying rate (P > 0.05). Average daily gain was also significantly higher, mostly due to higher breast percentage (P < 0.05). Hens fed low ME Lay diets probably needed a higher lipoprotein production in the liver to meet the egg production demand, indicated by higher plasma cholesterol (P = 0.07) and triglyceride (P < 0.05) levels, and heavier liver weights (P < 0.05). In conclusion, molting significantly influenced the body composition of laying hens, with reduced breast, liver and ovary weights. Lower postmolt ME Lay diets increased breast, liver and ovary weights and increased egg weights and egg mass production. High AFD Lys diets only showed a short-term positive effect on the laying rate.