Exploring optimal folic acid supplementation levels for lactating-pregnant rabbit does with different litter size

The lactation-pregnancy overlap in the industrialized 49-d breeding model increases nutritional demands for lactating-pregnant rabbit does. This study examined the effects of folic acid (FA) on the production performance and intestinal microflora of does with different litter sizes (LS, or number of kits). A total of 144 third-parity Hyplus does, aged 11 mo and weighing approximately 5.00 ± 0.07 kg, were divided into 4 treatment groups: control group (basal diet) and FA groups (basal diet + 15, 30, and 45 mg/kg FA). Does with LS of 5, 7, and 9 were selected after weaning. Our findings revealed that 1) Increased FA supplementation initially increased and then decreased the pre-lactation body weight of 21-d-old kits and prolactin (PRL) level of lactating does, while showing an inverse trend for estrogen (E) level of does. Increased LS significantly (P < 0.05) correlated with reduced milk yield of does. There was a significant (P < 0.05) interaction between FA and LS affecting PRL, E, growth hormone levels of does, and the pre-lactation body weight of 21-d-old kits; 2) Increased FA supplementation initially increased and then decreased the post-lactation body weight of 21-d-old and 35-d-old kits, elevated plasma FA (PFA) level of lactating-pregnant does, and significantly (P < 0.05) impacted the pregnancy rate of does. Increased LS was associated with reduced post-lactation body weight in 21-d-old and 35-d-old kits (P < 0.05). There was a significant (P < 0.05) interaction between FA and LS affecting plasma insulin-like growth factor-1 (IGF-1), homocysteine (HCY), PFA levels of does, the post-lactation body weight of 21-d-old kits, and weaning body weight of 35-d-old kits; and 3) FA supplementation promoted the growth of Ruminococcaceae and Rikenellaceae_RC9_gut_group in the feces of lactating-pregnant does, as well as the proliferation of Lachnospiraceae_NK4A136_group in the feces of pregnant does, suggesting enhanced anti-inflammatory properties and improved crude fiber breakdown. In summary, FA supplementation improved conception rates, regulated lactation-related hormone synthesis and secretion, increased body weight of 21-d-old kits, and enhanced intestinal anti-inflammatory functions.