The present study evaluated the effect of increasing levels of forage physically effective neutral detergent fiber ( pef NDF) on performance of feedlot lambs fed ground corn-based diets. The experimental design was a randomized complete block (grouping 6 treatments into 10 blocks), defined by initial BW and age at the beginning of the experiment. The experimental treatments were defined by the inclusion of forage pef NDF at 0 (0 pef NDF); 26 (26 pef NDF); 52 (52 pef NDF); 78 (78 pef NDF); 104 (104 pef NDF) and 130 (130 pef NDF) g/kg DM of a flint corn-based diet. The source of the forage was coast cross ( Cynodon sp. ) hay. There was a quadratic effect for dry matter intake [DMI (kg/d; P = 0.03)] and average daily gain [ADG (P < 0.01)], peaking at 78 pef NDF. However, the feed efficiency (FE) decreased linearly (P = 0.03) with increasing pef NDF levels. Intake time increased linearly (P < 0.001), but rumination (P < 0.0001) and chewing times (min/d) (P < 0.001) showed a quadratic effect, with the greatest values observed at 78 pef NDF when expressed in min/g DM. The slaughter body weight (SBW) showed a quadratic effect (P < 0.01), with the greatest values observed for 78 pef NDF. However, the greatest (P < 0.01) hot carcass weight (HCW) and hot carcass yield (HCY) were observed for lambs fed a diet 52 pef NDF. Diets containing ground flint corn as the main energy source for feedlot lambs can be formulated with smaller pef NDF levels than found in the literature, from 52 to 78 g/kg DM of pef NDF, depending on the objective. • Intake and daily gain peaked at 78 g/kg of effective forage fiber in the diet of lambs. • Feed efficiency decreased linearly as the effective forage fiber level increased. • Rumination and chewing times were greatest at 78 g/kg of effective forage fiber. • Carcass weight and yield were maximized at 52 g/kg of effective forage fiber. • Optimal forage fiber is 52-78 g/kg to balance animal gain and carcass yield.

From an economic perspective, meat sheep farms depend on several components, such as reproductive traits, which can be influenced by numerous factors of variation. In this study, the effects of two kinds of oestrus, a) induced oestrus (IO) and b) the first natural return oestrus (FRO), and of the genotype encoding a hyperprolific gene (FecL) on prolificacy and fertility were tested. The genetic parameters of these two traits were also estimated using BLUP animal models with ASReml software. The data related to 68,652 matings were recorded from 2010 to 2020 for 20,770 ewes from the Lacaune Ovi-Test population. The oestrus type, which is associated with the use of hormonal treatment, improved the two reproductive traits studied. As expected, one copy of the mutated allele of the FecL gene had an effect on the prolificacy of +0.5 lambs per lambing but had no effect on fertility. Moreover, the animal variances estimated during this study for prolificacy and fertility after IO and on FRO were low but still explained a share of the phenotype variance of ±0.5 lambs per litter and ±0.20 fertility points around the average. The repeatabilities and heritabilities of prolificacy and fertility were low but consistent with the literature. Owing to the structural relationship between the oestrus type in our dataset, less data were available on FRO, making its genetic evaluation less accurate than the genetic evaluation on IO. • Reproduction results according to the oestrus type differed in this study. • Hormonal treatments, used to synchronize oestrus, improved fertility and litter size. • Mutation of an allele of the FecL gene increased litter size by + 0.5 lambs. • The FecL gene had no impact on fertility. • The repeatability and heritability of prolificacy and fertility were low.