Selection of dairy sheep based on production levels has caused a loss of rusticity, which might compromise their future resilience to nutritional challenges. Although refocusing breeding programs toward improved feed efficiency (FE) is expected, more-efficient ewes also seem to be more productive. As a first step to examine the relationship between FE and resilience in dairy sheep, in this study we explored the variation in the response to and the recovery from an acute nutritional challenge in high-yielding Assaf ewes phenotypically divergent for FE. First, feed intake, milk yield and composition, and body weight changes were recorded individually over a 3-wk period in a total of 40 sheep fed a total mixed ration (TMR) ad libitum. Data were used to calculate their FE index (FEI, defined as the difference between the actual and predicted intake estimated through net energy requirements for maintenance, production, and weight change). The highest and lowest FE ewes (H-FE and L-FE groups, respectively; 10 animals/group) were selected and then subjected to the nutritional challenge (i.e., withdrawing the TMR and limiting their diet only to the consumption of straw for 3 d). Afterward, sheep were fed again the TMR ad libitum. Temporal patterns of variation in performance traits, and ruminal fermentation and blood parameters were examined. A good consistency between FEI, residual feed intake, and feed conversion ratio was observed. Results supported that H-FE were more productive than L-FE sheep at similar intake level. Average time trends of milk yield generated by a piecewise model suggest that temporal patterns of variation in this trait would be related to prechallenge production level (i.e., H-FE presented quicker response and recovery than L-FE). Considering all studied traits, the overall response to and recovery from underfeeding was apparently similar or even better in H-FE than in L-FE. This would refute the initial hypothesis of a poorer resilience of more-efficient sheep to an acute underfeeding. However, the question remains whether a longer term feed restriction might impair the ability of H-FE ewes to maintain or revert to a high-production status, which would require further research.

Despite the important role of digesta mean retention time (MRT) on digestive efficiency of ruminants, it is poorly investigated in total gastrointestinal tract (GIT) of growing ruminants, especially in goats. The objective of this study was to evaluate the effect of body weight (BW) and sex on GIT MRT of particles and solutes in growing Saanen goats. A dataset from two studies, comprising 103 individual records of castrated males (n = 36), females (n = 34), and intact males (n = 33) Saanen goats slaughtered at 15, 22, 30, 37, and 45 kg BW, was used. Goats were fed basically with total mixed ration composed by dehydrated corn plant (Zea mays) milled to pass a 10- mm screen, cracked corn grain, and soybean (Glycine max) meal. Variables evaluated were BW, feed intake, feed intake level, composition of ingested diet, wet weight of GIT tissues, wet digesta pool size, digesta composition (dry matter, neutral detergent fiber (NDF)), indigestible NDF:NDF ratio of ingested diet and GIT digesta, MRT of particles (MRT_iNDF) and solutes (MRT_Cr), and reticulorumen selectivity factors (large particles/solutes). Reticulorumen, omasum, abomasum, small intestine, cecum, and colon-rectum segments were evaluated. The dataset was analyzed as mixed models considering sex, BW, and sex × BW interaction as fixed effects, and study and residual error as random effects. Sex did not affect MRT_iNDF in any GIT segments. Females and intact males presented similar reticulorumen MRT_Cr (5.6 h; P = 0.92) and they presented lower reticulorumen MRT_Cr than castrated males (7.0; P ≤ 0.04). Total GIT MRT_Cr was similar between castrated males and females (15.7 h; P = 0.11) and between females and intact males (14.2 h; P = 0.76). Body weight did not affect MRT_iNDF in reticulorumen and colon-rectum and total GIT MRT_Cr (P ≥ 0.11). Reticulorumen and omasum MRT_Cr increased as BW increased (P < 0.01), and abomasum MRT_Cr decreased as BW increased (P = 0.02). Feed intake, and wet tissues and wet pool size of all GIT segments increased as BW increased, except abomasum wet pool size (P ≤ 0.01). The mechanism related to sex effect on MRT has to be elucidated. Reticulorumen MRT_iNDF and total GIT MRT_Cr is modulated by intake and GIT capacity. On the other hand, reticulorumen MRT_Cr seems to be regulated by reticulo-omasal orifice opening and saliva secretion.

Visceral organs play an important role in animals' energy requirements, so their growth must be well understood. The objective of the current study was to fit and compare growth curves that best describe body and visceral organ growth over time in Saanen goats of different sexes. Data were synthesized from seven studies in which curves were fitted to visceral organ growth over time for female, intact male and castrated male Saanen goats from 5 to 45 kg body weight. The liver, pancreas, spleen, rumen–reticulum, omasum, abomasum, small intestine, large intestine and mesenteric adipose tissue (MAT) data were fitted to eight models: simple linear regression, quadratic, monomolecular, Brody, Von Bertalanffy, logistic, Gompertz and Richards. The best-fit model was chosen based on the corrected Akaike information criterion and the concordance correlation coefficient. Model parameters for each sex were compared. Overall, the model that best described visceral organ growth was the logistic model. Sex did not influence the parameters that predicted organ growth (g), except for MAT, where females presented a lower tissue deposition rate and greater inflection point than males. Irrespective of sex, at the beginning of the growth curve, the liver accounted for 28 ± 1.1 g/kg of empty body weight, and the inflection point occurred at 1.7 months. The rumen–reticulum and large intestine presented higher growth rates in the first 2 months of life. Knowledge of the visceral organ growth curve is useful in improving the understanding of the effect of nutritional requirements for goats and must be used to optimize the nutritional plans.