This study aimed to quantify the effects of physiological status (PS) and the potential interaction of this factor with days of pregnancy (DOP) on beef heifers' weight variation, intake, and digestion kinetics. Twelve rumen-cannulated zebu beef heifers (n = 7 pregnant; n = 5 nonpregnant) were used. Heifers were placed in individual pens and fed medium-quality corn silage plus a protein-based supplement. Heifers' body weight was assessed at the beginning and end of each collection period. The feed intake was measured daily. Target outcomes were evaluated at 107, 170, 208, 240, 267, and 286 DOP. The apparent total-tract digestibility was determined considering the fecal spot collection, during 5 d within each collection period. Omasal and ruminal digesta samples were collected at 107, 208, and 267 DOP for determination of partial digestibilities and diet component outflow. For these purposes, Co-EDTA and indigestible neutral detergent fiber (iNDF) were used as indicators. All data were analyzed using a mixed model framework, considering the PS and DOP as fixed effects and the animal as a random effect. Significant differences were declared when P ≤ 0.05. Pregnant heifers showed an increase in body reserves (+35 kg) from 107 to 240 DOP, but experienced a decrease in shrunk body weight (-36 kg) from 240 to 286 DOP. The intake of dry matter (DM), organic matter (OM), crude protein (CP), and ash- and protein-free neutral detergent fiber (apNDF) increased as pregnancy progressed (P < 0.04). The apparent total-tract digestibility of DM tended to be lower (P = 0.09), and the apparent total-tract digestibility of apNDF was reduced (P < 0.01) in pregnant heifers. The digestibility of CP was higher (P < 0.01) on days 267 and 286 in pregnant cows compared to nonpregnant heifers. The ruminal digestibility of OM tended to be lower (P = 0.09) in pregnant beef heifers, while the ruminal digestibility of apNDF was lower (P = 0.02) in pregnant heifers compared to nonpregnant ones. The ruminal pool of wet matter and DM was lower (P ≤ 0.01) in pregnant heifers at 267 DOP. In all periods, the outflow tended to be greater (P = 0.06) in pregnant heifers compared to nonpregnant heifers. Pregnant beef heifers exhibited a faster (P = 0.01) digesta passage rate during late gestation. In conclusion, late-gestating beef heifers are less efficient in extracting energy from feed compared to nonpregnant animals.
Keywords: dry matter intake; gestation; passage rate; ruminal capacity; ruminal digestibility; ruminal retention time.
Gestation is a physiological condition characterized by a high demand for nutrients to support fetal development, particularly during the late stages of pregnancy. During this time, the ruminal capacity is limited due to the loss of space by the growing pregnant uterus, potentially affecting feed intake. However, in beef cows, it has been observed that the decrease in feed intake during late pregnancy is not proportional to the reduction in rumen capacity. This suggests the existence of compensatory mechanisms that prevent severe depression in feed intake and ensure adequate fetal development. In the current study, we investigated the kinetics and ruminal parameters in cannulated zebu beef heifers throughout the second third of gestation until the prepartum period, considering their physiological status (pregnant or not).
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