Effects of pregnancy on quantitative aspects of nutrition, physiology and metabolism of beef heifers

Abstract

There is a lack of information about the effect of gestation and gestation time on intake, total and partial digestion and nutrient utilization, reduction of ruminal volume and changes in the feed passage rate in Zebu cows. Brazil has 55 million heads that could be benefited from the knowledge advances in this area. Therefore, this research was carried out to quantify the effects of pregnancy on the nutrition, physiology and metabolism of beef heifers. Twelve ruminally cannulated Zebu beef heifers were divided at random into two groups [pregnant (n = 7) and non-pregnant (n = 5)]. All heifers received the same diet throughout the experiment. Pregnant heifers accumulated body reserves (+ 35 kg) up to 240 days of pregnancy (DOP), then started mobilizing tissues (- 36 kg) until 286 DOP. The intake of dry matter (DM), organic matter (OM), crude protein (CP), neutral detergent fiber (NDF) and total digestible nutrients (TDN) reduces (P < 0.06) with the course of pregnancy. The apparent total-tract digestibility of DM and NDF and ruminal digestibility of OM and NDF was lower (P ≤ 0.09) for pregnant compared with non-pregnant heifers. Crude protein apparent total-tract digestibility at 267 and 286 DOP was greater (P < 0.01) to pregnant compared with non-pregnant heifers. The ruminal pool of wet matter (- 7.10 kg), DM (- 1.30 kg) and NDF (- 0.63 kg) was lower (P < 0.02) to pregnant than non-pregnant heifers at 267 DOP. In all experimental periods, DM passage rate was greater (P < 0.09) to pregnant than non-pregnant heifers. No difference was found on nitrogen balance and ruminal fermentation parameters. Pregnant heifers were more efficient over time to synthetize microbial protein. The heart rate of late-pregnant heifers compared to controls increased by eight beats/min when evaluated just before morning feeding, and the difference reached 11 beats/min when evaluated four hours after morning feeding. Glucose levels before morning feeding were similar during all collection periods with an exception at 286 days (DOP × physiological status interaction; P = 0.05) when glucose was lower in pregnant (83 mg/dL) compared to non-pregnant (107 mg/dL) heifers. Both genes related to remodeling, inflammation, and volatile fatty acids, H+, HCO3-, and glucose transport in the ruminal epithelium were downregulated at late gestation. These results suggest that the ruminal epithelium saves energy at late pregnancy to benefit fetal development. In addition, the increase in heart rate coupled with tissue mobilization can be considered homeorhetic mechanisms that help meet the fetal nutrient requirements. The physiological status, as well as the stage of gestation, should be included in performance prediction models since late-gestating beef heifers are less efficient at extracting energy from feed compared to non-pregnant animals, changing the feed predicted total digestible nutrients.