Effect of lactation stage on quantitative aspects of digestion and physiology of beef cows

Abstract

The lactation period is critical to the cow, since this phase is marked of greatest nutrient demand by beef cows. This demand is associated with milk yield. Information about quantitative adjustments in the uptake and utilization of nutrients according to the stage of lactation in ruminants are scarce. Our objective was to quantify the effects of the physiological status (PS, lactation) and of the stage of lactation (DIM = days in milk) on dry matter (DM) intake (DMI), total apparent, ruminal, and intestinal digestibilities, as well as on metabolism of beef cows. The experiment was conducted in the feedlot facilities of the Department of Animal Science of the Federal University of Lavras. Twelve beef heifers with an average body weight (BW) of 482 ± 129 kg fitted with rumen cannula were used. Seven lactating (LA) cows were compared with their non-lactating (NLA) pairs (n = 5, heifers from the same contemporary group) to estimate the physiological effect of lactation at different time points over time (3, 10, 35 and 100 days of lactation). Before calving the animals were housed in individual pens with 50 m² per animal, with 16 m² of covered area to facilitate animals handling from calving. At being allocated in the pens, the heifers started the adaptation phase to the experimental conditions, in which the quantities of DM offered were gradually increased until voluntary intake was reached. At calving, it was installed in the bottom of each pen, a structure that allows the calves to move to a common pasture area, without the possibility of cows to move. The animal received the same diet composed by (DM basis) corn silage (92.3%) and concentrate supplement (7.7%) prepared from soybean meal (4.55%), ground corn (0.25%), urea (1.58%) and mineral mixture (1.32%). The nutritional composition of the experimental diet was designed to allow ad libitum intake without large accumulation of body reserves and adequate maintenance of lactation. Data was analysed through the mixed models methodology, considering the physiological effect (lactating and non-lactating) and the days in milk (DIM) as classificatory fixed effects and the animal as the random effect. Measurements were taken repeatedly over time (animal as experimental unit). The LA group increased DMI (g/kg BW) from DIM10 to DIM35 by 26% (P < 0.001), following with intake relatively constant until DIM100 (P = 0.205), whereas the NLA group increase by 18% in DMI during all trial period (P = 0.079). Ruminal digestibilities of DM, organic matter (OM), and ash- and protein-free neutral detergent fiber (apNDF) were lower for the LA group than for the NLA group from DIM10 to DIM35 (P < 0.1). However, there were no differences in ruminal digestibilities of all nutrients until DIM100 (P > 0.1). Physiological status did not affect the intestinal digestibility of all nutrients (P > 0.1). The mean ruminal pool of DM was greater than 31.5% for LA group that NLA group during the trial period. Lactating cows increased by 6.2% their ruminal pool between DIM10 and DIM100. Non-lactating cows showed ruminal pool relatively constant until DIM100 (mean = 4.55 ± 0.38). Passage rate (kp, h -1 ) of DM was affected by PS (P = 0.024). Lactating group presented greater passage rate (0.045 ± 0.0022) than NLA group (0.037 ± 0.0026). However, higher difference in passage rate between the groups was observed at DIM35 (kp, h -1 = 0.011). The physiological status as well as the stage of lactation should be included in performance prediction models, since early lactating beef cows are less efficient at extracting energy from feed compared to non-lactating animals, changing the feed predicted total digestible nutrients (TDN) values.