Influence of feed restriction and subsequent recovery on lactating Charolais cows

International audience

English. One of the potential consequences of global climate change is that forage resources may become more limited atcertain times of the year. In this study, 340 primiparous Charolais cows from two experimental farms partici-pated in a feed restriction challenge in order to better understand how lactating beef cattle can adapt to thislimitation. Therefore, half of the animals were fed 3 forage units (FU) less than their expected needs (around 30%reduction) from 10 days after calving until they were turned out on grass in April (average duration of 85 days).All animals were then kept together until mid-July (recovery period, average duration of 89 days). Regularmeasurements were taken of dam weight, body condition score (BCS), and calf weight; three estimates weremade of dam milk production during lactation; and the date of each cow’s resumption of cyclicity was recorded.On one of the farms, the experiment was extended for two more lactations, for a total of 592 lactations among allthe animals examined. The effects of dietary restriction were analyzed using generalized linear models of thephenotyped traits. We found that winter feed restriction in early-lactation Charolais cows affected all traits, withgenerally similar impacts on the mobilization of body reserves and limitation of milk production. At the end ofthe winter period, diet-restricted dams were an average of 55 kg lighter than their unrestricted counterparts(corresponding to 0.66 standard deviations, s.d.) and had BCS values that were 0.81 points lower (0.55 s.d.).Similarly, calves from restricted dams were also lighter (20 kg difference, corresponding to 0.68 s.d.), reflectingthe reduced milk production of their mothers (-1.7 L at 90 days in milk, corresponding to 0.77 s.d.). On average,feed-restricted primiparous cows resumed cyclicity almost an entire cycle later than their control counterparts(difference of 17 days, corresponding to 0.48 s.d.). For cow traits, the differences between groups remainedsignificant but decreased in magnitude as spring went on, especially for milk production; by the start of the next