Postprandial rhythms of circulating urea in lactating dairy cows: feeding timing and diet effects

Abstract

The objective of the first experiment was to determine effects of feeding time and dietary forage to concentrate ratio on diurnal averages and rhythms of blood urea (BU). Four multiparous (body weight, BW = 652 ± 14 kg, mean ± SD; body condition score, BCS = 2.87 ± 0.14; days in milk, DIM = 83 ± 22) and four primiparous (BW = 667 ± 110 kg; BCS = 3.19 ± 0.66; DIM = 81 ± 23) Holstein cows were utilized in a 4 × 4 Latin square design study with a 2 × 2 factorial arrangement of feeding time and diet type. A higher concentrate (forage to concentrate ratio = 38.5:61.5) or a lower concentrate (forage to concentrate ratio = 50.6:49.4) total mixed ration (TMR) was offered at either 2100 h or 0900 h. The study had four 21-d periods, with 14 d of adaptation. Blood was sampled every 2 h for two 24-h periods during sampling weeks. Feeding at 2100 h vs. 0900 h increased feed consumed within 3 h of feeding time, from 26% to 37% of total daily intake. Total daily dry matter intake (DMI) was similar among treatments. BU exhibited significant diurnal rhythms in both groups, which depended on diet type and its interactions with sampling hour and time of feeding (TF). The objective of the second experiment was to establish diurnal and post-feeding responses in BU levels to feeding time in lactating cows following 28-d adaptation periods. Four multiparous and four primiparous lactating Holstein cows (82 DIM) were used in a cross-over design study with two 6-wk periods. Each period had 4 wk of adaptation. A TMR with 49.8% concentrate was offered at either 0900 or 2100 h to permit 5–10% feed refusals. Jugular blood was sampled via catheters every 2 h for two 24-h period during wk 5 of each period. The proportion of daily TMR intake consumed within 3 h of feeding time was 55% in cows fed at 2100 h, and 46% in cows fed at 0900 h (P < 0.05). Rumen ammonia was higher at 2 h but lower at 6 h post-feeding and remained lower between 6 and 20 h post-feeding in cows fed at 2100 h vs. 0900 h. Feeding time did not affect daily averages of BU. BU increased shortly after feeding in morning-fed cows but not in evening-fed cows. BU was higher for about 12 h pre-feeding in evening- vs. morning-fed cows. Results establish that TF alters postprandial and diurnal patterns of feed intake, rumen ammonia, and peripheral BU. Time of eating and its interaction with diet type can therefore affect splanchnic and peripheral nitrogen metabolism and efficiency in lactating cows. Considering the comparative nature of the splanchnic metabolism in optimal mechanistic understanding of the gut function in any given species, these data could serve as a theoretical and in several aspects as an applied metabolic model for other high-producing ruminants and humans.