Biological rhythms are repeating patterns that are driven by time-keeping mechanisms within the animal and are adaptive as they coordinate physiology and metabolism with the external environment. The dairy cow has a well-recognized natural daily pattern of feed intake and milk synthesis and an annual rhythm of milk composition, but regulation of these rhythms has not been well described in the literature or well considered in current dairy management. We commonly assume that feeding a total mixed ration creates constant ruminal conditions, but the large variation in the rate of feed intake across the day causes large fluctuations in rumen fermentation and absorbed nutrients. Milk composition also differs across the day due to both dynamics in nutrient absorption and biological regulation attempting to match milk yield and composition with calf requirements across the day. Managing feeding times provides the opportunity to modify feed intake across the day, but behavior responses are complex.
Rather than simply responding to an environmental stimulus, endogenous timekeepers in the hypothalamus allow the animal to anticipate daily and yearly environmental changes before they occur. The timekeepers create rhythms that then drives adaptive changes in metabolism and physiology that increase survival. Two important aspects are the timing of the rhythms are set or “entrained” by environmental signals, such as light dark cycles, and the rhythms will persist if the animal is held under constant conditions because it is running within the body. Two major rhythms of importance to the dairy cow are circadian and annual rhythms.
Circadian rhythms refer to 24-hour repeating cycles followed by most physiological functions. Circadian rhythms are created by endogenous timekeeping mechanisms and are adaptive as they temporally coordinate behaviors and physiological processes with daily changes in the environment. Anyone who has flown across time zones or lost a night of sleep, or even just changed clocks to daylight savings time, appreciates the physiological and psychological importance of circadian rhythms. Their importance is also strongly supported by scientific evidence. For example, epidemiological data in humans clearly shows that disruption of circadian rhythms by night-shift work increases mortality and morbidity and is especially associated with many conditions normally associated with stress.
The “biological clocks” that keep track of what time it is exist in most tissues in the body. The biological clocks in metabolically important tissues (e.g. adipose and liver) are responsive both to timing of light-dark cycles that controls the master clock in the brain, but also the timing of food availability. Interestingly, in experimental models the timing of food intake can alter the synchrony between the central master timekeeper and peripheral clocks, resulting in development of numerous disorders including obesity, insulin resistance, and metabolic diseases (Reviewed by Takahashi et al., 2008). We have demonstrated that there is a biological clock in the mammary gland that responsive to the timing of feed intake.
Daily pattern of feed intake
Feeding behavior is centrally regulated through integration of many factors including hunger, satiety, physiological state, environment, and endogenous circadian rhythms (Allen et al., 2005). Grazing cows have a well described “crepuscular” feeding pattern with a large proportion of intake consumed at dawn and dusk (Reviewed by Albright, 1993). It is important to remember ruminants are prey animals and daily feeding patterns are expected to have been impacted evolutionarily by changes in risk of predators and nutritional value of forages over the day. Importantly, pasture forages are highest in sugar and amino acids in the afternoon after photosynthesis has occurred. A circadian rhythm of intake with greater intake during the afternoon synchronizes hunger with maximal forage quality.
Using an automated observation system, we have observed the effect of feeding time and diet composition on the daily rhythm of intake. The daily pattern of intake in high producing cows and the effect of feeding time is well illustrated in an experiment where we fed cows 1x/d at 0830 h or 2030 h (Niu et al., 2014). Over 20 and 34% of daily intake was consumed in the 2 h after feeding in cows fed at 0830 and 2030 h, respectively. The intake rate at other times of day did not differ greatly, with both groups having lower intake overnight and higher intake in the afternoon. Before this work we commonly thought that cows consumed feed mostly during the day because that is when we delivered feed and it was the freshest. Delivery of fresh feed is a strong stimulus for feed intake. However, it is interesting to note that cows fed in the evening had low intake during the overnight (not different from morning fed cows) and waited till the following afternoon when feed was over 16 h old to increase intake to about twice that of the overnight period. This experiment highlights that cows have a strong natural drive to consume feed during the afternoon and early evening and timing of feed delivery is a strong stimulant to modify this pattern and has been replicated in other studies.
Physiological significance of the circadian pattern of intake
The ruminant has a rather consistent absorption of nutrients over the day because of more frequent meals, the size of the rumen, and the slow rate of ruminal digestion. However, highly fermentable diets are commonly fed to maximize energy intake and microbial protein production and result in a rapid production of volatile fatty acids (VFA) after consumption (Allen, 1997). Additionally, differences in the rate of feed intake over the day results in a large difference in the amount of fermentable substrate entering the rumen over the day
The dynamic nature of rumen fermentation throughout the day is supported by high resolution observations of rumen pH by our lab and others (e.g. Yang and Beauchemin, 2006, DeVries et al., 2007, Harvatine, 2012), which clearly show a daily pattern of rumen pH with a nadir approximately 10 h after feeding. We also observed that ruminal digesta weight and starch concentration were 24% and 87% higher, respectively, 4 h after feeding compared to 1.5 before feeding. Additionally, we have observed that ruminal starch and NDF concentration over the day fit a cosine function with a 24 h period demonstrating a daily rhythm (Ying et al., 2015). We are not aware of a characterization of the rate or composition of duodenal flow throughout the day, but a daily rhythm has also been reported for fecal particle size, neutral detergent fiber (NDF), indigestible NDF, and starch concentration (Maulfair et al., 2011). We have also observed that the rhythm of fecal NDF was dependent on the time of feeding (Niu et al., 2014). Taken together, there is strong support for a circadian rhythm of nutrient absorption.
Evidence of circadian regulation of milk synthesis
Dairymen commonly recognize that morning and evening milking differ in milk yield and composition. Quist et al. (2008) conducted a survey of the milking-to-milking variation in milk yield and composition on 16 dairy farms. Milk yield and milk fat concentration showed a clear repeated daily pattern over the 5 days of observation in herds that milked 2 and 3 x/d. We have also observed milk yield and milk composition at each milking while milking every 6 h and feeding cows 1 x/d at 0800 h or in 4 equal feedings every 6 h (Rottman et al., 2014). This demonstrated the daily pattern of milk synthesis in cows and identified an interaction with the timing of feed intake. We have further demonstrated shifts in the timing of milk synthesis through fasting cows for a short period during the day compared to the night (Salfer and Harvatine, 2020).
Recent work at Purdue tested the effect of light-dark phase shifting on metabolic health in transition dairy cows (Suarez-Trujillo et al., 2020). They observed that light phase shifting reduced the circadian rhythms of core body temperature and melatonin. Phase shifted cows also had increased total resting time, but decreased resting bout durations. Phase shifted cows did increase milk yield 2.8 kg/d over the first 60 d of lactation, although this may be due to a change in nutrient partitioning and the long-term effect was not investigated.
Lastly, automated milking systems (AMS) provide an opportunity to observe a natural preference for milking time. Care is needed in interpretation of cow behavior in AMS because of the confounding factors of demand for the robot and the entrainment by multiple factors. However, the frequency of cows entering the milking system appears to follow a circadian pattern (e.g. Hogeveen et al., 2001, Wagner-Storch and Palmer, 2003). For example, Wagner-Storch et al. (2003) reported 2% of cows in the holding area between 0000 and 0500 h compared to 8 to 12% of cow between 0800 and 1900 h. The preference for milking time may be due to a natural circadian synchronization with environmental factors or simply support a natural low activity period of the day.
Effects of Photoperiod on Milk Production
Extensive research has examined the impact of altering photoperiod length on milk synthesis of the dairy cow. The first report of increased milk production after 16 h light: 8 h dark (16L:8D) photoperiod was make by Dr. Tucker’s lab at Michigan State (Peters et al., 1978). Since this initial discovery, several subsequent experiments have confirmed these findings (Dahl et al., 2000, Dahl et al., 2012). The effect occurs after implementation of any photoperiod greater than 12L: 12D, however the response is greatest at 16L: 8D. The mechanism of long day lighting on milk synthesis is likely through the same molecular mechanism regulating other circadian rhythms in the dairy cow.
Take Home Messages
- “Biological clocks” within the cow are keeping track of what time of day it is and create daily rhythms.
- This robust system coordinates physiology and metabolism with the external environment and likely helps the cow optimally produce milk while adjusting to changing conditions across the day.
- The dairy cow has a clear daily pattern of feed intake and milk synthesis. The timing of feed intake has a large impact on rumen environment including pH.
- The timing of feed delivery and feed management are our best opportunities to modify the daily pattern of feed intake with the goal of spreading feed intake across a larger part of the day.
Presented at the 2021 Animal Nutrition Conference of Canada. For information on the next edition, click here.