G.E. Dahl

Since the initial report in 1978 of galactopoietic effects of a photoperiod of 16 h of light:8 h of darkness, numerous studies have confirmed long-day stimulation of milk yield. The endocrine factor(s) responsible for increased milk yield, however, has eluded identification. Recent studies suggest that insulin-like growth factor-I (IGF-I) may mediate the galactopoietic response to long day photoperiod. Indeed, long days increase IGF-I in heifers and lactating cows; in the latter case, the response preceded an increase in milk yield. In heifers and cows, the increase in IGF-I is independent of changes in circulating growth hormone. Melatonin feeding to mimic a short-day photoperiod suppressed the increase of IGF-I in heifers induced by long days. However, melatonin feeding had no effect on milk yield in cows. Despite lack of resolution of the endocrine mechanism, dairy producers are interested in how photoperiod management can be integrated with current practices throughout the lactation cycle. There is strong evidence that milk yield responses to long days persist through an entire lactation. Also, long days can be combined with bovine somatotropin (bST) to produce additive increases in milk yield. During the dry period, long days increase the periparturient surge of prolactin. However, relative to long days, short-day treatment during the dry period produces the largest magnitude of response in milk yield during the subsequent lactation. The response to short days during the dry period may be due to a priming effect on the photoperiodic response system. In summary, IGF-I has emerged as a possible mediator of the increase of milk yield in response to long-day photoperiod. Photoperiod can be combined effectively with other management techniques such as bST. Consideration of photoperiod management during the dry period is essential to maximize responses during the subsequent lactation.

Cows exposed to short day photoperiod during the dry period produce significantly more milk in their subsequent lactation than cows exposed to long days. The mechanism(s) underlying this effect are unknown. Because concentrations of prolactin (PRL) in circulation are consistently affected by changes in photoperiod, we hypothesized that alterations in the prolactin axis and sensitivity of the mammary gland to prolactin signaling may mediate photoperiodic effects in dry cows. The objective of this study was to determine the effects of exposure to different lengths of daylight during the dry period on circulating PRL and PRL receptor (PRL-R) mRNA expression in lymphocytes and mammary tissue during the transition to lactation. Multiparous Holstein cows were dried off 62 d before calving and assigned to long day (16 h light: 8 h dark) or short day photoperiod (8 h light: 16 h dark). During the dry period, PRL and PRL-R mRNA were analyzed biweekly in plasma and lymphocytes, respectively. Expression of PRL-R mRNA was assessed in mammary biopsies during the dry and periparturient periods. Dry matter intake (DMI) was recorded through 21 d of lactation, and milk yield was recorded until 120 d in milk. Short day photoperiod was associated with reduced PRL, whereas milk yield and expression of PRL-R mRNA in lymphocytes and mammary tissue were increased. Cows on short days had higher DMI during the dry period but did not differ in DMI after parturition. These data support the concept that greater responsiveness and sensitivity to PRL during transition to lactation may be associated with an increase in subsequent milk yield.

Relative to a short daily (24-h) photoperiod, exposure to a long daily photoperiod increases the milk yield of dairy cows. However, the endocrine basis for this phenomenon is unknown. The present study was designed to test the hypothesis that a long daily photoperiod is associated with increased circulating insulin-like growth factor (IGF)-I, a hormone that is galactopoietic in ruminants. Forty lactating cows were exposed to either a natural photoperiod (< or = 13 h of light/d) or to a long daily photoperiod (18 h of light and 6 h of darkness) between January and April 1995. Cows were fed for ad libitum intake a total mixed diet formulated to meet the nutritional demands of lactation. Milk yield and dry matter intake were quantitated each day, and blood samples were collected by coccygeal venipuncture every 14 d. Plasma was harvested and assayed for IGF-I. The long photoperiod increased milk yield relative to the natural photoperiod (36.1 +/ 0.6 vs. 33.9 +/ 0.6 kg/d); the increase became significant after 28 d of treatment and was maintained for the duration of the study. In addition, cows exposed to a long photoperiod had greater circulating concentrations of IGF-I than did cows exposed to the ambient natural photoperiod (60.1 +/ 2.0 vs. 52.6 +/ 2.0 ng/ml). Concentrations of IGF binding protein -2 and -3 in plasma did not differ between treatments. These results support the hypothesis that a long daily photoperiod increases circulating concentrations of IGF-I in lactating cows and reveal a possible endocrine mechanism for the galactopoietic response to a long daily photoperiod.

An acute stress response can be provoked by abrupt social, nutritional, and environmental changes associated with weaning, and this may disrupt homeostasis and thus compromise well-being. Manipulating environmental factors, such as photoperiod, might provide a simple way to reduce the physiological consequences that piglets experience due to weaning stress. The objective of this study was to evaluate the impacts of photoperiod manipulation across various weaning ages on leukocyte populations, lymphocyte proliferation, natural killer cytotoxicity (NK), chemotaxis, phagocytosis, and immunoglobulin G, cortisol, and BW of piglets during the nursery phase. Sixty-eight crossbred piglets were obtained from sows kept on a short-day (8 h of light/d) photoperiod from d 90 of gestation until weaning. Piglets were weaned at 14, 21, or 28 d of age and kept on a short or long (16 h of light/d) photoperiod until 10 wk of age. Piglet BW and blood samples were collected at weaning and at 6, 8, and 10 wk of age. Pigs weaned at 28 d had reduced neutrophil counts (P < 0.001), phagocytosis (P < 0.001), and lymphocyte proliferation (P < 0.05) at weaning compared with those weaned at 14 and 21 d. Pigs weaned at 21 d tended to have lower (P = 0.08) lymphocyte counts than did pigs weaned at 14 or 28 d. Pigs weaned at 14 d had reduced (P < 0.01) NK relative to those weaned at 21 or 28 d. Photoperiod also influenced pig BW and immune status. Generally, those pigs on the long-day photoperiod and weaned at 28 d were heavier (P < 0.001) than their counterparts weaned at 14 or 21 d. At 6 wk of age, NK was greater (P = 0.002) in pigs kept on a long day and weaned at 14 or 21 d than in pigs weaned at 28 d. Phagocytosis was less (P = 0.005) at 6 wk of age, but was greater at 8 wk, in piglets kept on the long day and weaned at 28 d than in long-day pigs weaned at 14 or 21 d. These results suggest that photoperiod differentially influences immune responses in piglets weaned at different ages and indicate an inverse relationship between growth and immune status. Here, weaning at 28 d and a long-day photoperiod was the treatment combination that was most physiologically beneficial to piglets, whereas a 14-d weaning and short-day photoperiod was least physiologically beneficial.

Thirty-four lactating Holstein cows were dried off 60 d prior to their expected calving date, paired by calving date, and randomly assigned to one of two photoperiod treatments: long-day photoperiod (n = 18; 16 h light: 8 h dark/d) or a short-day photoperiod (n = 16; 8 h light: 16 h dark/d) to determine if manipulation of photoperiod during the dry period would impact milk yield in the subsequent lactation. At parturition, calf weight, height, and length were measured and the cows were moved into ambient lighting conditions (November 1996 to January 1997 and November 1997 to January 1998, 39 degrees latitude) with the rest of the University of Maryland herd. After parturition, milk yield and composition were measured for 16 wk. Prolactin concentrations were higher by 11.7 ng/ml in the long-day photoperiod treatment group than for the cows on a short-day photoperiod. During lactation cows previously exposed to a short-day photoperiod produced 3.2 kg/d more milk than long-day photoperiod cows. Energy-corrected milk yield was greater in short-day photoperiod than in long-day photoperiod cows. Treatment did not affect calf growth, milk composition, or plasma insulin growth factor I concentrations. In conclusion, manipulation of photoperiod during the dry period of dairy cows may be a useful management tool for increasing milk yield in the subsequent lactation.