great lakes dairy sheep symposium - the Department of Animal ...

light treatments, when all ewes were switched to equilibrated day lengths (12 h light), the LDPP
ewes experienced a 37% decrease in milk production over the next six weeks while SDPP ewes
only experienced an 8% decrease in milk production (Bocquier et al., 1997). A similar effect of
LDPP on established lactation has been observed in dairy cows over a range of production levels
and stages of lactation (reviewed by Dahl et al., 2000).
While there is evidence that photoperiod affects milk production during lactation, recent
studies in dairy cows have shown that prepartum photoperiod also affects mammary
development and subsequent milk production. Cows experiencing short days during the dry
period produced more milk than those exposed to long days. Milk production in short-day
treated dairy cattle was 3.2 kg/d greater than long-day treated cattle during the first 16 weeks of
lactation, following a prepartum treatment period of 60 d (Miller et al., 2000). The proposed
mechanism of action is the effect of photoperiod on circulating prolactin.
Based on previous work on the interaction of prolactin and the mammary gland, there is an
inverse relationship between circulating prolactin levels and the expression of prolactin receptor
mRNA in the liver, lymphocytes and mammary tissue of steers (Auchtung et al., 2003) and cows
(Auchtung et al., 2005). Auchtung et al. (2005) found that cows in SDPP treatments for 60 d
prepartum had lower levels of circulating prolactin than those in LDPP at both 33 and 5 days
before calving. Prolactin returned to similar levels 2 days after calving. Cows experiencing
SDPP also had increased expression of prolactin receptor mRNA. Thus, SDPP cows would be
more sensitive to the natural periparturient prolactin surge, possibly experiencing enhanced
proliferation and survival of mammary cells (Auchtung et al., 2005; Wall et al., 2005) due to the
critical role of prolactin in mammary gland development.
During mammogenesis, prolactin promotes expansion of mammary ducts in rats (Akers,
2002). The expansion of these alveolar ducts is critical for complete development of the
mammary gland and maximization of milk producing surface area on secretory epithelial cells.
In sheep, when prolactin production is blocked for 30 days prepartum, ewes produce
significantly less milk during the first two weeks of lactation than untreated ewes (Hooley et al.,
1978). Prolactin is thought to play a role in the commitment of mammary cells to produce milk,
and manipulation of the signaling to these cells through prolactin mRNA may affect the number
of cells available for milk production.
Initial analysis of prepartum prolactin levels in dairy sheep have found prepartum responses
to photoperiod. Pregnant ewes (90 days in gestation) exposed to LDPP had higher prepartum
plasma prolactin levels than ewes exposed to SDPP after 30 days of light treatment (Perier et al.,
1986). Bassett (1992) found this difference in prolactin levels after three weeks of light
treatment, but found no difference in growth of lambs nursing ewes in each photoperiod
treatment. While this study noted an increase in udder size of long day ewes, this was
substantially influenced by the number of fetuses, with larger udders found in ewes bearing two
or more fetuses and may not have been affected by photoperiod. This study was conducted to
determine the effect of pre-lambing photoperiod on circulating prolactin, milk production and
milk composition in dairy ewes.
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