There is evidence indicating that seasonal changes in IGF-1 concentrations are driven by changes in photoperiod. Previous studies have shown that dry matter intake and milk production increase in dairy cattle in response to long day photoperiods, and this has been found in association with increased IGF-1, but not growth hormone. All animals were maintained under short day conditions (8 h light, 16 h dark). Two groups of eight lactating, pregnant cows were exposed to electromagnetic fields for 16 h per day (8 h light period plus first 8 h of the dark period) in either of two sequences. Each sequence consisted of three consecutive 28 days periods. For the first group: 28 days non-exposed, followed by 28 days exposed and then 28 days non-exposed (OFF-ON-OFF). For the second group: 28 days exposed, 28 days non-exposed, and 28 days exposed (ON-OFF-ON).
Dry matter intake and plasma IGF-1 were increased during exposure. IGF-1 concentration was comparable to that reported in experiments with artifical long day photoperiods. The meangrowth hormone concentration was not affected, but a "treatment x hour interaction" was found, with growth hormone lower for the exposedanimals during the first 16 h of the sampling period, and higher for the last 8 h (24 h measurement). Overall, the milk yield or its components were not affected by electromagnetic fieldexposure, but milk yield was significantly higher for the exposedanimals during week 4 of treatment (at the end of the experimental period).