]. Liver and adipose tissue IGF-I response to GH administration is lower in cows with negative energy balance, as during early lactation, likely caused by down-regulation of GH receptors [27,28]. IGF-I decreased the blood fatty acid profile during early lactation after the administration of bovine somatotropin [28], which is in contrast to the previous research that adipose tissue stores were minimal as a result of the negative energy balance. Furthermore, protozoal infection in calves has been shown to cause prolonged thyroid deficiency dictated by reduced feed consumption, subsequent to impaired secretion of thyroid hormones and metabolism [29]. Because triiodothyronine (T3) is essential for GH-dependent IGF-I synthesis in the liver, decreased thyroid activity as a consequence of parasitism can have direct repercussions on growth and reproduction. In summary, nutrition mostly affects reproductive functions such as age at puberty, fertility, calving, mammary growth and milk production. In general, a high concentrate/high protein diet is recommended in heifer calves and cows during states of negative energy balance. Further research is needed to better understand the mechanisms by which specific nutrients influence hormones affecting systems physiology. This may also enhance biotechnology both in cattle and other livestock.