Minerals and reproduction
The mechanism of mineral-reproduction interactions is not fully understood because  of the complexity of neuro-hormonal dialogue. Some minerals act directly on the gonads, while others act through hypophyseal - pituitary - gonadal axis. Elements like Se once considered toxic, is known to improve both male and female fertility when supplemented in organic form as selenomethionine. During reproductive events reactive metabolites of oxygen are produced and are removed through antioxidant process by Se and vitamin E and provide a convenient environment for reproduction. Similarly other trace elements like Cu, Zn, Mn, Cr and I also act as co–factors or activate enzymes and helps in hormone synthesis and hence influence biochemical functions associated with reproduction. Because of their role in the endocrine system and in tissue integrity, minerals have a beneficial role to play in resumption of follicular growth and fertility in dairy cows and buffaloes. The potential for minerals to play a significant role in herd fertility is indisputable. The minerals that affect reproduction in ruminants are generally found within the trace element group, although deficiencies of calcium and phosphorous can also affect the fertility. Reproductive problems are frequently reported in association with trace mineral deficiencies, particularly copper, zinc, selenium and manganese.

Ca and P: Deficiency of Ca may delay uterine involution and increase incidence of dystocia, retained placenta and prolapse of uterus. Excess Ca may impair reproductive function by causing secondary deficiency of P, K Mg, Zn, Cu and other trace minerals by inhibiting their absorption in the intestine. Calcium-dependent mechanisms are involved in steroid biosynthesis in the tests, adrenal glands and ovaries. Calcium plays a role in the utilization of cholesterol by mitochondria or by stimulating the conversion of pregnenolone to progesterone. Phosphorus is often associated with reproductive abnormalities in cattle although infertility due to P deficiency is usually manifested after other signs are readily apparent. Phosphorus deficiency induces lowered conception rate, irregular estrus, anestrus, decreased ovarian activity, increased incidence of cystic follicles and generally depressed fertility. The involvement of P in phospholipid and c-AMP synthesis may be a key to its effect on reproduction.

Zn and Cu : Zinc deficiency in ruminants has been postulated to weaken the skin and other stratified epithelia as well as reducing the basal metabolic rate following infectious challenge. Zinc is a co-factor for many proteins and enzymes involved in acute phase response to infection and inflammation. Because the mammary gland is a skin gland, it is likely that zinc will have a positive role in its protection. Skin integrity of the teat has been shown to be specially linked with mastitis prevention. Zinc activates several enzyme systems and is a component of many metalloenzymes. I t  p la ys a  v i ta l  r o le  in  ho r mo n e  s e c r e t i o n ,  e s p e c ia l ly  r e l a t e d  t o  g r o wth , r ep r o duc tio n, immunoc o mp e tenc e a nd str e ss. Zinc is a lso invo lve d in the generation of keratin and in skin nucleic acid and collagen synthesis as well as in the maintenance of normal vitamin A concentration in plasma and in ovarian function. Many animals therefore require supplemental zinc in the diet for normal body function because of either low levels in the dietary ingredients or the presence of antagonistic factors, which decrease the bioavailability of the element. Antagonism might be due to metals ion interactions such as iron or copper. Source of fibre has also been reported  to decrease the availability of zinc. Indian studies have shown that repeat breeding and /or anoestrus conditions in livestock could be controlled by improving Cu and Zn nutrition.

I : The need for iodine for the thyroid activity and for the prevention of goiter has been well recognized. Reproductive failure often is a secondary manifestation of thyroid dysfunction resulting from iodine deficiency in cattle. Fetal development during iodine deficiency may be arrested at any stage and lead to early embryonic death, fetal resorption, stillbirth or birth of goitrous, weak or dead fetus. Hypothyroidism also can reduce gonadotrophin output by the pituitary. Iodine deficiency in bulls is associated with depressed libido and deterioration of semen quality-several studies have revealed tha t sup p le menta tio n o f io d ine ha s imp r o ve d  fer tility, r e d uc e d stillb ir ths, abortions and incidence of retained placenta. Infertility in dairy cattle resulting from irregular or suppressed estrus is often responsive to iodine therapy. Iodine supplementation is necessary in many areas of deficiency but toxic amounts of iodine are not favourable.

Mn: Manganese is involved in the activities of several enzyme systems including hydrolases, kinases, decarboxylases and transferases as well as Fe-containing enzymes which require Mn in their activity. It is therefore involved in carbohydrate, lipid and protein metabolism. It is also needed for bone growth and maintenance of connective and skeletal tissue. Mn also plays a role in reproduction and in immunological function. Mn deficiency results in abnormal skeletal growth, increased fat deposition, reproductive problems and reduced milk production.

Se and Co: Selenium is a semi-metal that is very similar to sulphur in its chemical properties. It is an essential component of glutathione enzyme system, and a deficiency of selenium will leave the cell vulnerable to oxidation and increase the requirement of vitamin E. It has therefore been usual to supplement in the diets of all classes of animals, because of its antioxidant properties. Cobalt is an essential trace element in ruminant diets for the production of vitamin B12, which has 4% cobalt in its chemical
structure, by the rumen microbes to meet the vitamin B12  requirements of both the ruminal bacteria and the host animal. This means that a cobalt deficiency is really a vitamin B12 deficiency that would lead to anaemia.