VET VOICE PREMATING CHECKS
Cows not cycling? Check ovary health Vet Lisa Whitfield explains how ovarian hormones can malfunction and create non-cycling cows.
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any people forget that ovarian health is one of the key drivers of a cow achieving an efficient turnaround between calving, resumption of cycling and becoming pregnant again. A complex hormonal cascade – the hypothalamo-pituitary-ovarian axis – both drives and is limited by ovarian health. Excessive or insufficient levels of the hormones that drive this cycle will result in the fine hormonal balance being off, and thus a cow that is unlikely to be able to become pregnant. The main drivers of this cascade are: • GnRH - Gonadotropin releasing hormone – produced in the hypothalamus • LH - Luteinising hormone – produced in the pituitary gland • FSH – Follicle stimulating hormone – produced in the pituitary gland • Oestrogen – produced in the ovaries • Progesterone – produced in the ovaries The ovaries are responsible for the production of the key sex hormones including oestrogen and progesterone.
OESTROGEN Oestrogen is produced by granulosa cells in maturing follicles. Oestrogen production drives the physical display of heat behaviour – as a follicle matures, oestrogen production peaks and the cow comes on heat, thus enabling insemination to occur as the follicle undergoes ovulation. Too much oestrogen, which occurs in cows with cystic follicles, can result in either hypersexuality – a cow that is on heat constantly – or, more often, in complete suppression of heat behaviour, thus the cow presents as a non-cycler.
PROGESTERONE Progesterone is produced by the corpus luteum, a structure that forms from the remains of an ovulated follicle. A corpus 70
luteum forms and will THE HYPOTHALAMO-PITUITARY-OVARIAN AXIS: produce progesterone for Hypothalamus about 18 days following heat. Progesterone is critical GnRH for controlling the rate of follicular development and Pituitary Gland the timing of ovulation (-) (-) as well as for establishing LH/FSH pregnancy. Insufficient progesterone will usually (+) be the result of a cow that Ovaries does not resume cycling after calving. Excessive progesterone Progesterone (+) Estrogen (+) will also result in Corpus Luteum Follicle suppression of heat and a non-cycling cow. This can level of oestrogen will usually result in occur in a cow that has a luteal cyst. heat behaviour, excessive levels for a long period of time will exhaust the feedback ANOVULATORY ANOESTRUS loop mechanism to the hypothalamus and Failure to achieve a first ovulation result in complete suppression of cycling. following calving is one of the most Luteal cysts on the other hand are thickcommon reasons for a cow to walled, fluid-filled structures on the present as a non-cycler – ovary, which act like a persistent these cows are experiencing corpus luteum. They pump out anovulatory anoestrus. high levels of progesterone, When looking at the which feeds back to the ovaries of a cow with hypothalamus and shuts down anovulatory anoestrus the hormonal axis. we see very little follicular It is possible to identify ovarian activity occurring. Follicles are conditions by getting ovary scanning Lisa Whitfield small and do not reach maturity, done. This service allows for tailored and so there is little of either treatment and monitoring of nonoestrogen or progesterone being produced. cyclers. However, it is not commonly This condition usually results from cows performed in New Zealand because of experiencing nutrition and energy deficits having to scan a large number of cows and losing too much body condition at once. In many cases, the treatment between calving and mating. for these ovarian conditions is the same, and that is to use the normal hormonal CYSTIC OVARIES non-cycler treatment programmes that are Cystic ovaries are a less common cause around for these cows. If you want more of non-cycling cows in New Zealand, but information about what is causing nonit is good to be aware of this condition. cycling in your herd, ovary scanning may Follicular cysts are basically thin-walled, be a useful way to investigate this. oversized follicles that do not ovulate, and that sit on the ovary, often pumping • Lisa Whitfield MVM (Distinction) BVSc out high levels of oestrogen. While a high Production Animal Veterinarian. Dairy Exporter | www.nzfarmlife.co.nz | October 2020
