Age affected: Gilts.
Causes: Mismanagement; malnutrition; genetic.
Effects: No heat by 180 days of age.
Puberty is the point of sexual maturity and is generally considered to be the first heat (oestrus) in the gilt. In hybrid gilts, puberty usually occurs at around 160 days of age but it is breed dependent. Puberty can occur at around 115 days in the Meishan and in miniature pigs, or later than usual in pure gilts of western breeds. Puberty may be delayed by poor environmental conditions, cold, sunburn and poor light. Overcrowding and the associated bullying and stress may result in delayed puberty. Poor nutrition acts by reducing growth rates or by causing deficiencies in particular nutrients. Disease may affect the gilt by reducing body condition or by causing pain. Lameness reduces bodily condition and precludes behaviour associated with oestrus. The early management of growing gilts is important. Dirty housing, group sizes below 6, uneven lighting patterns and poor observation for signs of oestrus may result in real or apparent delays. Finally, poor management of peer group contacts may delay puberty. Puberty may be delayed by housing with young boars or by contact with old boars too early. As the expected time of first oestrus approaches, inadequate boar contact or contact with board with low levels of boar odour may delay puberty.
Not transmissible. The problem may recur in successive batches where no attempt has been made to eliminate likely causes.
Clinical signs of delayed puberty consist simply of the failure of gilts to show oestrus (enlargement of the vulva, reddening of the vulva, remaining still for back pressure, clustering round a boar) by the time they would be expected to have reached puberty based on previous experience with the breed or hybrid on the farm concerned, or industry norms. For this to be ascertained, records of the chronological age of the animals must be available, or an estimate of their age made based on weight, size for age, or time since selection for the breeding pool. Clinical signs of contributory factors such as disease or poor condition may be present. Anatomical evidence of hermaphroditism may be obvious Delayed puberty may occur in individual animals in a group or in whole groups of animals. If oestrus has not occurred or been noted by 240 days of age, it is unlikely that it will occur.
Investigation must establish whether puberty has been delayed and if so, what has caused it. The age of the animals under examination must be known accurately and individual animals must be identified or removed from the group when oestrus is first detected. Daily inspection of the group is required. This should take place in adequate light and animals should be examined for physical signs of oestrus, signs of mounting and include behaviour towards boars. Evidence that puberty has been reached may be obtained from determination of blood hormone levels. Peaks of oestrogen indicate that follicles are maturing or that ovulation is occurring and high levels of progesterone indicate that ovulation has taken place. Ultrasound may be sued to detect mature follicles or corpora lutea in the ovary. Pregnancy is a common cause of apparent anoestrus in gilts with access to entire boars and its presence should be eliminated. Inspection of the accommodation, temperature, lighting, relationship to boars and ability to observe oestrus should be made and the general condition and health of the animals recorded in order to determine the cause.
These are usually recorded at slaughter, as animals are usually of saleable size or culling can be planned to ensure that this is the case. Examination of the ovaries in culled gilts for follicles and corpora lutea will confirm whether or not puberty has occurred. Pregnancy will be obvious.
Prevention of delayed puberty depends upon correcting the causal management factors. Gilts should be reared to 5-6 months of age in groups of 6-30, isolated from boars, with 12-16 hours light of adequate intensity at a temperature of about 20˚C and given food to appetite. Disease should be controlled. Purchased gilts should be housed under these conditions on arrival. As mycoplasma arthritis frequently develops within 14 days of the arrival of clean gilts in infected herds, treatment with lincomycin, tiamulin or valnemulin is sometimes given to eliminate infection before the clinical signs become apparent. Gilts which have reached 160-210 days of age should be exposed to vasectomised boars or housed in sight, smell and touch of a smelly older boar. Oestrus should then be observed. If not, then gilts can be tested with another boar daily, taking the gilt to the boars.
Treatment with gonadotrophic hormones induces oestrus in pre-pubertal gilts. Gilts should be reared to 5-6 months of age in isolation from boars, injected with gonadotrophic hormone preparations and then allowed contact with boars for 15-20 minutes per day. Ninety percent of gilts will develop oestrus within 5-7 days. They should not be mated at this oestrus.