There may be only 1.5 grams of trace minerals and vitamins in each kilogram of feed eaten by the sow, but they are extremely important to her reproduction. It is now possible to propose that adjustments in the daily intake of these micro-nutrients can reduce the decline in litter size usually associated with advancing reproductive age.

In a report last month I suggested how sow productivity could benefit from an age-specific approach to nutrition in the breeding herd. The report argued the case for organising the sows of the herd into younger and older sub-populations, to allow age-based feeding that would address their different nutritional requirements for both the total quantity and the type of nutrients fed per day.

Adjusting the feed according to the sow's age has the opportunity of correcting age-related problems of litter size and sow viability. The basic problem with most sows is that the total number of pigs per litter (and numbers born alive) levels out and then declines progressively after early increases in Litters 1, 2 and 3. The decline typically begins after Litter 6 or 7. This parity-related decrease in litter size seems premature from a reproductive standpoint and may be due at least partly to the changing micro-nutrient status of the sow as she ages.

A paper published 10 years ago by Mahan and Newton (The Ohio State University, USA) carried the message of the progressive de-mineralisation of sows because of the demands of bearing and nursing large litters of piglets. They found that minerals in sows' body tissues were depleted after 3 parities compared to others of an equivalent age that had not reproduced. For a sow that had produced litter weaning weights in excess of 55kg, for example, the depletion of calcium, phosphorus, zinc and selenium was between 15-18%.

The fault here seems to lie with the common practice of giving all sows the same daily feed allowance in pregnancy after their body condition has been restored. As the sow becomes heavier at every parity, this practice results in a reduction in the intake of trace minerals and vitamins per kilogram of bodyweight. Therefore there is a continual erosion of the safety margin between the amount fed daily and the increasing quantity required by the sow for body maintenance and reproduction.

De-mineralisation (in other words, the reduction in mineral status) can only grow worse as the sow's reproductive age increases. It is based on the assumption that the feeds contain 0.149% of a vitamin-trace minerals premix and are given in a fixed daily amount regardless of whether the animal is in the gilt development unit (GDU) weighing just 105kg or has farrowed up to 9 litters. Daily intake levels and sow weights were those given by PIC USA data for 2002.


As mentioned above, Hanor has farms where sows having 3 or more litters are segregated from younger females. Their organisation according to parity structure has allowed them to serve as the basis for field tests of adjusted feeding in which the micro-nutrient intake was equalised for the older sow quadrants.

Initially we wanted to test the principles of the concept before initiating larger-scale trials. For this, we took 5300 sows from each of 2 control farms and compared them with 5300 from 2 test herds. Those acting as controls received the usual vitamin-mineral supplementation of 0.15% of the diet. Test sows were fed increased amounts of the premix and also choline. Increasing the concentration in the test diets achieved the same daily intake per kilogram of bodyweight in later litters that had applied to sows at Parity 3.

This gave the first recorded illustration that the age-related decline in litter size could be flattened by dietary adjustment for older sows. Total piglet born per litter and the number born alive were higher in the senior sows receiving an adjusted intake of minerals and vitamins. The difference translated into an extra 0.6 piglets weaned per litter or another 1.44 pigs weaned per sow/year.

A point to mention is that the trace mineral forms used in the test were from inorganic sources, although organic selenium was fed at the same level for all sows. Since the early trial we have started to look more closely into the whole age-feeding concept and not least whether the balance of inorganic and organic trace mineral forms has a bearing on results. Further increases in sulphate forms of the minerals would lead to an increase in total free radicals. There are preliminary data from Ohio research that increased free radicals in the sow would be counterproductive to improved litter size.

More generally for sow nutrition, we should accept that there is a risk inherent in the usual practice of expressing nutrient inputs in terms of the amount present in the diet. Actual inputs of micro-nutrients become biased towards the younger sow, when they are translated into the quantity per kilogram of bodyweight. They can be predicted to lead to a progressive demineralisation of the sow with each extra parity, unless there is a strong compensating mechanism of increased absorption and retention of micro-nutrients as the animal ages and this seems unlikely, given what is known from the pet nutrition sector for the geriatric cat and dog.