Thirty sows were allocated at their first farrowing to five litter size treatments. Litter sizes of six, eight, 10,12 and 14 were established by cross-fostering within 48 h post partum. Milk yield (Y, kg/day) of sows was linearly related to litter size (L) and was described by the equations; Y = 5-98 (s.e. 0-48) + 0-689 (s.e. 0-046)L, R 2 = 0-99; residual s.d. = 0-29; P < 0-001 and Y = 8-20 (s.e. 0-46) + 0-324 (s.e. 0-044)1, R 2 = 0-95; residual s.d. = 0-28; P < 0-002 for early (day 10 to day 14) and late (day 24 to day 28) lactation, respectively. The composition of milk from sows suckling various litter sizes remained relatively stable but suckling frequency in early lactation increased linearly in response to increased litter size. Litter size significantly affected the average growth rate of individual piglets; piglet growth rate from birth to weaning at 28 days of age decreased from 283 giday to 202 giday in response to increasing litter size from six to 14. The relationships between milk yield, average piglet growth rate and litter size indicate that the number of functional glands is the major factor influencing milk yield of sows. Increasing the number of functional glands by increasing litter size more than compensates for any decrease in milk output from individual glands.
The objective of this study was to determine whether sow milk yield per gland could be increased by reducing the interval between suckles (suckling interval). Eighteen sows were allocated at their first farrowing to three treatments comprising litter sizes of 6 or 12 piglets or a cross-suckling treatment that was imposed to increase suckling frequency. The cross-suckled treatment comprised two groups of six piglets each. Each suckling group was allowed to suckle the sow during 30-min intervals each day between d 6 and d 28 of lactation. The suckling interval was shorter (P < .05) for cross-suckled sows than for sows suckling single litters of 6 or 12 piglets during early lactation (d 10 to 14) and late lactation (d 24 to 28). Average piglet growth rate between birth and 28 d of age was greatest (P < .05) for piglets in the single litters of six and lowest for piglets in the cross-suckled treatments. Single litters of 12 piglets had the highest (P < .05) litter growth rates, followed by the cross-suckled litters and then the single litters of six piglets. The concentration of lactose and fat in milk from sows remained relatively stable, although milk from the cross-suckled sows contained more protein in early lactation (P < .05). Milk yield of sows was not significantly increased (P > .05) by the cross-suckle treatment, although during early lactation, milk yield tended to be greater from sows in the cross-suckle treatment than from sows suckling single litters of six (8,920 g/d vs 7,819 g/d, P < .1). The concentration of DNA and total RNA and the RNA:DNA ratio in mammary glands was unaffected by treatment (P > .05). Sows with single litters of 12 piglets had the greatest total DNA in their udders (P < .05). However, individual gland weights were heavier (P < .05) in cross-suckled sows than in sows with single litters of 6 or 12 piglets. Increased suckling frequency seemed to play a role in increased mammary gland weight and milk production during lactation.
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