University of Minnesota researchers recently conducted an experiment at two University experiment stations for two years to determine the lysine requirement for first to third parity lactating sows. Numerous studies across the U.S. have estimated lysine requirement for lactating sows based on litter weight gain. However, recent information suggests additional lysine may be required to shorten interval from weaning to rebreeding and maximize litter size at next farrowing. Thus, the question of whether lysine requirement for maximal subsequent reproduction is higher than that for maximal lactation performance becomes very important.

The experiment used 208 primiparous sows (PIC Camborough 22). Sows were fed one of five lactation diets containing .60, .85, 1.10, 1.35 and 1.60% total lysine. These sows were on trial for three parities. No attempt was made to keep the sows on the same diet throughout the study.

All diets contained corn, soybean meal, 5% corn gluten meal, 5% soybean oil and 0 to 8% rice hulls. Rice hulls were included to equalize energy content across the experimental diets. Sows had free access to their assigned diets from parturition to weaning (19.5 days postpartum). All sows were fed a common gestation diet (14% crude protein and .68% lysine) from weaning to next farrowing.

Litter weight gain was lower when sows were fed a diet containing .6% lysine versus .85% lysine in all three parities. Litter growth rate did not increase further when lysine levels were above .85%. Actual lysine intake varied from 33 to 73 g/day in parity 1, from 39 to 79 g/day in parity 2 and from 41 to 99 g/day in parity 3.

Maximal litter growth rate of 4.5, 5.2 and 5.5 lb./day occurred at about 44, 55 and 56 g/day of lysine intake for parities 1, 2 and 3 sows, respectively. Lysine intake had no influence on sow body weight change in all three parities and no effect on backfat change in parities 2 and 3, but tended to increase backfat loss in parity 1.

All primiparous sows did not experience much body weight change (less than 11 lb.) during lactation, and most second and third parity sows gained body weight during lactation. This might be due to high feed intake of sows (Table 9).

Daily feed intake was 10 to 12 lb. in parity 1, 13-14 lb. in parity 2, and 14-15 lb. for parity 3.

Interestingly, increasing dietary lysine intake decreased feed intake of sows in all three parities. The factor(s) responsible for the decreased feed intake with the increase of lysine intake is unknown. Perhaps, very high lysine (or protein) intake may have elevated nitrogen load that must be excreted. When sows consume less feed, nitrogen load is reduced.

Increasing dietary lysine intake did not affect wean-to-estrus interval (3.9 to 6.6 days) and farrowing rate (72 to 77%) in all three parities. However, lysine intake during the first and second lactation did influence their subsequent litter size. When sows were fed 33 to 54 g/day of lysine in the first lactation, their second litter size was similar, but further increasing lysine intake to 73 g/day decreased second litter size by about 1.5 pigs (Table 9). This suggests feeding primiparous sows excessive dietary lysine during lactation may be detrimental to their subsequent litter size.

Similar results were observed by researchers from the University of Missouri. However, Australian workers reported that high lysine intake during the first lactation increased second litter size by 1.5 pigs. In the present study, increasing lysine intake from 55 to 93 g/day during the second lactation increased total born and born alive in the third litter. "To our surprise, third litter size of sows with the lowest lysine intake (39.0 g/day) during second lactation was similar (13.3 vs. 13.6 total born) to that of sows with the highest lysine intake (93 g/day)," says Hong Yang, University of Minnesota animal scientist. "An explanation for this observation is not readily apparent. " It appears the response of litter size at farrowing of third and fourth parities to previous lactation lysine intake is similar. These results suggest that parity influences the response of subsequent litter size to previous lactation lysine intake, and that increasing dietary lysine intake above the current level during lactati on does not seem to improve subsequent litter size. "Therefore, we suggest that lysine requirement for maximal subsequent reproduction and maximal litter growth is the same 44, 55 and 56 g/day for first, second and third parity lactating sows, respectively, in this study," Yang relates.

A second experiment was conducted to understand the mechanisms underlying the effects of lysine intake on reproduction. Sows were fed one of three diets containing .4% (low), 1.0% (normal) and 1.6% (high)lysine. Actual lysine intakes over an 18-day lactation were 16, 36 and 56 grams/day for sows fed low, normal and high, respectively.

Luteinizing hormone (LH) is an important factor responsible for stimulating follicle development and resumption of return to estrus postweaning.

Estradiol is a hormone produced mainly by follicles, and thus could be used as an indicator of follicle development. Increasing lysine intake may improve follicular quality, and thus may stimulate egg maturation and increase the chance for the embryo to survive during early gestation. This may help researchers to understand and solve the high embryo mortality problem in the swine industry. The researchers found that very low lysine intake decreased secretion of LH and estradiol during lactation and decreased the number of large follicles shortly before estrus. Very low lysine intake also decreased the ability of the follicle to support egg maturation (follicle quality) compared with normal and high lysine treatments. High lysine intake did not further improve follicular quality compared with medium lysine intake.

These results suggest that very low lysine intake may reduce embryo survival and subsequent litter size. Very high lysine intake may not increase subsequent litter size further compared to medium lysine intake. These results were consistent with the results from the first experiment in which the researchers found that second litter size was similar when sows were fed from 33 to 54 g/day of lysine during the first lactation.

"Producers should realize that the very low lysine intake in this study (16 g/day) is much lower than that in the first study (33 g/day)," says Yang. "Taken together, very low lysine intake during the first lactation reduces litter weight gain and impairs subsequent reproduction, but increasing lysine intake above the current industry level appears not to improve subsequent litter size."

Researchers: Hong Yang, Jim E. Pettigrew, Lee J. Johnston, Jerry C. Shurson and Roger D. Walker, University of Minnesota, and George R. Foxcroft, University of Alberta, Edmonton, Canada. Phone Yang at: (612) 625-3743.