In order to determine how the environmental factors would affect the growth and development of lambs and to consider whether the growth pattern as described by MIMURA (1956) would be the same or different for lambs produced in normal season and one produced outside the normal season, twenty-one lambs were selected and reared among twenty-seven lambs which had been produced outside the normal season reported by MIMURA (1959, 1964) with eight lambs produced normally. They were reared during twelve months by female and during six months by male. The growth in live weight and the development in linear measurements investigated along with the growing of lambs, and the skin area on the part which was tattooed on the mid-side of lambs by the method reported by BURNS (1935) and MIMURA (1956) was also measured and the growth investigated. After this the lambs were classified into five groups; spring lambs, early summer lambs, summer lambs, autumn lambs and winter lambs according to their lambing seasons, and the data were compared each other.
The results are as follows.
1. Although there are remarkably growth- differences after the the third month, a straight line shows until the lambs grow up to the third month, not only spring lambs but in other season lambs with the exception of autumn lambs. Autumn lambs grow up straight forward until the forth month.
2. The growth and development of lambs after about the 3rd month shows a considerably higher rate under spring condition, and a lower rate under summer condition followed by early autumn and winter conditions.
3. The results, therefore, indicate that the general pattern of growth and development suggested by MIMURA (1956) and the present experiments coincide, but that seasonal effects considerably affect the growth and development of lambs after the early stage. The remarkable decrease of growth rate, for example, in early summer lambs and summer lambs from the 4th to the 9th months against the clear increases from the 9th to the 12th months are pointed out.
The seasons of summer and early autumn in the west districts of Japan are characterized as hot and dry. The growth and development of lambs are suppressed not only by climatic effects, but mainly by nutritional fall in feeding under these seasons. Lambs in Japan are also under-feeding in winter.
4. It is very interesting to observe that the general pattern of growth in early stages was not affected by lambing seasons, probably due to unknown endogenous factors, milk supply and others would overcome the environmental factors. Concerning the exception with autumn lambs from the 3rd month to the 4th month, it must be considered that when the lambs after the 3rd month are under spring condition the decrease of growth rate will be relieved. Then it will be correct to say that the growth will slow down to about the third month or the fourth month, if one want to discuss the point of inflection of growth on the lambs produced all over a year.
5. There was a significant correlation between the expansion ratio of skin area on the part tattooed and the growth in live weight, although the expansion ratio changed during the growing of lambs on a larger scale than that of the growth.
6. From the investigation of specific growth rate, under summer condition suppressing influences were shown in body length, rump length and rump width of winter lambs, but under spring condition advantageous influences were shown in those of autumn lambs as compared with that of lambs produced under normal season.
7. It is very interesting to observe that from the present results autumn lambs will be most advantageous commercially if the lambs could be produced under treatment.