Dry period and milk yield
Authors: Lonka T. 
Published: July 2018

The relation between dry period milk yield among Finnish Ayrshire cattle has been studied in this investigation. The question has been handled particularly from the standpoint of breeding selection in weighing whether the length of the preceding dry period is to be taken into consideration in breeding selection and how exactly the length of the dry period due to genotype characteristic to each cow can be determined. The calculations made have given the following results: I. The length of the preceding dry period has comparatively small effect on the milk yield of the next lactation. The milk yield does in fact increase somewhat as the dry period lengthens up to sixty days (Figure 1) but this effect is so slight that it has no significance in breeding selection. 11. The determination of the length of the dry period characteristic to each cow is made difficult by the fact that its length is dependent not only upon the cow’s genotype but also to a very great extent upon non-genetic factors. The most important of these are; I. Feeding and care, 2. Length of calving interval, and 3. Number of previous lactations. 1. The quality and quantity of the feeding as well as the care of the cows has an apparently greater effect on the length of the dry period. In breeding selection it is especially confusing because it is very difficult to make even an approximate estimate of the effect of the different feeding and care on different farms on the length of the dry period, and it cannot be levelled by using mean correction factors or other levelling methods. This touches also the different feeding during the several seasons. One of the practical measures of cattle raising which brings about variation in the length of the dry period is that cows of great persistency are forced into dry period at the latest four to six weeks previous to the next calving. This variation is relatively small, however, compared to that arising from other non-genetic causes. 2. The length of the dry period is dependent upon the calving interval in that, as the calving interval increases the dry period increases also, and vice versa (r = +0.30±0.02, Table and Figure 3). In order to level the length of the dry period to correspond to the 365 day long calving interval, the mean correction factors have been calculated in this investigation (Table 4). 3. There is correlation between the number of previous lactations and dry period in so far that the first dry period is on an average 15 days shorter than the later dry periods. The abovementioned and many other non-genetic factors cause so much variation in the length of dry period that it for the great part conceals the variation due to different genotypes. Thus the greatest correlation coefficient between dam’s and daughter’s dry period is only +0.24±0.04 (Table 6). In this case the shortest available dry period chosen from among the dry periods whose corresponding calving interval has been 450 days at the most has been used as the dry period of a single cow. Such a method for the elimination of variation caused in the length of the dry period by the calving interval has proved to be at least as good as the application of mean correction factors, for from the dry periods levelled by correction factors a correlation coefficient between dam and daughter has been obtained amounting to + 0.23±0.03. On the other hand it is, however, to be noted that the effect of both the correction methods mentioned above is very small for the corresponding correlation coefficients computed from uncorrected dry period is +0.21±0.04. In spite of the low values the correlation coefficients presented are on about the same level as those obtained from the Ayrshire cows’ lactation milk yields between dam and daughter. As to the measure indicating the shape of the lactation curve, the so-called five months’ comparative milk yield, which is in use in Finland, it is considerably much less dependable. In the material at our disposal there is hardly any correlation (r = +0.08±0.04) between the five months’ comparative milk yields of dam and daughter. The dry period also expresses the most important and apparently the most varying part of the shape of the lactation curve, which is proved e. g. by the fact that in our feeding conditions the extent of the lactation milk yield depends to a considerable extent on the length of the dry period (r = —0.40±0.02, Table 5). As in addition the dry period is such a simple measure that all cattle-owners know how to apply it in breeding selection, it can well maintain its position among the measures indicating the shape of lactation curve.