7.1. Gestation length

found that the average milk yield in kg after calving was found to be 6219 for single-calving

In a study performed in the USA [41], the effects of twin calving on the lactation in Holsteins were studied from a compilation of calving records from the Eastern Artificial Insemination Cooperative. The records of cows were grouped according to whether or not the twin calving was associated with dystocia. Records of each twin-calving cow were paired with records of a singlecalving control herdmate. In the twin group of 175 cows that had difficult calvings, twin pregnancy was not found to have an effect on the cows' production; milk and fat yield in the lactation initiated by twinning were depressed; however, this was not significant. In the group of 367 twincalving cows with no dystocia at calving, milk and fat production were depressed by 285 and 14 kg in the lactation after the calving. It was shown that twinning associated with dystocia seemed to depress the milk yield in the subsequent lactation (year 2). Compared to controls, twin-calving cows outperformed their herdmates in both the lactation before the twin calving (year 1) and the lactation after the next successful calving (year 3). Separate analysis of the 172 individuals with milk records for years 1 and 2 gave mean yields for year 1 of 7848 kg for twincalving cows and 7722 kg for the control cows. For year 2, the mean for twin-calving cows was 7794 kg compared to a mean of 7811 kg for control cows. Although these means also indicate a depressive effect of twinning, this interaction was not considered significant. Mean values for cows where twinning was not associated with dystocia showed that twinning also depressed subsequent milk yield. The twin calvers outperformed their herdmates in years 1 and 3. No depression was noted for milk yield in the twin pregnancy lactation (year 1). There was also significant depression in fat percentage with twinning combined with a depression in milk yield to depress fat yield. In a group of 305 pairs in which no dystocia with twinning was observed, fat yield also was depressed in the twin pregnancy lactation. Twin calvers outproduced their herdmates in year 3, the year not directly affected by a twin calving. Both groups showed roughly the same effect of twinning on the number of open days following a twinning (Table 2). The 100 twin calvers associated with dystocia showed a noticeable increase in the number of open days following twinning. The 157 twin-calving cows with records for years 2 and 3 were open for an average of 105 days prior to conception of twins and 132 days after birth of twins as compared to means for the controls of 101 and 106 days. Analysis of the conception rate of 212 pairs of cows in which twin calvers had no difficulty calving showed that conceptions with twins averaged 9 days earlier when compared to conceptions of single-calving herdmate controls. After the twin birth, twin calvers took 22 days longer to conceive than their herdmates. When 316 pairs were examined for years 2 and 3 only, twin calvers showed a mean number of days open prior to conception of twins of 100 days and a mean of 131 days to conception following the twinning as compared to

Twinning in dairy cattle can have both favorable and unfavorable results, but it is not seen as a desirable trait in dairy cattle. Delayed rebreeding and other reproductive problems in twinning cows are some of the reasons for culling by farmers, whereas cows calving singles are culled more due to low milk production [26]. Twinning in the dairy herd population appears

cows, 6434 for twin-calving cows, and 7097 for cows calving triplets.

70 Ruminants - The Husbandry, Economic and Health Aspects

means of the controls of 105 and 108 days.

7. Consequences of twinning

It was found in a study [27] that the average length of gestation before calving on the studied farm was 276.3 days. Here we experienced significant (p < 0.001) difference of approximately 5 days by type of calving. Twin-calving cows' gestation length (274 days) has proven to be shorter than that of nontwinning (278.7 days) herdmates. As their gestation is shorter and their reproductive performances are better, with shorter regeneration time and higher conception rates, they have a short calving interval. However, the benefits of this shortened calving interval before twinning are negated by the reproductive problems following twinning. There are more factors influencing the profitability of dairy cows than calving interval alone.
