The effects of the lactation, pregnancy and calendar month on monthly milk records of 1st and 3rd lactation Israeli-Friesian dairy cows were studied. The analyses involved 717 966 monthly records in 76 788 lactations of cows diagnosed as pregnant in herds providing 20 or above lactations per herd per year and lactation number. The records were grouped into 18 lactation classes according to yield post partum: high, medium and low cows within high, medium and low herds, separately for 1st and 3rd lactations.

The objects of the investigation were to estimate factors for relating persistency to peak yield, and to examine how interactions between peak yield and persistency should be taken account of in post partum breeding policies.

The major factor affecting yield was the number of months post partum; heifers dropped in yield by 0·173 from the 1st to the 10th month of lactation and cows by 0·46. The herd level affected the shape of the lactation curve. In high-yielding herds peak yields were achieved later than in low-yielding herds, but the rates of yield decline from the 2nd to the 10th month were similar in high- and low-yielding herds. Within herds, there were large differences in the rate of decline between cows with high and low peak yields. The monthly decline within herds from the 2nd to the 10th lactation month for animals of high and low peak yield were –0·0347 and –0·0217 for heifers and –0-0633 and –0·0520 for cows.

Effects of pregnancy on milk yields were observed from the 1st month post conception, but the effects were small. The effects of 7 months of pregnancy on test-day yields were 3 to 4 kg milk, independent of the yield level of the cow. It was suggested that the effect of herd level could be disregarded but that account should be taken oflactation number and the yield level of the cow within her herd. Heifers and cows in high-yielding herds may be mated later than cows in low-yielding herds. Since pregnancy had a similar effect on high- and low-producing cows, and within herds the higher yielding cows were less persistent in yields than the remainder, it might be a reasonable management policy to breed most of the cows in a herd of a particular lactation at approximately the same time post partum.