Lactations were divided into three periods: early (1 to 99 d), mid (10
0 to 199 d), and late (200 to 299 d). One hundred Holsteins were rando
mly split into four groups that were balanced for parity. Groups 222 a
nd 333 were milked twice and three times a day, respectively, througho
ut lactation. Group 233 was switched from twice to three times daily m
ilking at 100 d, and group 223 was switched at 200 d. Compared with gr
oup 222, milk yield for group 333 increased by 10.4%, and fat and prot
ein yields increased by 4.7 and 7.3%, respectively. Mean milk SCC for
all groups was <175,000 cells/ml within each lactation period. The per
centage of CP was lower for cows milked three times a day than for cow
s milked twice a day during each stage of lactation (early, 2.78 and 2
.91; mid, 3.08 and 3.19; and late, 3.16 and 3.28, respectively). Casei
n as a percentage of CP was significantly higher for cows milked three
times a day during midlactation. The acid degree values (milliequival
ents of FFA/ 100 g of fat) were significantly higher for milk from cow
s milked three times a day than for cows milked twice a day during ear
ly and midlactation, (early, 0.75 and 0.55; mid, 0.82 and 0.61; and la
te, 0.88 and 0.75, respectively). No differences were detected in milk
flavor or plasmin activity because of milking frequency. Casein as a
percentage of CP decreased, and plasmin activity increased, as parity
and stage of lactation increased.