Variations in protein, solids-not-fat, and fat of milk

In this research, 1541 milk samples from 140 cows were tested for protein, solids-not-fat, and fat during a 17-month period. When all cows were included, the total protein percentage was 3.81; solids-not-fat, 9.21; and fat, 5.14. The cows milking beyond the 10-month lactation raised the average percentage. The average for 1271 tests from 140 cows during the first 10 months of lactation was 3.67 percent for total protein and 9.21 percent for solids-not-fat. Results from 66 cows with 10 months of completed lactation showed that stage of lactation had highly significant effect on percentage of total protein which increased progressively from a low of 3.28 during the second month to 4.15 for the tenth month - an average of 3.66 percent. The average solids-not-fat was again 9.21 percent, and this too showed a highly significant increase as lactation advanced. For total production during the 10-month lactation, the 66 cows averaged 93.74 lbs. milk containing 4.54 lbs. of butterfat, and 342, 863, and 1317 lbs. of total protein, solids-not-fat and total solids, respectively. Gestation did not affect total protein percentage, but did affect solids-not-fat percentage with a marked increase for pregnant cows. The decline in milk production directly affected the increase of protein percentage with a highly significant negative correlation of -0.987. Seasonal effects were reflected in high total protein in fall and winter and low protein in spring and summer. The solids-not-fat percentage was low during the pasture season from May through September. It was high during the remaining seven successive months of the year. Age showed a significant effect on total protein which increased from 3.70 to 3.79 from the first to the second lactation, and then dropped to 3.56 percent for cows of advanced age. The differences for solids-not-fat with advancing age were highly significant - a high of 9.43 percent for cows in the first lactation and 9.20 for the fourth lactation and beyond. Age was definitely a major source of the decrease in solids-not-fat content of milk, with a consistent trend in the drop of the solids-not-fat tests as the cows became older.