Extending lactation in pasture-based dairy cows: I. Genotype and diet effect on milk and reproduction.

The aim of this study was to test the feasibility of extended lactations in pastoral systems by using divergent dairy cow genotypes [New Zealand (NZ) or North American (NA) Holstein-Friesian (HF)] and levels of nutrition (0, 3, or 6 kg/d of concentrate dry matter). Mean calving date was July 28, 2003, and all cows were dried off by May 6, 2005. Of the 56 cows studied, 52 (93%) were milking at 500 d in milk (DIM) and 10 (18%) were milking at 650 DIM. Dietary treatments did not affect DIM (605 +/- 8.3; mean +/- SEM). Genotype by diet interactions were found for total yield of milk, protein, and milk solids (fat + protein), expressed per cow and as a percentage of body weight. Differences between genotypes were greatest at the highest level of supplementation. Compared with NZ HF, NA HF produced 35% more milk, 24% more milk fat, 25% more milk protein, and at drying off had 1.9 units less body condition score (1 to 10 scale). Annualized milk solids production, defined as production achieved during the 24-mo calving interval divided by 2 yr, was 79% of that produced in a normal 12-mo calving interval by NZ HF, compared with 94% for NA HF. Compared with NZ HF, NA HF had a similar 21-d submission rate (85%) to artificial insemination, a lower 42-d pregnancy rate (56 vs. 79%), and a higher final nonpregnancy rate (30 vs. 3%) when mated at 451 d after calving. These results show that productive lactations of up to 650 d are possible on a range of pasture-based diets, with the highest milk yields produced by NA HF supplemented with concentrates. Based on the genetics represented, milking cows for 2 yr consecutively, with calving and mating occurring every second year, may exploit the superior lactation persistency of high-yielding cows while improving reproductive performance.