Effect of Stocking Rate on Pasture Production, Milk Production, and Reproduction of Dairy Cows in Pasture-Based Systems

Literature with data from dairy cows on pasture was reviewed to evaluate the effects of supplementation on intake, milk production and composition, and ruminal and postruminal digestion. Low dry matter intake (DMI) of pasture has been identified as a major factor limiting milk production by high producing dairy cows. Pasture DMI in grazing cows is a function of grazing time, biting rate, and bite mass. Concentrate supplementation did not affect biting rate (58 bites/min) or bite mass (0.47 g of DM/bite) but reduced grazing time 12 min/d per kilogram of concentrate compared with unsupplemented cows (574 min/d). Substitution rate, or the reduction in pasture DMI per kilogram of concentrate, is a factor which may explain the variation in milk response to supplementation. A negative relationship exists between substitution rate and milk response; the lower the substitution rate the higher the milk response to supplements. Milk production increases linearly as the amount of concentrate increases from 1.2 to 10 kg DM/d, with an overall milk response of 1 kg milk/kg concentrate. Compared with pasture-only diets, increasing the amount of concentrate supplementation up to 10 kg DM/d increased total DMI 24%, milk production 22%, and milk protein percentage 4%, but reduced milk fat percentage 6%. Compared with dry ground corn, supplementation with nonforage fiber sources or processed corn did not affect total DMI, milk production, or milk composition. Replacing ruminal degradable protein sources with ruminal undegradable protein sources in concentrates did not consistently affect milk production or composition. Forage supplementation did not affect production when substitution rate was high. Fat supplementation increased milk production by 6%, without affecting milk fat and protein content. Increasing concentrate from 1.1 to 10 kg DM/d reduced ruminal pH 0.08 and NH3-N concentration 6.59 mg/dl, compared with pasture-only diets. Replacing dry corn by high moisture corn, steam-flaked or steam-rolled corn, barley, or fiber-based concentrates reduced ruminal NH3-N concentration 4.36 mg/dl. Supplementation did not affect in situ pasture digestion, except for a reduction in rate of degradation when high amounts of concentrate were supplemented. Supplementation with energy concentrates reduced digestibility of neutral detergent fiber and intake of N but did not affect digestibility of organic matter or flow of microbial N.

Relationships among milk production, body condition score (BCS), body weight (BW), and reproduction were studied using logistic regression on data from 6433 spring-calving Holstein-Friesian dairy cows in 74 commercial herds. Multivariate models were adjusted for herd, breeding value for milk yield, proportion of Holstein-Friesian genes, lactation number, calving period, and degree of calving assistance. Significant associations between reproductive measures and components of energy balance were identified. Higher 200-d milk protein content and higher protein-to-fat ratio at start of breeding were associated with increased likelihood of submission for breeding in the first 21 d of the breeding season (SR21). High 100-d cumulative milk yield as a proportion of estimated 305-d milk yield (low persistency) was associated with a lower likelihood of pregnancy to first service (PREG1), whereas cows reaching peak milk yields earlier tended to have higher PREG1. Cows that reached nadir milk protein content relatively late in lactation had lower PREG1. Milk yield at first service and 305-d milk protein content were positively associated with the likelihood of pregnancy after 42 d of breeding (PR42). Higher 305-d milk lactose content was associated with increased PREG1 and PR42. Mean BCS at 60 to 100 d of lactation was positively associated with both SR21 and PR42, whereas nadir BCS was positively associated with PREG1. Cows with precalving BCS > 3.0 that also lost > 0.5 BCS unit by first service had lower PR42. More BW gain for 90 d after start of breeding was associated with higher SR21 and PREG1; more BW gain for 90 d after first service was associated with higher PR42. Milk protein and lactose content, BCS, and BW changes are important tools to identify cows at risk of poor reproduction.