Full lactation response of cows fed diets with different sources and amounts of fiber and ruminal degradable protein

Macromineral digestion by lactating dairy cows: estimating phosphorus excretion via manure

Data from 8 experiments involving 39 diverse dietary treatments and 162 Holstein cows were used to derive equations that estimate manure excretion of P by lactating dairy cows. Within an experiment, diets were formulated to have similar P concentrations and to meet or slightly exceed standard recommendations for P. The data were generated from total collection digestion trials. The only sources of supplemental P used were dicalcium phosphate and monosodium phosphate. The concentration of dietary P ranged from 0.34 to 0.45% of dry matter (DM). Cows varied in milk production (8 to 59 kg/d), DM intake (12.4 to 30.5 kg/d), and P intake (45 to 133 g/d). Apparent digestibility of P averaged 40.4%, fecal output of P averaged 47 g/d, and apparent P retention averaged 4 g/d. Two equations were derived to estimate excretion of P via manure (g/d): 1) Manure P = -2.5 + 0.64 x P intake, and 2) Manure P = 7.5 + 0.78 x P intake - 0.702 x Milk; where P intake is in g/d and milk is kg/d. Both equations were evaluated using literature data, and both equations had acceptable accuracy for field use. The requirements for P of lactating dairy cows from the National Research Council (NRC) were also evaluated, and for most diets, those requirements were adequate based on retention of P.

Influence of feed intake and source of dietary carbohydrate on milk yield and composition, nitrogen balance, and plasma constituents of lactating goats

The influence of diet (synchronized or not synchronized for the degradation rate of the carbohydrate and N fractions) and amount of feed offered [2.40 +/- 0.10 or 2.00 +/- 0.10 kg of dry matter (DM)/d] on milk yield and composition, N balance, and some plasma constituents was studied using 32 multiparous dairy goats (100 +/- 16 d in milk) that were fed a mixed diet for 9 wk. Diets were 40% concentrate with rapidly degraded starch and rapidly degraded N or highly digestible fibers and slowly degraded N. Nitrogen balance was determined at wk 4 and 8. Goats that were fed greater amounts of DM had higher yields of milk, fat-corrected milk, and protein, but lower milk fat concentrations during the whole trial. Raw milk yield was higher at wk 4 and from wk 6 to 7 for goats consuming the rapidly degraded diet than for goats fed the slowly degraded diet at a high feed intake. Concentrations of milk fat tended to be greater for goats fed the rapidly degraded diet at wk 5, 7, and 8. At wk 5 and 6, an interaction between feed intake and diet was observed for milk protein concentration. Nitrogen digestibility, milk N, and N balance were increased for goats fed at high intakes. The output of N in urine and the efficiency of N use for milk output was greater, and N balance was lower, for goats fed the rapidly degraded diet. Plasma concentrations of nonesterified fatty acids, beta-hydroxybutyrate, and urea (wk 2 to 6) were lower, and insulin concentrations were transiently increased, for goats fed at high intakes. Goats fed the rapidly degraded diet compared with goats fed the slowly degraded diet had higher plasma concentrations of urea, which may indicate inefficient use of ruminal N.

Estimating the available energy content of feeds for dairy cattle

The available energy content of feeds must be known before diets can be formulated, but the difficulty in measuring energy availability requires that estimated values be used. Based on production data, the current system using net energy for lactation (NEL) or total digestible nutrients (TDN) over-valued the energy in feeds by 7 to 8%. A standard discount of 8 TDN units was used for both NEL and TDN. When the discount was calculated from actual intake (4 units per increment of maintenance), feed values for NEL and TDN were overestimated by 5 to 6%. Most feed values for NEL and TDN are estimated from acid detergent fiber. These models do not account for all sources of variation in digestibility, do not allow for variable discounts based on intake and associative effects, and are specific for a given population. Mechanistic models account for many sources of variation and can be modified to include variable discounts based on digestion and passage kinetics. The equation used to convert TDN to NEL does not consider the source of the digestible energy. The NEL of the fat that is contained within feeds is under-valued, and fiber is probably overvalued. Single-component equations are not adequate to describe the complex processes that determine energy availability. The current NEL and TDN systems overvalue the energy in feeds, but both systems still work remarkably well. These systems need to be modified to improve accuracy, but they should not be discarded.