The effects of stocking rate and level of supplementation with concentrate on cows grazing intensively managed tropical pastures were measured with four treatments: grazing alone at 2.5 head per ha per year (T1); grazing plus concentrate at the rate of 1 kg per 2 kg of milk irrespective of daily yield (T2); grazing plus concentrates at the rate of 1 kg per 2 kg above 10 kg daily (T3); and grazing at 5 head per ha plus concentrates at the same level as cows in T2 (T4). All three supplemented groups (T2, T3 and T4) produced significantly greater yields of milk and fat. Average milk yields (kg, M.E. basis) were 3450, 5568, 4709 and 5462 for cows on T1, T2, T3 and T4, respectively. Cows on the higher level of supplement (T2 and T4) gained significantly more weight over the lactation period than cows on T1 and T3. The effect of the supplement appears to be greatest during the early part of lactation when physiological limitations on the consumption of pasture prevent sufficient nutrient intake to supply demand. Cows on T2 produced only 105 kg more milk than cows on T4, while the expected difference, based on increased pasture availability, was over 455 kg. The reduced difference in level of production is postulated as an effect of excessive pasture growth on both intake and diet selection. While milk production was highest for cows receiving the high levels of supplement (T2 and T4), expected return per lactation in income over the cost of supplements, lime and fertilizer were greatest for T3 (medium supplement). Expected returns on a per ha basis were greatest for T4 with 5 cows per ha. Results indicate that where land costs are high, most efficient use of pasture and concentrate resources may be reached by grazing at the rate of 5 head per ha and supplementing with concentrate at 1 kg per 2 kg of milk.

Records of milk yield, fat percent, body weight, reproduction and health of 506 calvings of 362 cows were used for estimating the efficiency of utilization of tropical grass pastures by lactating cows fed on grazing alone or grazing with various types of supplement. There were seven feeding systems: grazing alone (T1); grazing plus supplement with ground maize (T2); with molasses (T3); with concentrate (T5); or with urea-molasses (T6), at the rate of 1.0 kg per 2.0 kg milk in excess of 10 kg of milk per day and of 2.5 cows per ha; or grazing plus concentrate feeding of 1.0 kg per 2.0 kg milk irrespective of milk yield at a stocking rate either of 2.5 cows (T4) or 5-0 cows (T7) per ha. All supplement systems had significantly higher yields of milk, fat and fat-corrected milk than grazing alone. Level of fat percent paralleled dependence on intake of forage. Supplement also extended days in milk. System of feeding was significant for body weight gain, time to reach peak milk yield, the level of peak yield, persistency of milk yield, days open, time from first breeding to conception, and calving interval. High levels of supplement (T4, T7) increased weight gains, time to reach peak yield, and persistency, but lowered breeding efficiency. On medium levels (T3, T5, T6), the efficiency (Meal/kg dry matter) of utilization of supplement for milk production was satisfactory, but unsatisfactory on high supplement levels (T4, T7). Supplements as high or higher than those in the grass treatments supplemented with non-protein nitrogen (urea-molasses) or crude protein (concentrate) gave a more efficient utilization than either maize or molasses. During the first 150 days of lactation, cows on grazing alone averaged 14.3 kg pasture grass dry matter intake per day, or 2.9% of body weight. Cows on low supplement (T2, T3, T5, T6) averaged 24 to 29% less; and cows on high supplement, nearly 60% less intake. Type of supplement had little influence on pasture grass dry matter (PGDM) intake. When the genetic potential for milk yield of cows exceeds 3,000 kg, supplementary feeding appears economically feasible. Even under the high levels of nitrogen fertilization employed, there was a rise in average milk yield with intakes of protein from the supplementary feed. Supplementary feeding with tropical grass pastures caused a high rate of substitution; hence, the efficiency of use of PGDM is lowered unless stocking rate is carefully adjusted.

Samples from five tropical grass species: Guinea (Panicum maximum), Pangola (Digitaria decumbens), Congo (Brachiaria ruziziensis), Merker (Pennisetum purpureum), and Star (Cynodon nlemfuensis), were harvested during nine weeks at ages from 7 to 63 days in southwestern Puerto Rico. The grasses were fertilized at the rate of 4480 kg/ha per year with a 15-5-10 fertilizer. The crude protein content of all grasses exceeded 10 percent up through 28 days of regrowth. Grasses declined in crude protein from a mean of 18.1 percent at 7 days to 5.6 percent at 63 days. A narrower range was observed between grasses from 42 to 63 days. The five tropical grasses possessed high contents of structural carbohydrates, principally cellulose and lignin, which increased with age, except in Pangola grass. Silica did not change uniformly with age in any of the grasses while hemicellulose was characterized by marked fluctuations. Digestibility values were lower at all stages of growth than in temperate forages of similar ages. Cellulose was negatively correlated with in vitro digestibility in all grasses except Pangola. Lignin appeared to be the predominant factor in determining digestibility. The relationship of silica to digestibility varied between species, being positive in some (Guinea and Congo) and negative in others (Star, Pangola, and Merker). The grasses may be ranked as follows with regard to their chemical composition and digestibility: Merker > Congo > Star > Guinea > Pangola. Pangola grass, though lowest in in vitro digestibility, declined least with advancing age, thus maintaining a more constant quality for a longer period of time.

Nine forages of known nutritive value, as determined in conventional digestibility trials with cattle, were used as controls to further evaluate the artificial rumen or in vitro cellulose fermentation technique at 12 and 24 hours with the following results. The correlation of the relative intake (R.I.) and of the nutritive value index (N.V.I.) with 12-hour in vitro cellulose digestion was by no means the best. Low correlation coefficients (r) of 0.48 and 0.41, respectively, were obtained. Further research is needed to improve the efficiency and precision of the 12-hour fermentation. The 24-hour in vitro cellulose digestion in the artificial rumen gave the best estimates of the apparent digestibility of forages. Regression equations to predict biological T.D.N. (Y) and digestible energy (K cal./gm.) (F) from 24-hour in vitro cellulose digestion (X) were calculated: Y = 12.2 + 1.19X and Y = 0.75 + 0.05-X, and highly significant correlations of 0.98 and 0.90 were obtained. By means of these equations, the biological T.D.N, and digestible energy were calculated for all samples, and the results were tested statistically by means of the analysis of variance. N.V.I, correlated to 24-hour in vitro cellulose digestion and to 12-hour X 24-hour in vitro cellulose digestion gave coefficients of 0.70 and 0.71, respectively. In addition to the control samples, different forages, bromegrass, orchardgrass, and timothy from 13 different plots harvested on 4 first-cutting dates at 3 different locations in northern, central, and southern New Jersey, and fertilized with a complete fertilizer containing 50 pounds of nitrogen per acre, 1 plot receiving no nitrogen and 1 plot 100 pounds, were used in this study. They were evaluated by means of the artificial-rumen or in vitro cellulose-fermentation technique as a biological means of estimating their nutritive value. Advancing harvest dates (May 5 to June 6) were found to decrease the biological T.D.N, of orchardgrass, timothy, and bromegrass by 0.80 T.D.N, percentage units per day. It can be concluded that the 24-hour in vitro cellulose digestion can be used as a means of estimating the digestibility of forages or to predict T.D.N., or digestible energy, but the use of this 12-hour in vitro cellulose-digestion technique to predict intake or N.V.I, for cattle requires further refinement and precision.

The effect of three harvest intervals (30, 45 and 60 days) on the yield, crude protein (CP), digestible protein (DP), and mineral (calcium, Ca; phosphorus, P; potassium, K; and magnesium, Mg) composition of 10 forage grasses was determined during a 1-year period at the Isabela Substation. The species studied included three Brachiarias, five Digitarias and two Cynodons. As the 30-day harvest interval, the dry matter yield (DMY) ranged from 25,859 to 19,041 pounds per acre per year. The highest DMY was obtained by Transvala digitgrass (Digitaria decumbens, PRPI. 6439) Stargrass (Cynodon nlemfuensis PRPI. 2341) had the highest CP and DP contents. The highest DMY at the 45-day interval was obtained by Transvala digitgrass, being significantly different (P < .05) from other grasses. The DMY ranged from 29,532 to 20,962 pounds per acre per year. Digitaria eriantha had the highest CP and DP contents. Stargrass had the highest DMY and CP at the 60-day harvest interval. The DMY ranged from 37,504 to 30,269 pounds per acre per year. Mean Ca and Mg contents in the grasses increased at the 45-day interval, with no Ca increase but a Mg decrease at 60 days. Mean P and K contents decreased as the harvest intervals increased. The yielding ability of the grasses was encouraging, especially in the Transvala digitgrass and Stargrass species. The CP and DP contents tended to vary as plants matured, suggesting that studies of mineral absorption and/or utilization of forage grasses by niminants should be undertaken.