Effects of season and genotype on grazing behaviour, forage intake and digesta kinetics of pure Ankole and Ankole X Holstein crossbred cows
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Grazing behaviour and forage utilization of pure Ankole and Ankole x Holstein crossbreds was evaluated in the dry and rainy seasons. The grazed pastures were evaluated for quantity and quality according to the seasons and pasture species. Grazing behavior was determined by scan sampling and continuous monitoring of 10 pure Ankole and 10 Ankole x Holstein crossbred cows for 10 days per season. The cows were pulse-dosed with 80g of chromium mordant in both dry and rainy seasons. Rectal feacal grab samples were collected over a period of 84 hours post dose. Digesta kinetics and forage dry matter intake were derived by fitting fecal chromium excretions to nonlinear G4G1compartment model with gamma age dependency in the fast compartment. Results indicated that pasture quantity, crude protein and digestibility were higher in the rainy season than the dry season although the neutral detergent fibre contents were above 50% in both seasons, rendering the pasture low in quality. Among the pasture species, Brachiaria is the best in quality and quantity. Grazing constituted the highest proportion of the activities of Ankole and Ankole x Holstein crossbred cows in both seasons but the crossbreds grazed proportionally longer than Ankole (p<0.05). The short grazing period of Ankole permitted them to lie longer (p<0.01) on pasture than the crossbred cows especially in rainy season (p<0.05). The Ankole also ruminated longer (p<0.05) than the crossbreds. Unproductive behaviors such as walking (p<0.001), and social activities (p<0.001) occurred mainly in the rainy season. The proportions of various pasture species in the diets of pure Ankole and its Holstein crossbred cows also varied according to the seasons but were similar and positively correlated to crude protein, abundance and biomass of the pasture species. Patches containing Brachiaria and Cynodon were grazed longer than other pasture species patches. The forage ingested by Ankole and the crossbreds were lower in crude protein and dry matter digestibility in the dry season (p<0.001) than rainy season. Thus, digesta passage rates in both genotypes were low (p<0.01) in the dry season. Overall, the level of forage dry matter intake was less than 3% of their body weights although the crossbred ingested more forage than the Ankole. It was concluded that seasonal variations, especially in forage quality, is the main factor affecting the grazing behaviour, digesta kinetics and intake of pure Ankole and Ankole x Holstein crossbred cows. Therefore, improvements in the nutrition of pure Ankole and Ankole-Holstein crossbred cows on pasture would require increasing the prevalence of Brachiaria in the sward, supplementation and allowing the crossbreds ample time on pasture, especially in the dry season.